grit_lib/odb/

mod.rs

//! Loose object database: reading and writing zlib-compressed Git objects.
//!
//! Git stores objects as files under `<git-dir>/objects/<xx>/<38-hex-chars>`,
//! where the path is derived from the SHA-1 digest. Each file is a zlib-
//! compressed byte sequence whose decompressed form is:
//!
//! ```text
//! "<type> <size>\0<data>"
//! ```
//!
//! # Usage
//!
//! Files-backed repositories (default):
//!
//! ```no_run
//! use std::path::Path;
//! use grit_lib::objects::odb::Odb;
//!
//! let odb = Odb::new(Path::new(".git/objects"));
//! ```
//!
//! Custom primary store — see [`OdbBuilder`] and [`crate::repo::RepositoryBuilder`].

pub mod builder;
mod commit;
mod peel;
pub mod store;
mod stream;

#[doc(inline)]
pub use builder::OdbBuilder;

pub(crate) use store::loose::{
    decompress_zlib_loose_bytes, for_each_loose_object_id, parse_object_bytes,
    read_loose_object_info,
};
#[doc(inline)]
pub use store::LooseStore;
pub use store::{CompositeStore, FilesSource, ObjectStream};
#[doc(inline)]
pub use store::{MemoryStore, ObjectStore, WritableObjectStore};

use crate::git_path::CanonicalizeExt;
use std::collections::{HashMap, HashSet};
use std::fs;
use std::hash::{Hash, Hasher};
use std::ops::ControlFlow;
use std::path::{Path, PathBuf};
#[cfg(test)]
use std::sync::atomic::AtomicUsize;
use std::sync::atomic::{AtomicBool, AtomicU64, Ordering};
use std::sync::{Arc, Mutex, OnceLock, RwLock};

use crate::config::ConfigSet;
use crate::diagnostics::NullDiagnostics;
use crate::error::{Error, Result};
use crate::hash;
use crate::midx::validate_midx_referenced_packs_with_diagnostics;
use crate::objects::{
    oid_hash_set, HashAlgo, Object, ObjectId, ObjectInfo, ObjectKind, OidHashMap, OidHashSet,
};
use crate::pack;
use crate::pack_store::PackStore;
use filetime::{set_file_mtime, FileTime};
use flate2::Compression;

pub(crate) use store::BoundedRead;
use store::{LocalSizedRead, PackedObjects};

/// How repack/gc retains unreachable objects stored only in packs about to be deleted.
#[derive(Debug, Clone, Copy, PartialEq, Eq)]
pub(crate) enum UnreachablePackRetention {
    /// Keep every unreachable object (matches `git repack -ad` with `-k` / `gc.pruneExpire=never`).
    KeepAll,
    /// Keep unreachable objects whose pack mtime is not strictly older than the cutoff.
    KeepGraceProtected(std::time::SystemTime),
    /// Drop unreachable objects when their pack is removed (immediate prune).
    DropAll,
}

type MemOdbOverlayStore = Arc<RwLock<Option<Arc<MemoryStore>>>>;

/// Cached alternate [`CompositeStore`] keyed by file/env/submodule alternate state.
struct AlternateSourcesCache {
    file_generation: u64,
    env_fingerprint: u64,
    submodule_generation: u64,
    sources: Option<Arc<CompositeStore>>,
}

struct AlternateSourcesKey {
    file_generation: u64,
    env_fingerprint: u64,
    submodule_generation: u64,
}

/// Cached `info/alternates` chain for one [`Odb`] (generation bumps on invalidation/reload).
struct FileAlternatesCache {
    generation: u64,
    snapshot: Option<Arc<Vec<PathBuf>>>,
}

/// Options for [`Odb::write_with_options`].
#[derive(Debug, Clone, Copy, Default, PartialEq, Eq)]
pub struct WriteOptions {
    /// When set, do not touch mtimes on existing objects (Git `WRITE_OBJECT_SILENT`).
    pub silent: bool,
    /// When set, skip the full `Odb::exists` scan (packs/alternates) and only
    /// test for a loose file in this repository's object directory before writing.
    ///
    /// Bulk staging of new worktree blobs uses this so each insert does not walk
    /// every pack on a miss.
    pub assume_loose_only_existence: bool,
    /// When set, skip existence probes and write when the loose path is absent
    /// (bulk add of all-new blobs).
    pub trust_new_loose: bool,
    /// When set, skip `create_dir_all` on the loose prefix directory (call
    /// [`Odb::ensure_all_loose_prefix_dirs`] first).
    pub loose_prefix_dirs_ready: bool,
}

/// [`WriteOptions`] for parallel staging of new worktree blobs (bulk `git add`).
pub const BULK_NEW_LOOSE_WRITE: WriteOptions = WriteOptions {
    assume_loose_only_existence: true,
    trust_new_loose: true,
    silent: false,
    loose_prefix_dirs_ready: true,
};

/// Parallel restage of tracked modifications (objects may already be loose or packed).
pub const BULK_TRACKED_RESTAGE_LOOSE_WRITE: WriteOptions = WriteOptions {
    assume_loose_only_existence: true,
    trust_new_loose: false,
    silent: false,
    loose_prefix_dirs_ready: true,
};

/// True when `oid` is stored as a loose object or in a **non-promisor** local pack.
///
/// Non-promisor pack membership is checked before loose storage so packed objects
/// avoid a per-probe loose `stat` on the hot path. A non-promisor pack hit already
/// implies the object is materialized locally, including when a loose copy also exists.
fn exists_materialized_in_objects_dir(objects_dir: &Path, oid: &ObjectId) -> bool {
    store::local_object_materialized(objects_dir, oid)
}

/// The oids from `oids` for which `keep` answers `Ok(true)`, stopping at the first error.
fn filter_oids(
    oids: impl IntoIterator<Item = ObjectId>,
    mut keep: impl FnMut(&ObjectId) -> Result<bool>,
) -> Result<Vec<ObjectId>> {
    oids.into_iter()
        .filter_map(|oid| keep(&oid).map(|k| k.then_some(oid)).transpose())
        .collect()
}

fn objects_dir_has_pack_index_files(objects_dir: &Path) -> bool {
    let pack_dir = objects_dir.join("pack");
    let Ok(entries) = fs::read_dir(pack_dir) else {
        return false;
    };
    entries
        .filter_map(|e| e.ok())
        .any(|ent| ent.path().extension().is_some_and(|ext| ext == "idx"))
}

/// A loose-object database rooted at a given `objects/` directory.
///
/// # Examples
///
/// ```
/// use grit_lib::objects::{ObjectKind, odb::Odb};
/// use std::path::Path;
///
/// # fn main() -> grit_lib::error::Result<()> {
/// let dir = tempfile::tempdir()?;
/// let objects = dir.path().join("objects");
/// std::fs::create_dir_all(&objects)?;
/// let odb = Odb::new(&objects);
/// let oid = odb.write(ObjectKind::Blob, b"hello")?;
/// let obj = odb.read(&oid)?;
/// assert_eq!(obj.data, b"hello");
/// # Ok(())
/// # }
/// ```
#[derive(Clone)]
pub struct Odb {
    objects_dir: PathBuf,
    /// Work tree root for resolving relative alternate env paths.
    work_tree: Option<PathBuf>,
    /// Embedded submodule object stores registered for this read pass (Git `register_all_submodule_sources`).
    submodule_alternate_dirs: Arc<Mutex<Vec<PathBuf>>>,
    /// When set, used to read `core.multiPackIndex` (and related) for MIDX-backed object reads.
    config_git_dir: Option<PathBuf>,
    /// Cache for `core.multiPackIndex` — populated on first lookup.
    ///
    /// Reading this config requires loading the system/global/local config cascade and reparsing
    /// every file; the value cannot change for a process that has opened a single repository, so
    /// caching it here avoids re-loading the cascade for every object read.
    core_multi_pack_index_cache: Arc<OnceLock<bool>>,
    /// Whether `objects/pack` contains at least one `.idx` file (directory probe, once per Odb).
    local_pack_indexes_seen: Arc<OnceLock<bool>>,
    /// `info/alternates` chain resolved lazily; generation bumps on invalidation/reload.
    file_alternate_dirs_cache: Arc<RwLock<FileAlternatesCache>>,
    #[cfg(test)]
    exists_probe: Arc<AtomicUsize>,
    #[cfg(test)]
    pub(crate) hot_path_test_metrics: Arc<crate::hot_path_test_metrics::HotPathTestMetrics>,
    /// Explicit env alternates from [`Self::with_env_alternate_dirs`] (tests / callers); empty uses lazy env read.
    env_alternate_dirs: Vec<PathBuf>,
    /// Lazily parsed `GIT_ALTERNATE_OBJECT_DIRECTORIES` (first alternate lookup only).
    env_alternate_lazy: Arc<OnceLock<Arc<Vec<PathBuf>>>>,
    /// When `Some`, object writes are redirected into this in-memory overlay instead of being
    /// persisted to the loose store (Git's tmp-objdir). Reads consult the overlay first. This
    /// mirrors `git merge-tree --quiet`, which performs a full merge but must leave the object
    /// database untouched (no new loose objects).
    mem_overlay: MemOdbOverlayStore,
    /// Whether [`Self::enable_mem_overlay`] activated the in-memory overlay (avoids locking on reads).
    mem_overlay_active: Arc<AtomicBool>,
    /// Lazily built [`FilesSource`] for [`Self::objects_dir`].
    primary_source: Arc<OnceLock<Arc<FilesSource>>>,
    /// Alternate object dirs as a [`CompositeStore`] (generation-bumped with file alternates).
    alternate_sources_cache: Arc<RwLock<AlternateSourcesCache>>,
    /// Whether MIDX pack validation has run for this [`Odb`] (Git warns once per process).
    midx_packs_validated: Arc<OnceLock<()>>,
    /// The repository's object hash algorithm (`extensions.objectformat`),
    /// detected lazily from the config and cached. Determines the hash used
    /// when writing objects. Defaults to SHA-1 when no config is available.
    hash_algo_cache: Arc<OnceLock<HashAlgo>>,
    /// Zlib level for loose-object writes, resolved from config and cached.
    loose_zlib_cache: Arc<OnceLock<Compression>>,
    /// Zlib level for pack-objects output, resolved from config and cached.
    pack_zlib_cache: Arc<OnceLock<Compression>>,
    /// `core.bigFileThreshold`, resolved from config and cached.
    big_file_threshold_cache: Arc<OnceLock<u64>>,
    /// Shared with [`crate::repo::Repository`] so config is loaded once per open handle.
    shared_config_state: Option<crate::repo::RepositoryConfigSnapshot>,
    /// One-time sync of `core.deltaBaseCacheLimit` into the process-wide pack delta-base LRU.
    delta_base_cache_sync: Arc<OnceLock<()>>,
    /// Environment paired with [`Self::shared_config_state`] for repository-scoped config cache.
    shared_environment: Option<Arc<crate::environment::Environment>>,
    /// Fallback when [`Self::shared_environment`] is unset (repo-local config only).
    empty_config_env: Arc<OnceLock<crate::environment::Environment>>,
    /// Pack files whose mtimes were already bumped for object freshening on this [`Odb`]
    /// (Git's `packed_git->freshened`: at most one `utimensat` per pack per process).
    freshened_packs: Arc<Mutex<HashSet<PathBuf>>>,
    /// Repository-scoped pack/MIDX read caches (shared by [`Odb`] clones).
    pack_store: Arc<PackStore>,
    /// Cached [`PackStore`] handles for alternate object directories (lazy).
    alternate_pack_stores: Arc<Mutex<HashMap<PathBuf, Arc<PackStore>>>>,
    /// Bumps when [`Self::register_submodule_object_directories_from_index`] repopulates submodule sources.
    submodule_sources_generation: Arc<AtomicU64>,
    /// When set, replaces the lazy [`FilesSource`] primary ([`OdbBuilder::primary`]).
    custom_primary: Option<Arc<dyn WritableObjectStore>>,
    /// Read-only layers from [`OdbBuilder::push_read_source`], after primary, before alternates.
    extra_read_sources: Vec<Arc<dyn ObjectStore>>,
    /// When false, alternate object directories are not consulted ([`OdbBuilder::alternates`]).
    alternates_enabled: bool,
    /// Avoid re-reading the same loose object repeatedly during one repository session.
    session_loose_reads: Arc<std::sync::Mutex<SessionLooseReads>>,
    /// Parsed tree entries reused for the lifetime of this [`Odb`] handle (Git object cache).
    session_tree_entries: Arc<Mutex<SessionTreeEntries>>,
}

/// Largest loose object [`Odb::read_unless_larger`] keeps in the session read cache. Its callers
/// (checkout) promise bounded memory, so a cache of up to a few hundred entries must not be
/// able to pin hundreds of near-limit blobs.
const SESSION_BOUNDED_LOOSE_CACHE_MAX: usize = 64 * 1024;

/// Largest blob a general [`Odb::read`] keeps in the session read cache. Tiny blobs that are
/// read again and again (attributes, submodule and mailmap files) stay cached, while the
/// blobs a diff or merge reads once each pin at most this much per cache entry.
const SESSION_READ_BLOB_CACHE_MAX: usize = 4 * 1024;

/// Objects the session read cache holds before it starts over.
const SESSION_LOOSE_CACHE_MAX_ENTRIES: usize = 256;

/// Payload bytes the session read cache holds before it starts over, so no mix of reads can
/// make it pin more than this however large the objects are.
const SESSION_LOOSE_CACHE_MAX_BYTES: usize = 16 * 1024 * 1024;

/// Recently read objects reused within one [`Odb`] session, bounded by entry count and by
/// total payload bytes. When either bound would be exceeded the cache starts over.
#[derive(Default)]
struct SessionLooseReads {
    objects: OidHashMap<Object>,
    bytes: usize,
}

impl SessionLooseReads {
    fn get(&self, oid: &ObjectId) -> Option<&Object> {
        self.objects.get(oid)
    }

    fn insert(&mut self, oid: ObjectId, obj: &Object) {
        let len = obj.data.len();
        if len > SESSION_LOOSE_CACHE_MAX_BYTES {
            return;
        }
        if self.objects.len() > SESSION_LOOSE_CACHE_MAX_ENTRIES
            || self.bytes.saturating_add(len) > SESSION_LOOSE_CACHE_MAX_BYTES
        {
            self.objects.clear();
            self.bytes = 0;
        }
        if let Some(old) = self.objects.insert(oid, obj.clone()) {
            self.bytes = self.bytes.saturating_sub(old.data.len());
        }
        self.bytes += len;
    }

    #[cfg(test)]
    fn is_empty(&self) -> bool {
        self.objects.is_empty()
    }
}

/// Recently parsed trees for [`Odb::cached_tree_entries`], bounded by total entry count.
#[derive(Default)]
struct SessionTreeEntries {
    trees: OidHashMap<Arc<Vec<crate::objects::TreeEntry>>>,
    entries: usize,
}

impl SessionTreeEntries {
    /// Total tree entries kept before the cache starts over; a bigger tree is never
    /// kept. A parsed entry costs roughly 80 bytes, so this holds a few MiB: enough
    /// for the trees a diff of neighbouring commits revisits, without growing with
    /// history size.
    const ENTRY_BUDGET: usize = 1 << 16;

    fn insert(&mut self, oid: ObjectId, tree: Arc<Vec<crate::objects::TreeEntry>>) {
        if tree.len() > Self::ENTRY_BUDGET {
            return;
        }
        if self.entries + tree.len() > Self::ENTRY_BUDGET {
            self.trees.clear();
            self.entries = 0;
        }
        self.entries += tree.len();
        self.trees.insert(oid, tree);
    }
}

#[cfg(test)]
mod session_tree_entries_tests {
    use super::*;
    use crate::objects::TreeEntry;

    fn tree(entries: usize) -> Arc<Vec<TreeEntry>> {
        let entry = TreeEntry {
            mode: 0o100644,
            name: b"f".to_vec(),
            oid: ObjectId::zero(),
        };
        Arc::new(vec![entry; entries])
    }

    fn oid(n: u8) -> ObjectId {
        ObjectId::from_bytes(&[n; 20]).expect("oid")
    }

    #[test]
    fn cache_starts_over_when_the_entry_budget_is_exceeded() {
        let mut cache = SessionTreeEntries::default();
        let half = SessionTreeEntries::ENTRY_BUDGET / 2;
        cache.insert(oid(1), tree(half));
        cache.insert(oid(2), tree(half));
        assert_eq!(cache.entries, SessionTreeEntries::ENTRY_BUDGET);
        assert_eq!(cache.trees.len(), 2);

        cache.insert(oid(3), tree(1));
        assert_eq!(cache.trees.len(), 1);
        assert!(cache.trees.contains_key(&oid(3)));
        assert_eq!(cache.entries, 1);

        cache.insert(oid(4), tree(SessionTreeEntries::ENTRY_BUDGET + 1));
        assert!(
            !cache.trees.contains_key(&oid(4)),
            "an oversized tree is not cached"
        );
        assert!(cache.trees.contains_key(&oid(3)));
        assert!(cache.entries <= SessionTreeEntries::ENTRY_BUDGET);
    }
}

const _: () = {
    fn assert_send_sync<T: Send + Sync>() {}
    let _ = assert_send_sync::<Odb>;
};

impl std::fmt::Debug for Odb {
    fn fmt(&self, f: &mut std::fmt::Formatter<'_>) -> std::fmt::Result {
        f.debug_struct("Odb")
            .field("objects_dir", &self.objects_dir)
            .field("work_tree", &self.work_tree)
            .field("submodule_alternate_dirs", &"<mutex>")
            .field("config_git_dir", &self.config_git_dir)
            .finish()
    }
}

/// Resolve the repository hash algorithm from `extensions.objectformat` in `git_dir`'s config.
///
/// Uses the same config parser as [`Odb::hash_algo`], including case-insensitive section and
/// key names. Defaults to [`HashAlgo::Sha1`] when the extension is absent or unreadable.
#[must_use]
pub fn hash_algo_for_git_dir(env: &crate::environment::Environment, git_dir: &Path) -> HashAlgo {
    ConfigSet::load(env, Some(git_dir), true)
        .ok()
        .and_then(|cfg| cfg.get("extensions.objectformat"))
        .and_then(|v| HashAlgo::from_name(&v))
        .unwrap_or(HashAlgo::Sha1)
}

/// Like [`hash_algo_for_git_dir`] but takes an `objects/` directory path.
#[must_use]
pub fn hash_algo_for_objects_dir(
    env: &crate::environment::Environment,
    objects_dir: &Path,
) -> HashAlgo {
    objects_dir
        .parent()
        .map(|d| hash_algo_for_git_dir(env, d))
        .unwrap_or(HashAlgo::Sha1)
}

impl Odb {
    /// Create an [`Odb`] pointing at the given `objects/` directory.
    ///
    /// The directory does not need to exist yet; it will be created on the
    /// first write operation. Does not read `GIT_ALTERNATE_OBJECT_DIRECTORIES`;
    /// use [`Self::env_alternate_dirs_from_var`] and [`Self::with_env_alternate_dirs`]
    /// (or open via [`crate::repo::Repository`], which resolves env alternates lazily on first use).
    #[must_use]
    pub fn new(objects_dir: &Path) -> Self {
        Self {
            objects_dir: objects_dir.to_path_buf(),
            work_tree: None,
            submodule_alternate_dirs: Arc::new(Mutex::new(Vec::new())),
            config_git_dir: None,
            core_multi_pack_index_cache: Arc::new(OnceLock::new()),
            local_pack_indexes_seen: Arc::new(OnceLock::new()),
            file_alternate_dirs_cache: Arc::new(RwLock::new(FileAlternatesCache {
                generation: 0,
                snapshot: None,
            })),
            env_alternate_dirs: Vec::new(),
            env_alternate_lazy: Arc::new(OnceLock::new()),
            mem_overlay: Arc::new(RwLock::new(None)),
            mem_overlay_active: Arc::new(AtomicBool::new(false)),
            primary_source: Arc::new(OnceLock::new()),
            alternate_sources_cache: Arc::new(RwLock::new(AlternateSourcesCache {
                file_generation: 0,
                env_fingerprint: 0,
                submodule_generation: 0,
                sources: None,
            })),
            midx_packs_validated: Arc::new(OnceLock::new()),
            hash_algo_cache: Arc::new(OnceLock::new()),
            loose_zlib_cache: Arc::new(OnceLock::new()),
            pack_zlib_cache: Arc::new(OnceLock::new()),
            big_file_threshold_cache: Arc::new(OnceLock::new()),
            shared_config_state: None,
            delta_base_cache_sync: Arc::new(OnceLock::new()),
            shared_environment: None,
            empty_config_env: Arc::new(OnceLock::new()),
            freshened_packs: Arc::new(Mutex::new(HashSet::new())),
            pack_store: Arc::new(PackStore::new(objects_dir.to_path_buf())),
            alternate_pack_stores: Arc::new(Mutex::new(HashMap::new())),
            submodule_sources_generation: Arc::new(AtomicU64::new(0)),
            custom_primary: None,
            extra_read_sources: Vec::new(),
            alternates_enabled: true,
            session_loose_reads: Arc::new(Mutex::new(SessionLooseReads::default())),
            session_tree_entries: Arc::new(Mutex::new(SessionTreeEntries::default())),
            #[cfg(test)]
            exists_probe: Arc::new(AtomicUsize::new(0)),
            #[cfg(test)]
            hot_path_test_metrics: crate::hot_path_test_metrics::HotPathTestMetrics::new(),
        }
    }

    /// Create an [`Odb`] with a work tree for resolving relative alternate paths.
    ///
    /// Does not read `GIT_ALTERNATE_OBJECT_DIRECTORIES`; pass env-derived paths with
    /// [`Self::with_env_alternate_dirs`] (see [`Self::env_alternate_dirs_from_var`]).
    #[must_use]
    pub fn with_work_tree(objects_dir: &Path, work_tree: &Path) -> Self {
        Self {
            objects_dir: objects_dir.to_path_buf(),
            work_tree: Some(work_tree.to_path_buf()),
            submodule_alternate_dirs: Arc::new(Mutex::new(Vec::new())),
            config_git_dir: None,
            core_multi_pack_index_cache: Arc::new(OnceLock::new()),
            local_pack_indexes_seen: Arc::new(OnceLock::new()),
            file_alternate_dirs_cache: Arc::new(RwLock::new(FileAlternatesCache {
                generation: 0,
                snapshot: None,
            })),
            env_alternate_dirs: Vec::new(),
            env_alternate_lazy: Arc::new(OnceLock::new()),
            mem_overlay: Arc::new(RwLock::new(None)),
            mem_overlay_active: Arc::new(AtomicBool::new(false)),
            primary_source: Arc::new(OnceLock::new()),
            alternate_sources_cache: Arc::new(RwLock::new(AlternateSourcesCache {
                file_generation: 0,
                env_fingerprint: 0,
                submodule_generation: 0,
                sources: None,
            })),
            midx_packs_validated: Arc::new(OnceLock::new()),
            hash_algo_cache: Arc::new(OnceLock::new()),
            loose_zlib_cache: Arc::new(OnceLock::new()),
            pack_zlib_cache: Arc::new(OnceLock::new()),
            big_file_threshold_cache: Arc::new(OnceLock::new()),
            shared_config_state: None,
            delta_base_cache_sync: Arc::new(OnceLock::new()),
            shared_environment: None,
            empty_config_env: Arc::new(OnceLock::new()),
            freshened_packs: Arc::new(Mutex::new(HashSet::new())),
            pack_store: Arc::new(PackStore::new(objects_dir.to_path_buf())),
            alternate_pack_stores: Arc::new(Mutex::new(HashMap::new())),
            submodule_sources_generation: Arc::new(AtomicU64::new(0)),
            custom_primary: None,
            extra_read_sources: Vec::new(),
            alternates_enabled: true,
            session_loose_reads: Arc::new(Mutex::new(SessionLooseReads::default())),
            session_tree_entries: Arc::new(Mutex::new(SessionTreeEntries::default())),
            #[cfg(test)]
            exists_probe: Arc::new(AtomicUsize::new(0)),
            #[cfg(test)]
            hot_path_test_metrics: crate::hot_path_test_metrics::HotPathTestMetrics::new(),
        }
    }

    /// Parsed tree entries for `oid`, cached on this [`Odb`] handle.
    pub(crate) fn cached_tree_entries(
        &self,
        oid: &ObjectId,
    ) -> Result<Arc<Vec<crate::objects::TreeEntry>>> {
        if let Ok(guard) = self.session_tree_entries.lock() {
            if let Some(entries) = guard.trees.get(oid) {
                return Ok(Arc::clone(entries));
            }
        }
        let obj = self.read_impl(oid, false)?;
        if obj.kind != ObjectKind::Tree {
            return Err(Error::CorruptObject(format!(
                "expected tree, got {}",
                obj.kind.as_str()
            )));
        }
        let parsed = crate::objects::parse_tree_with_oid_len(&obj.data, self.hash_algo().len())?;
        let arc = Arc::new(parsed);
        if let Ok(mut guard) = self.session_tree_entries.lock() {
            guard.insert(*oid, Arc::clone(&arc));
        }
        Ok(arc)
    }

    fn session_cached_loose_read(&self, oid: &ObjectId) -> Option<Object> {
        self.session_loose_reads.lock().ok()?.get(oid).cloned()
    }

    /// Keep `obj` in the session read cache. Bounded checkout reads call this directly for
    /// the small blobs they reuse; general reads go through [`Self::finish_session_loose_read`].
    fn remember_session_loose_read(&self, oid: &ObjectId, obj: &Object) {
        if let Ok(mut guard) = self.session_loose_reads.lock() {
            guard.insert(*oid, obj);
        }
    }

    /// Return a freshly read object, caching commits, trees, tags and tiny blobs for reuse
    /// when `remember` is set.
    ///
    /// Larger blobs are not cached here (see [`SESSION_READ_BLOB_CACHE_MAX`]): diffs, merges
    /// and blame read each blob about once, so a cached copy only costs an extra allocation
    /// and keeps every blob of an operation resident until the cache starts over. Callers
    /// that do re-read small blobs opt in through [`Self::read_unless_larger`].
    fn finish_session_loose_read(
        &self,
        oid: &ObjectId,
        obj: Object,
        remember: bool,
    ) -> Result<Object> {
        if remember
            && (obj.kind != ObjectKind::Blob || obj.data.len() <= SESSION_READ_BLOB_CACHE_MAX)
        {
            self.remember_session_loose_read(oid, &obj);
        }
        Ok(obj)
    }

    /// Hot-path regression counters (crate-internal tests only).
    #[cfg(test)]
    #[must_use]
    pub(crate) fn hot_path_metrics(&self) -> &crate::hot_path_test_metrics::HotPathTestMetrics {
        &self.hot_path_test_metrics
    }

    #[cfg(test)]
    #[must_use]
    pub(crate) fn hot_path_metrics_arc(
        &self,
    ) -> Arc<crate::hot_path_test_metrics::HotPathTestMetrics> {
        Arc::clone(&self.hot_path_test_metrics)
    }

    /// Share the repository's lazy config snapshot (see [`crate::repo::Repository::config`]).
    #[must_use]
    pub(crate) fn with_shared_config_state(
        mut self,
        state: crate::repo::RepositoryConfigSnapshot,
        env: Arc<crate::environment::Environment>,
    ) -> Self {
        self.pack_store.set_repo_caches(Arc::clone(&state));
        self.shared_config_state = Some(state);
        self.shared_environment = Some(env);
        self
    }

    fn diagnostics_handle(&self) -> crate::diagnostics::DiagnosticsHandle {
        self.shared_config_state
            .as_ref()
            .map(|state| state.diagnostics_handle())
            .unwrap_or_else(|| Arc::new(NullDiagnostics))
    }

    fn load_config_cascade(&self) -> Result<ConfigSet> {
        if let (Some(caches), Some(env)) = (&self.shared_config_state, &self.shared_environment) {
            let git_dir = self
                .config_git_dir
                .as_deref()
                .or_else(|| self.objects_dir.parent());
            let cfg = caches.load_config(env.as_ref(), git_dir, true)?;
            return Ok(cfg.as_ref().clone());
        }
        let git_dir = self
            .config_git_dir
            .as_deref()
            .or_else(|| self.objects_dir.parent());
        if let Some(git_dir) = git_dir {
            ConfigSet::load(self.config_environment(), Some(git_dir), true)
        } else {
            Ok(ConfigSet::new())
        }
    }

    fn config_environment(&self) -> &crate::environment::Environment {
        if let Some(env) = self.shared_environment.as_deref() {
            return env;
        }
        self.empty_config_env
            .get_or_init(crate::environment::Environment::empty)
    }

    fn env_alternate_dirs_snapshot(&self) -> Arc<Vec<PathBuf>> {
        if !self.env_alternate_dirs.is_empty() {
            return Arc::new(self.env_alternate_dirs.clone());
        }
        Arc::clone(self.env_alternate_lazy.get_or_init(|| {
            Arc::new(Self::env_alternate_dirs_from_var(
                self.config_environment(),
                self.work_tree.as_deref(),
            ))
        }))
    }

    /// Parse `GIT_ALTERNATE_OBJECT_DIRECTORIES` once for [`Self::with_env_alternate_dirs`].
    ///
    /// Relative entries are resolved against `resolve_base` (typically the work tree root).
    #[must_use]
    pub fn env_alternate_dirs_from_var(
        env: &crate::environment::Environment,
        resolve_base: Option<&Path>,
    ) -> Vec<PathBuf> {
        match env.var("GIT_ALTERNATE_OBJECT_DIRECTORIES") {
            Some(val) if !val.is_empty() => {
                let mut dirs = parse_alternate_env(&val);
                if let Some(base) = resolve_base {
                    for dir in &mut dirs {
                        if dir.is_relative() {
                            *dir = base.join(&dir);
                        }
                    }
                }
                dirs
            }
            _ => Vec::new(),
        }
    }

    /// Attach env-derived alternate object directories (see [`Self::env_alternate_dirs_from_var`]).
    #[must_use]
    pub fn with_env_alternate_dirs(mut self, dirs: Vec<PathBuf>) -> Self {
        self.env_alternate_dirs = dirs;
        self.clear_alternate_sources_cache();
        self
    }

    fn clear_alternate_sources_cache(&self) {
        if let Ok(mut guard) = self.alternate_sources_cache.write() {
            guard.sources = None;
        }
    }

    fn alternate_sources_key(&self) -> AlternateSourcesKey {
        AlternateSourcesKey {
            file_generation: self
                .file_alternate_dirs_cache
                .read()
                .map(|g| g.generation)
                .unwrap_or(0),
            env_fingerprint: self.env_alternate_fingerprint(),
            submodule_generation: self.submodule_sources_generation.load(Ordering::Relaxed),
        }
    }

    fn env_alternate_fingerprint(&self) -> u64 {
        let mut hasher = std::collections::hash_map::DefaultHasher::new();
        for path in self.env_alternate_dirs_snapshot().iter() {
            path.hash(&mut hasher);
        }
        hasher.finish()
    }

    /// Drop the cached `info/alternates` chain so the next lookup re-reads the file.
    pub fn invalidate_alternates_cache(&self) {
        if let Ok(mut guard) = self.file_alternate_dirs_cache.write() {
            guard.generation = guard.generation.wrapping_add(1);
            guard.snapshot = None;
        }
        self.clear_alternate_sources_cache();
    }

    /// Reload `info/alternates` from disk into this [`Odb`]'s cache (used after external writes).
    ///
    /// # Errors
    ///
    /// Propagates failures from reading the alternates chain (I/O or malformed `info/alternates`).
    pub fn refresh_file_alternates_from_disk(&self) -> Result<()> {
        let snapshot = Arc::new(pack::read_alternates_recursive(&self.objects_dir)?);
        if let Ok(mut guard) = self.file_alternate_dirs_cache.write() {
            guard.generation = guard.generation.wrapping_add(1);
            guard.snapshot = Some(snapshot);
        }
        self.clear_alternate_sources_cache();
        Ok(())
    }

    /// Append `alternate_objects_dir` to `objects/info/alternates` (deduped) and refresh the cache.
    ///
    /// # Errors
    ///
    /// [`Error::Io`] when the alternates file cannot be read or written.
    pub fn append_file_alternate(&self, alternate_objects_dir: &Path) -> Result<()> {
        Self::append_alternate_objects_line(&self.objects_dir, alternate_objects_dir)?;
        self.refresh_file_alternates_from_disk()?;
        Ok(())
    }

    /// Append one absolute `objects` directory line to `objects/info/alternates` (deduped).
    ///
    /// Does not update any in-memory cache; pair with [`Self::refresh_file_alternates_from_disk`]
    /// on the relevant [`Odb`] when the same process keeps using it.
    pub fn append_alternate_objects_line(
        objects_dir: &Path,
        alternate_objects_dir: &Path,
    ) -> Result<()> {
        let info = objects_dir.join("info");
        fs::create_dir_all(&info)?;
        let alt_path = info.join("alternates");
        let alt_abs = alternate_objects_dir
            .canonicalize_simplified()
            .unwrap_or_else(|_| alternate_objects_dir.to_path_buf());
        let line = format!("{}\n", alt_abs.display());
        let existing = match fs::read_to_string(&alt_path) {
            Ok(s) => s,
            Err(e) if e.kind() == std::io::ErrorKind::NotFound => String::new(),
            Err(e) => return Err(Error::Io(e)),
        };
        if existing
            .lines()
            .any(|l| l.trim() == alt_abs.to_string_lossy())
        {
            return Ok(());
        }
        let mut out = existing;
        out.push_str(&line);
        fs::write(alt_path, out).map_err(Error::Io)
    }

    fn file_alternate_dirs_snapshot(&self) -> Arc<Vec<PathBuf>> {
        loop {
            let observed_gen = self
                .file_alternate_dirs_cache
                .read()
                .map(|g| g.generation)
                .unwrap_or(0);
            if let Ok(guard) = self.file_alternate_dirs_cache.read() {
                if guard.generation == observed_gen {
                    if let Some(snapshot) = &guard.snapshot {
                        return Arc::clone(snapshot);
                    }
                }
            }
            if let Ok(mut guard) = self.file_alternate_dirs_cache.write() {
                if guard.generation != observed_gen {
                    continue;
                }
                if let Some(snapshot) = &guard.snapshot {
                    return Arc::clone(snapshot);
                }
                let snapshot = Arc::new(
                    pack::read_alternates_recursive(&self.objects_dir).unwrap_or_default(),
                );
                guard.snapshot = Some(Arc::clone(&snapshot));
                return snapshot;
            }
            break;
        }
        Arc::new(Vec::new())
    }

    #[cfg(test)]
    pub(crate) fn exists_probe_count(&self) -> usize {
        self.exists_probe.load(Ordering::Relaxed)
    }

    #[cfg(test)]
    pub(crate) fn reset_exists_probe(&self) {
        self.exists_probe.store(0, Ordering::Relaxed);
    }

    /// Enable the in-memory write overlay (Git's tmp-objdir): subsequent [`Self::write`]/
    /// [`Self::write_local`] calls keep objects only in memory, and [`Self::read`] consults that
    /// overlay before the on-disk store. Used by `merge-tree --quiet` so a full merge can run
    /// without persisting any new loose objects.
    pub fn enable_mem_overlay(&self) {
        self.mem_overlay_active.store(true, Ordering::Release);
        if let Ok(mut guard) = self.mem_overlay.write() {
            if guard.is_none() {
                *guard = Some(Arc::new(MemoryStore::new(self.hash_algo())));
            }
        }
    }

    /// Disable the in-memory write overlay, discarding any objects accumulated in it.
    pub fn disable_mem_overlay(&self) {
        self.mem_overlay_active.store(false, Ordering::Release);
        if let Ok(mut guard) = self.mem_overlay.write() {
            *guard = None;
        }
    }

    /// Whether the in-memory write overlay is currently enabled.
    #[must_use]
    pub(crate) fn is_mem_overlay_active(&self) -> bool {
        self.mem_overlay_active.load(Ordering::Acquire)
    }

    fn overlay_active(&self) -> bool {
        self.mem_overlay
            .read()
            .ok()
            .and_then(|g| g.as_ref().map(|_| ()))
            .is_some()
    }

    fn overlay_store(&self) -> Option<Arc<MemoryStore>> {
        if !self.mem_overlay_active.load(Ordering::Acquire) {
            return None;
        }
        self.mem_overlay.read().ok().and_then(|g| g.clone())
    }

    /// Number of objects stored in the active mem overlay (tests only).
    #[cfg(test)]
    pub(crate) fn mem_overlay_len_for_tests(&self) -> Option<usize> {
        let store = self.overlay_store()?;
        let mut count = 0usize;
        store
            .for_each_object(&mut |_| {
                count += 1;
                ControlFlow::Continue(())
            })
            .ok()?;
        Some(count)
    }

    /// If the in-memory overlay is active, store `(kind, data)` under `oid` there and return
    /// `true`; otherwise return `false` so the caller falls through to the on-disk path.
    fn overlay_store_object(&self, oid: ObjectId, kind: ObjectKind, data: &[u8]) -> bool {
        let Some(store) = self.overlay_store() else {
            return false;
        };
        store.insert_object(oid, kind, Arc::from(data.to_owned()));
        true
    }

    /// Read `oid` from the in-memory overlay, if active and present.
    fn overlay_read(&self, oid: &ObjectId) -> Option<Object> {
        let store = self.overlay_store()?;
        store.read(oid).ok().flatten()
    }

    /// Primary on-disk object source for this repository's `objects/` directory.
    ///
    /// # Errors
    ///
    /// Propagates failures while opening pack, MIDX, or loose backends.
    pub fn primary(&self) -> Result<Arc<FilesSource>> {
        self.require_files_primary("primary")?;
        if let Some(src) = self.primary_source.get() {
            return Ok(Arc::clone(src));
        }
        let src = self.build_files_source(
            &self.objects_dir,
            self.multi_pack_index_reads_enabled(),
            self.midx_tip_listing_enabled(),
        )?;
        match self.primary_source.set(Arc::clone(&src)) {
            Ok(()) => Ok(src),
            Err(existing) => Ok(existing),
        }
    }

    /// Whether any alternate object directory is configured for this database.
    #[must_use]
    fn alternate_object_lookup_needed(&self) -> bool {
        if !self.file_alternate_dirs_snapshot().is_empty() {
            return true;
        }
        if !self.env_alternate_dirs_snapshot().is_empty() {
            return true;
        }
        self.submodule_alternate_dirs
            .lock()
            .ok()
            .is_some_and(|guard| !guard.is_empty())
    }

    /// Alternate object directories (`info/alternates`, env, submodules) as one composite store.
    ///
    /// # Errors
    ///
    /// Propagates failures while opening any alternate files source.
    pub fn sources(&self) -> Result<Arc<CompositeStore>> {
        if !self.alternates_enabled {
            return Ok(Arc::new(CompositeStore::new(Vec::new(), self.hash_algo())));
        }
        let key = self.alternate_sources_key();
        if let Ok(guard) = self.alternate_sources_cache.read() {
            if guard.file_generation == key.file_generation
                && guard.env_fingerprint == key.env_fingerprint
                && guard.submodule_generation == key.submodule_generation
            {
                if let Some(sources) = guard.sources.as_ref() {
                    return Ok(Arc::clone(sources));
                }
            }
        }
        let use_midx = self.multi_pack_index_reads_enabled();
        let midx_tip = self.midx_tip_listing_enabled();
        let mut stores: Vec<Arc<dyn ObjectStore>> = Vec::new();
        for alt_dir in self.file_alternate_dirs_snapshot().iter() {
            stores.push(self.build_files_source(alt_dir, use_midx, midx_tip)?);
        }
        for alt_dir in self.env_alternate_dirs_snapshot().iter() {
            stores.push(self.build_files_source(alt_dir, use_midx, midx_tip)?);
        }
        if let Ok(guard) = self.submodule_alternate_dirs.lock() {
            for alt_dir in guard.iter() {
                stores.push(self.build_files_source(alt_dir, false, false)?);
            }
        }
        let composite = Arc::new(CompositeStore::new(stores, self.hash_algo()));
        if let Ok(mut guard) = self.alternate_sources_cache.write() {
            guard.file_generation = key.file_generation;
            guard.env_fingerprint = key.env_fingerprint;
            guard.submodule_generation = key.submodule_generation;
            guard.sources = Some(Arc::clone(&composite));
        }
        Ok(composite)
    }

    fn midx_tip_listing_enabled(&self) -> bool {
        self.config_git_dir.is_some() && self.core_multi_pack_index_enabled()
    }

    fn build_files_source(
        &self,
        objects_dir: &Path,
        use_midx: bool,
        midx_tip_listing_for_exists: bool,
    ) -> Result<Arc<FilesSource>> {
        let loose = self.loose_at(objects_dir)?;
        let pack_store = self.pack_store_for(objects_dir);
        let diagnostics = if objects_dir == self.objects_dir.as_path() {
            self.diagnostics_handle()
        } else {
            Arc::new(NullDiagnostics)
        };
        FilesSource::open(
            pack_store,
            loose,
            use_midx,
            midx_tip_listing_for_exists,
            diagnostics,
        )
        .map(Arc::new)
    }

    /// Register `<submodule-git-dir>/objects` for every stage-0 gitlink in `index` that has a
    /// checkout under `work_tree`, so reads can resolve submodule commits stored only in the
    /// nested repository (matches Git's `odb_add_submodule_source_by_path` / `register_all_submodule_sources`).
    pub fn register_submodule_object_directories_from_index(
        &self,
        work_tree: &Path,
        index: &crate::index::Index,
    ) {
        use crate::diff::submodule_embedded_git_dir;

        let Ok(mut dirs) = self.submodule_alternate_dirs.lock() else {
            return;
        };
        dirs.clear();
        for e in &index.entries {
            if e.stage() != 0 || e.mode != crate::index::MODE_GITLINK {
                continue;
            }
            let path_str = String::from_utf8_lossy(&e.path);
            let abs = work_tree.join(path_str.as_ref());
            let Some(sub_git) = submodule_embedded_git_dir(&abs) else {
                continue;
            };
            let objects = sub_git.join("objects");
            if !objects.is_dir() {
                continue;
            }
            let canon = objects.canonicalize_simplified().unwrap_or(objects);
            if !dirs.iter().any(|p| p == &canon) {
                dirs.push(canon);
            }
        }
        self.submodule_sources_generation
            .fetch_add(1, Ordering::Relaxed);
        self.clear_alternate_sources_cache();
    }

    /// Attach a git directory so [`Self::read`] can honor `core.multiPackIndex` when resolving packed objects.
    #[must_use]
    pub fn with_config_git_dir(mut self, git_dir: PathBuf) -> Self {
        self.config_git_dir = Some(git_dir);
        self
    }

    /// The repository's object hash algorithm, detected from
    /// `extensions.objectformat` and cached for the lifetime of this `Odb`.
    ///
    /// The git directory is taken from the attached `config_git_dir` when set,
    /// otherwise inferred as the parent of the `objects/` directory. Defaults
    /// to [`HashAlgo::Sha1`] when no config can be read.
    ///
    /// With a custom [`OdbBuilder::primary`] this is the store's own algorithm. A
    /// successful [`RepositoryBuilder::open`](crate::repo::RepositoryBuilder::open) has
    /// already checked that it equals the repository's object format; a standalone
    /// [`OdbBuilder::build`] reports the store's algorithm unchecked.
    #[must_use]
    pub fn hash_algo(&self) -> HashAlgo {
        if let Some(primary) = &self.custom_primary {
            return primary.hash_algo();
        }
        *self.hash_algo_cache.get_or_init(|| {
            if self.config_git_dir.is_none() && self.objects_dir.parent().is_none() {
                return HashAlgo::Sha1;
            }
            let cfg = self.load_config_cascade().unwrap_or_default();
            cfg.get("extensions.objectformat")
                .and_then(|v| HashAlgo::from_name(&v))
                .unwrap_or(HashAlgo::Sha1)
        })
    }

    fn resolve_loose_zlib_level(&self) -> Result<u32> {
        if self.config_git_dir.is_none() && self.objects_dir.parent().is_none() {
            return Ok(ConfigSet::LOOSE_OBJECTS_ZLIB_DEFAULT as u32);
        }
        let cfg = self.load_config_cascade()?;
        Ok(cfg.loose_objects_zlib_level()? as u32)
    }

    /// Zlib compression used for loose-object writes ([`ConfigSet::loose_objects_zlib_level`]),
    /// cached for the lifetime of this `Odb`.
    ///
    /// # Errors
    ///
    /// Returns errors from loading the config cascade or from invalid compression settings.
    pub(crate) fn loose_compression(&self) -> Result<Compression> {
        if let Some(cached) = self.loose_zlib_cache.get() {
            return Ok(*cached);
        }
        let level = self.resolve_loose_zlib_level()?;
        let compression = Compression::new(level);
        let _ = self.loose_zlib_cache.set(compression);
        Ok(compression)
    }

    fn resolve_pack_zlib_level(&self) -> Result<i32> {
        if self.config_git_dir.is_none() && self.objects_dir.parent().is_none() {
            return Ok(6);
        }
        let cfg = self.load_config_cascade()?;
        cfg.pack_objects_zlib_level()
    }

    /// Zlib compression for pack-objects ([`ConfigSet::pack_objects_zlib_level`]).
    ///
    /// # Errors
    ///
    /// Returns errors from loading the config cascade or from invalid compression settings.
    pub(crate) fn pack_compression(&self) -> Result<Compression> {
        if let Some(cached) = self.pack_zlib_cache.get() {
            return Ok(*cached);
        }
        let level = self.resolve_pack_zlib_level()?;
        let compression = crate::pack_zlib::flate2_compression_for_pack_level(level);
        let _ = self.pack_zlib_cache.set(compression);
        Ok(compression)
    }

    /// `core.bigFileThreshold` ([`ConfigSet::big_file_threshold`]), cached for the lifetime of
    /// this `Odb`.
    ///
    /// Objects of at least this size are written by [`Self::write_stream`] into their own
    /// pack, and worktree files of at least this size that need no clean conversion are hashed
    /// and stored as a stream instead of being read into memory.
    ///
    /// # Errors
    ///
    /// Returns errors from loading the config cascade or from an invalid threshold value.
    pub fn big_file_threshold(&self) -> Result<u64> {
        if let Some(cached) = self.big_file_threshold_cache.get() {
            return Ok(*cached);
        }
        let threshold = if self.config_git_dir.is_none() && self.objects_dir.parent().is_none() {
            ConfigSet::BIG_FILE_THRESHOLD_DEFAULT
        } else {
            self.load_config_cascade()?.big_file_threshold()?
        };
        let _ = self.big_file_threshold_cache.set(threshold);
        Ok(threshold)
    }

    /// An operation-local view with bounded pack residency and tree delta bases.
    /// Custom stores and overlays are preserved; file sources and alternates are rebuilt
    /// against private caches so this view cannot evict or reconfigure a caller's caches.
    pub(crate) fn with_bounded_pack_cache(&self) -> Self {
        let mut odb = self.clone();
        odb.pack_store = Arc::new(PackStore::for_bounded_reads(self.objects_dir.clone()));
        odb.primary_source = Arc::new(OnceLock::new());
        odb.session_loose_reads = Arc::new(Mutex::new(SessionLooseReads::default()));
        odb.alternate_pack_stores = Arc::new(Mutex::new(HashMap::new()));
        odb.alternate_sources_cache = Arc::new(RwLock::new(AlternateSourcesCache {
            file_generation: 0,
            env_fingerprint: 0,
            submodule_generation: 0,
            sources: None,
        }));
        odb
    }

    fn sync_delta_base_cache_limit(&self) {
        if self.pack_store.bounded_reads() {
            return;
        }
        self.delta_base_cache_sync.get_or_init(|| {
            if self.config_git_dir.is_some() {
                let cfg = self.load_config_cascade().unwrap_or_default();
                self.pack_store.configure_delta_base_from_config(Some(&cfg));
            }
        });
    }

    pub(crate) fn pack_store_for(&self, objects_dir: &Path) -> Arc<PackStore> {
        if objects_dir == self.objects_dir.as_path() {
            return Arc::clone(&self.pack_store);
        }
        let new_store = || {
            Arc::new(if self.pack_store.bounded_reads() {
                PackStore::for_bounded_reads(objects_dir.to_path_buf())
            } else {
                PackStore::new(objects_dir.to_path_buf())
            })
        };
        if let Ok(mut guard) = self.alternate_pack_stores.lock() {
            return Arc::clone(
                guard
                    .entry(objects_dir.to_path_buf())
                    .or_insert_with(new_store),
            );
        }
        new_store()
    }

    /// Pack/MIDX read cache shared by this handle and its [`Clone`]s.
    pub fn pack_store(&self) -> Result<&Arc<PackStore>> {
        self.require_files_primary("pack_store")?;
        Ok(&self.pack_store)
    }

    /// Path to the largest pack index in the primary `objects/pack/` directory (by entry count).
    ///
    /// Used when writing a reachability bitmap for an all-into-one repack. Returns `Ok(None)`
    /// when no pack index exists.
    ///
    /// # Errors
    ///
    /// Returns [`Error::InvalidRef`] when the store is not files-backed.
    pub fn largest_pack_index_path(&self) -> Result<Option<PathBuf>> {
        self.require_files_primary("largest_pack_index_path")?;
        let mut indexes = pack::read_local_pack_indexes_cached(self.objects_dir())?;
        indexes.sort_by_key(|idx| std::cmp::Reverse(idx.len()));
        Ok(indexes.first().map(|idx| idx.idx_path.clone()))
    }

    /// Drop the resolved delta bases cached for the primary `objects/` packs, keeping
    /// indexes and pack bytes. Frees memory between phases of a long operation whose
    /// next reads will not revisit the same bases.
    pub(crate) fn clear_delta_base_cache(&self) {
        self.pack_store.delta_bases().clear();
    }

    /// Drop cached pack listings, parsed indexes, pack bytes, delta bases, and MIDX layers.
    ///
    /// Call after in-process repack, garbage collection, or [`crate::index_pack::install_pack_bytes`]
    /// so the next read rescans `objects/pack/`. Cloned [`Odb`] handles share one
    /// [`PackStore`] for the primary `objects/` directory; alternates have separate stores
    /// that are invalidated here as well.
    pub fn invalidate_packs(&self) {
        self.pack_store.invalidate_all();
        PackStore::invalidate_legacy_for_objects_dir(&self.objects_dir);
        if let Ok(guard) = self.alternate_pack_stores.lock() {
            for store in guard.values() {
                store.invalidate_all();
                PackStore::invalidate_legacy_for_objects_dir(store.objects_dir());
            }
        }
        self.invalidate_change_id_header_probe();
    }

    fn invalidate_change_id_header_probe(&self) {
        if let Some(caches) = self.shared_config_state.as_ref() {
            crate::change_id::invalidate_stored_change_id_header_probe(caches, self.objects_dir());
        }
    }

    fn note_commit_object_written(&self, kind: ObjectKind) {
        if kind == ObjectKind::Commit {
            self.invalidate_change_id_header_probe();
        }
    }

    /// Materialize unreachable objects from packfiles superseded by a new repack pack.
    ///
    /// Git gc stores grace-protected unreachable objects in cruft packs; grit writes them
    /// loose (with preserved mtimes) so later loose pruning can apply `gc.pruneExpire`.
    ///
    /// `reachable` is only walked when a superseded pack holds an object that is in
    /// neither the new pack nor a `.keep` pack. `retained_packs` protects ordinary
    /// packs that supply available promises from being superseded or loosened.
    ///
    /// # Errors
    ///
    /// Propagates pack index read failures, object read/write errors, I/O failures,
    /// and errors from the reachability walk.
    pub(crate) fn loosen_unreachable_from_superseded_packs(
        &self,
        pack_dir: &Path,
        keep_pack: &Path,
        reachable: &crate::gc::LazyReachable<'_>,
        retention: UnreachablePackRetention,
        retained_packs: &HashSet<PathBuf>,
    ) -> Result<usize> {
        if matches!(retention, UnreachablePackRetention::DropAll) {
            return Ok(0);
        }
        self.require_files_primary("repack")?;
        let keep_idx = keep_pack.with_extension("idx");
        let keep_pack_oids = if keep_idx.is_file() {
            PackedObjects::object_ids_from_index_path(&keep_idx)?
        } else {
            HashSet::new()
        };
        let kept_pack_oids = oids_in_marker_kept_packs(pack_dir, keep_pack)?;
        let mut loosened = 0usize;
        for (idx_path, pack_mtime) in superseded_pack_indexes(pack_dir, keep_pack, retained_packs)?
        {
            let oids = PackedObjects::object_ids_from_index_path(&idx_path)?;
            for oid in oids {
                let keep = match retention {
                    UnreachablePackRetention::KeepAll => true,
                    UnreachablePackRetention::KeepGraceProtected(cutoff) => pack_mtime >= cutoff,
                    UnreachablePackRetention::DropAll => false,
                };
                if !keep {
                    continue;
                }
                if keep_pack_oids.contains(&oid) || kept_pack_oids.contains(&oid) {
                    continue;
                }
                if reachable.get()?.contains(&oid) {
                    continue;
                }
                let loose_path = oid.loose_path_in(&self.objects_dir);
                let pack_ft = system_time_to_file_time(pack_mtime);
                match retention {
                    UnreachablePackRetention::KeepGraceProtected(cutoff) => {
                        if loose_path.is_file() {
                            let loose_mtime = fs::metadata(&loose_path)
                                .and_then(|m| m.modified())
                                .unwrap_or(std::time::SystemTime::UNIX_EPOCH);
                            if loose_mtime >= cutoff {
                                continue;
                            }
                            set_file_mtime(&loose_path, pack_ft).map_err(Error::Io)?;
                            loosened += 1;
                            continue;
                        }
                    }
                    UnreachablePackRetention::KeepAll => {
                        if loose_path.is_file() {
                            continue;
                        }
                    }
                    UnreachablePackRetention::DropAll => {}
                }
                let obj = self.read(&oid)?;
                self.write_loose_materialize(obj.kind, &obj.data)?;
                let primary = self.primary()?;
                let loose_path = primary.loose_store().object_path(&oid);
                set_file_mtime(&loose_path, pack_ft).map_err(Error::Io)?;
                loosened += 1;
            }
        }
        Ok(loosened)
    }

    fn core_multi_pack_index_enabled(&self) -> bool {
        // The system/global/local config cascade is expensive to load (the parser walks every
        // file from `/etc/gitconfig` through `.git/config`); calling it once per object lookup
        // dominated `status` runtime. Cache the result for the lifetime of this `Odb`.
        *self.core_multi_pack_index_cache.get_or_init(|| {
            if self.config_git_dir.is_none() {
                return false;
            }
            let cfg = self.load_config_cascade().unwrap_or_default();
            match cfg.get_bool("core.multiPackIndex") {
                Some(Ok(b)) => b,
                Some(Err(_)) => true,
                None => true,
            }
        })
    }

    fn multi_pack_index_reads_enabled(&self) -> bool {
        if self.custom_primary.is_some() {
            return false;
        }
        if self.config_git_dir.is_none() {
            return false;
        }
        if !*self
            .local_pack_indexes_seen
            .get_or_init(|| objects_dir_has_pack_index_files(&self.objects_dir))
        {
            return false;
        }
        self.core_multi_pack_index_enabled()
    }

    fn ensure_midx_prepared(&self) {
        if !self.multi_pack_index_reads_enabled() {
            return;
        }
        let _ = self.midx_packs_validated.get_or_init(|| {
            let diagnostics = self.diagnostics_handle();
            self.with_pack_store_for(&self.objects_dir, || {
                validate_midx_referenced_packs_with_diagnostics(
                    &self.objects_dir,
                    diagnostics.as_ref(),
                );
            });
        });
    }

    /// Direct pack read when [`PackStore::with_context`] matches the primary `objects/` tree.
    ///
    /// Skips [`ObjectStore`] dynamic dispatch on batch cat-file workloads; MIDX repos use the
    /// full primary path instead.
    fn try_read_packed_in_batch_context(&self, oid: &ObjectId) -> Result<Option<Object>> {
        if self.custom_primary.is_some()
            || !PackStore::read_context_matches_objects_dir(&self.objects_dir)
            || self.multi_pack_index_reads_enabled()
        {
            return Ok(None);
        }
        self.sync_delta_base_cache_limit();
        match pack::try_read_object_from_packs_with_options(
            &self.objects_dir,
            oid,
            pack::PackLookupOptions::ALL_LOCAL_PACKS,
        ) {
            Ok(obj) => Ok(Some(obj)),
            Err(Error::ObjectNotFound(_)) => Ok(None),
            Err(err) => Err(err),
        }
    }

    fn try_read_info_packed_in_batch_context(&self, oid: &ObjectId) -> Result<Option<ObjectInfo>> {
        if self.custom_primary.is_some()
            || !PackStore::read_context_matches_objects_dir(&self.objects_dir)
            || self.multi_pack_index_reads_enabled()
        {
            return Ok(None);
        }
        match pack::try_read_object_info_from_packs_with_options(
            &self.objects_dir,
            oid,
            pack::PackLookupOptions::ALL_LOCAL_PACKS,
        ) {
            Ok(info) => Ok(Some(info)),
            Err(Error::ObjectNotFound(_)) => Ok(None),
            Err(err) => Err(err),
        }
    }

    /// Return the path to the `objects/` directory.
    ///
    /// This is always the repository layout path (including for custom primaries that do not
    /// store objects on disk). Prefer [`Self::files_objects_dir`] when the operation requires a
    /// files-backed primary; filesystem maintenance returns
    /// [`Error::UnsupportedObjectStore`] otherwise.
    #[must_use]
    pub fn objects_dir(&self) -> &Path {
        &self.objects_dir
    }

    /// The repository's commit-graph chain, loaded at most once per repository handle.
    ///
    /// `None` when this database is not attached to a repository's caches (built
    /// standalone) or has no readable commit-graph; callers then read commits directly.
    pub(crate) fn commit_graph_chain(
        &self,
    ) -> Option<Arc<crate::commit_graph_file::CommitGraphChain>> {
        self.shared_config_state
            .as_ref()?
            .commit_graph_chain(self.objects_dir())
    }

    fn try_read_extra_sources(&self, oid: &ObjectId) -> Result<Option<Object>> {
        for src in &self.extra_read_sources {
            if let Some(obj) = src.read(oid)? {
                return Ok(Some(obj));
            }
        }
        Ok(None)
    }

    fn try_read_info_extra_sources(&self, oid: &ObjectId) -> Result<Option<ObjectInfo>> {
        for src in &self.extra_read_sources {
            if let Some(info) = src.read_info(oid)? {
                return Ok(Some(info));
            }
        }
        Ok(None)
    }

    fn try_open_stream_extra_sources(
        &self,
        oid: &ObjectId,
    ) -> Result<Option<store::ObjectStream<'_>>> {
        for src in &self.extra_read_sources {
            if let Some(stream) = src.open_stream(oid)? {
                return Ok(Some(stream));
            }
        }
        Ok(None)
    }

    /// Return the attached git directory, if one was set with
    /// [`Self::with_config_git_dir`].
    ///
    /// Used by config- and ref-aware operations (delta islands, on-disk delta
    /// reuse) that need the repository root rather than just the object store.
    #[must_use]
    pub fn config_git_dir(&self) -> Option<&Path> {
        self.config_git_dir.as_deref()
    }

    /// Return the filesystem path for a given object ID.
    #[must_use]
    pub fn object_path(&self, oid: &ObjectId) -> PathBuf {
        oid.loose_path_in(&self.objects_dir)
    }

    /// Pack indexes in this database's own `objects/pack` (not alternates), from
    /// the shared index cache.
    ///
    /// # Errors
    ///
    /// Propagates pack directory listing and index parse failures.
    pub(crate) fn local_pack_indexes(&self) -> Result<Vec<Arc<pack::PackIndex>>> {
        pack::read_local_pack_indexes_cached(&self.objects_dir)
    }

    /// Size in bytes of `oid`'s loose file in this database's own `objects/`,
    /// or `None` when it is not stored loose here.
    pub(crate) fn local_loose_file_size(&self, oid: &ObjectId) -> Option<u64> {
        fs::metadata(self.object_path(oid))
            .ok()
            .filter(fs::Metadata::is_file)
            .map(|meta| meta.len())
    }

    /// Whether the object exists under this database directory only (loose or local packs).
    ///
    /// Unlike [`Self::exists`], this ignores `info/alternates` and
    /// `GIT_ALTERNATE_OBJECT_DIRECTORIES`. Used for partial-clone bookkeeping where
    /// objects reachable via alternates are still treated as "missing" until copied locally.
    ///
    /// Objects stored **only** in promisor packs (sibling `.promisor` marker next to the
    /// `.pack`) are treated as absent: Git considers them fetchable on demand, and
    /// `rev-list --missing=print` lists them until materialized as loose objects or a
    /// non-promisor pack.
    ///
    /// The empty tree object is treated as present without a loose file (matches Git).
    ///
    /// A failing custom primary store counts as "absent"; decision paths inside the
    /// crate use the fallible form, which propagates the backend error.
    #[must_use]
    pub fn exists_local(&self, oid: &ObjectId) -> bool {
        self.try_exists_local(oid).unwrap_or(false)
    }

    /// Fallible [`Self::exists_local`]: errors from a custom primary store propagate.
    pub(crate) fn try_exists_local(&self, oid: &ObjectId) -> Result<bool> {
        if self.is_empty_tree(oid) {
            return Ok(true);
        }
        if let Some(primary) = &self.custom_primary {
            return primary.contains(oid);
        }
        Ok(self.with_pack_store_for(&self.objects_dir, || {
            exists_materialized_in_objects_dir(&self.objects_dir, oid)
        }))
    }

    /// Whether `oid` is stored locally in loose storage or any pack (including promisor).
    ///
    /// Unlike [`Self::exists_local`], objects present only in promisor-marked packs count
    /// as stored. Does not trigger lazy fetch or consult alternates.
    ///
    /// A failing custom primary store counts as "absent"; decision paths inside the
    /// crate use the fallible form, which propagates the backend error.
    #[must_use]
    pub fn exists_stored(&self, oid: &ObjectId) -> bool {
        self.try_exists_stored(oid).unwrap_or(false)
    }

    /// Fallible [`Self::exists_stored`]: errors from a custom primary store propagate.
    pub(crate) fn try_exists_stored(&self, oid: &ObjectId) -> Result<bool> {
        if oid.is_canonical_empty_tree() || self.overlay_read(oid).is_some() {
            return Ok(true);
        }
        if let Some(primary) = &self.custom_primary {
            return primary.contains(oid);
        }
        Ok(self.with_pack_store_for(&self.objects_dir, || {
            store::local_object_stored(&self.objects_dir, oid)
        }))
    }

    /// Whether `oid` is stored on disk in this repository's object store (loose or
    /// non-promisor pack), without treating the canonical empty tree as virtually
    /// present.
    ///
    /// Use this when deciding whether a peer already holds an object for incremental
    /// local transfer; [`Self::exists`] and [`Self::exists_local`] treat the empty
    /// tree as present even when no object file exists.
    ///
    /// A failing custom primary store counts as "absent".
    #[must_use]
    pub fn exists_materialized_local(&self, oid: &ObjectId) -> bool {
        if let Some(primary) = &self.custom_primary {
            return primary.contains(oid).unwrap_or(false);
        }
        self.with_pack_store_for(&self.objects_dir, || {
            exists_materialized_in_objects_dir(&self.objects_dir, oid)
        })
    }

    /// Bring this repository's cached pack listing up to date with `objects/pack/`.
    ///
    /// Call once before a batch of [`Self::exists_materialized_local_without_rescan`]
    /// probes so packs written before the batch are visible.
    pub(crate) fn reprepare_local_packs(&self) {
        if self.custom_primary.is_some() {
            return;
        }
        self.with_pack_store_for(&self.objects_dir, || {
            let _ = crate::pack::reprepare_pack_directory_on_miss(&self.objects_dir);
        });
    }

    /// Like [`Self::exists_materialized_local`], but a miss does not rescan
    /// `objects/pack/`.
    ///
    /// For walks that probe many objects that are expected to be absent (local
    /// clone/fetch/push boundary checks): each miss would otherwise list the pack
    /// directory again. Packs that appear after [`Self::reprepare_local_packs`] are
    /// not seen, so a miss can be stale; callers must treat it as "send anyway".
    pub(crate) fn exists_materialized_local_without_rescan(&self, oid: &ObjectId) -> bool {
        if self.custom_primary.is_some() {
            return self.exists_materialized_local(oid);
        }
        self.with_pack_store_for(&self.objects_dir, || {
            store::local_object_materialized_without_rescan(&self.objects_dir, oid)
        })
    }

    /// Whether `oid` is in this database: the primary store, any extra read source
    /// ([`OdbBuilder::push_read_source`]) or an alternate object directory.
    ///
    /// The canonical empty tree is always present. Sources are probed in the same order
    /// [`Self::read`] uses, and a source that fails stops the probe just as it stops a
    /// read, so `contains` never reports an object that `read` cannot return.
    ///
    /// # Errors
    ///
    /// The first source failure (for example an [`Error::Io`] from a network- or
    /// database-backed store), instead of reporting the object as present or missing.
    pub fn contains(&self, oid: &ObjectId) -> Result<bool> {
        self.probe_sources(oid, Err)
    }

    /// Probe every source for `oid` in lookup order; `on_err` decides whether a failing
    /// source aborts the probe (`Err`) or is skipped (`Ok`).
    fn probe_sources(
        &self,
        oid: &ObjectId,
        mut on_err: impl FnMut(Error) -> Result<()>,
    ) -> Result<bool> {
        #[cfg(test)]
        self.exists_probe.fetch_add(1, Ordering::Relaxed);
        if self.is_empty_tree(oid) || self.overlay_read(oid).is_some() {
            return Ok(true);
        }
        let mut found = |probe: Result<bool>| match probe {
            Ok(present) => Ok(present),
            Err(err) => on_err(err).map(|()| false),
        };
        if found(self.primary_writable().and_then(|p| p.contains(oid)))? {
            return Ok(true);
        }
        for src in &self.extra_read_sources {
            if found(src.contains(oid))? {
                return Ok(true);
            }
        }
        if !self.alternate_object_lookup_needed() {
            return Ok(false);
        }
        found(self.sources().and_then(|s| s.contains(oid)))
    }

    /// [`Self::contains`] for every id in `oids`, one answer per id in input order.
    ///
    /// Each source is asked once about the ids no earlier source holds, through
    /// [`ObjectStore::contains_many`], so a database- or network-backed primary answers
    /// the whole batch in one round trip instead of one per id.
    ///
    /// # Errors
    ///
    /// The first source failure, as for [`Self::contains`].
    pub(crate) fn contains_many(&self, oids: &[ObjectId]) -> Result<Vec<bool>> {
        #[cfg(test)]
        self.exists_probe.fetch_add(oids.len(), Ordering::Relaxed);
        let mut present: Vec<bool> = oids
            .iter()
            .map(|oid| self.is_empty_tree(oid) || self.overlay_read(oid).is_some())
            .collect();
        let mut pending: Vec<usize> = (0..oids.len()).filter(|&i| !present[i]).collect();
        store::resolve_pending(oids, &mut present, &mut pending, |batch| {
            self.primary_writable()?.contains_many(batch)
        })?;
        for src in &self.extra_read_sources {
            store::resolve_pending(oids, &mut present, &mut pending, |batch| {
                src.contains_many(batch)
            })?;
        }
        if !pending.is_empty() && self.alternate_object_lookup_needed() {
            store::resolve_pending(oids, &mut present, &mut pending, |batch| {
                self.sources()?.contains_many(batch)
            })?;
        }
        Ok(present)
    }

    /// The oids from `oids` that [`Self::contains`] reports present, in input order.
    ///
    /// # Errors
    ///
    /// Propagates the first [`Self::contains`] failure.
    pub(crate) fn present_oids(
        &self,
        oids: impl IntoIterator<Item = ObjectId>,
    ) -> Result<Vec<ObjectId>> {
        self.partition_oids(oids, true)
    }

    /// The oids from `oids` that [`Self::contains`] reports missing, in input order.
    ///
    /// # Errors
    ///
    /// Propagates the first [`Self::contains`] failure.
    pub(crate) fn missing_oids(
        &self,
        oids: impl IntoIterator<Item = ObjectId>,
    ) -> Result<Vec<ObjectId>> {
        self.partition_oids(oids, false)
    }

    /// The oids from `oids` whose [`Self::contains_many`] answer equals `want_present`.
    fn partition_oids(
        &self,
        oids: impl IntoIterator<Item = ObjectId>,
        want_present: bool,
    ) -> Result<Vec<ObjectId>> {
        let oids: Vec<ObjectId> = oids.into_iter().collect();
        let present = self.contains_many(&oids)?;
        Ok(oids
            .into_iter()
            .zip(present)
            .filter_map(|(oid, has)| (has == want_present).then_some(oid))
            .collect())
    }

    /// The oids from `oids` that are not stored locally per [`Self::exists_stored`]
    /// (alternates are not consulted), in input order.
    ///
    /// # Errors
    ///
    /// Propagates the first failure of a custom primary store.
    pub(crate) fn missing_stored_oids(
        &self,
        oids: impl IntoIterator<Item = ObjectId>,
    ) -> Result<Vec<ObjectId>> {
        filter_oids(oids, |oid| self.try_exists_stored(oid).map(|has| !has))
    }

    /// Lenient [`Self::contains`]: a source that fails is skipped, so a later source can
    /// still answer `true` and an object no healthy source holds counts as absent.
    ///
    /// Use only for advisory probes. Where "missing" changes what happens next (what to
    /// fetch or send, whether a ref is broken), call [`Self::contains`] so a backend
    /// outage surfaces as an error rather than as a missing object.
    #[must_use]
    pub fn exists(&self, oid: &ObjectId) -> bool {
        self.probe_sources(oid, |_| Ok(())).unwrap_or(false)
    }

    /// Run `f` with this [`Odb`]'s [`PackStore`] for `objects_dir` (primary or alternate).
    fn with_pack_store_for<R>(&self, objects_dir: &Path, f: impl FnOnce() -> R) -> R {
        let store = self.pack_store_for(objects_dir);
        PackStore::with_context(store, f)
    }

    /// Run `f` with pack read caches for this database's primary `objects/` directory.
    ///
    /// Use for batch workloads (for example `cat-file --batch`) so each object read does not
    /// reinstall thread-local pack context.
    pub fn with_pack_read_context<R>(&self, f: impl FnOnce() -> R) -> R {
        self.with_pack_store_for(&self.objects_dir, f)
    }

    /// Install the pack read context of [`Self::with_pack_read_context`] on the current
    /// thread until the returned guard drops (for long-lived worker threads).
    pub(crate) fn pack_read_context_guard(&self) -> crate::pack_store::PackReadContextGuard {
        crate::pack_store::PackReadContextGuard::install(self.pack_store_for(&self.objects_dir))
    }

    /// Open a reader over the single local pack for a long object walk.
    ///
    /// Returns `None` unless [`Self::with_pack_read_context`] is active for this database,
    /// there is no custom primary store, and exactly one local pack exists (the conditions
    /// under which the files store's [`Self::read`] path opens the same reader per call). A
    /// reader miss is not authoritative: callers must fall back to [`Self::read`] for
    /// objects it does not find.
    pub(crate) fn open_session_pack_reader(&self) -> Result<Option<pack::SingleLocalPackReader>> {
        if self.custom_primary.is_some()
            || !PackStore::read_context_matches_objects_dir(&self.objects_dir)
        {
            return Ok(None);
        }
        self.sync_delta_base_cache_limit();
        pack::SingleLocalPackReader::open_allow_loose(&self.objects_dir)
    }

    /// Touch the loose object file or pack file containing `oid`, matching Git's
    /// `odb_freshen_object` (updates mtime so age-based prune keeps recently re-referenced objects).
    ///
    /// Returns `true` if an on-disk object was found and touched.
    #[must_use]
    pub fn freshen_object(&self, oid: &ObjectId) -> bool {
        if self.is_empty_tree(oid) {
            return false;
        }

        let loose = self.object_path(oid);
        if loose.is_file() {
            return self.touch_object_mtime(&loose).is_some();
        }

        if self.freshen_object_in_objects_dir(&self.objects_dir, oid) {
            return true;
        }

        let file_alts = self.file_alternate_dirs_snapshot();
        for alt_dir in file_alts.iter() {
            if self.freshen_object_in_objects_dir(alt_dir, oid) {
                return true;
            }
        }

        for alt_dir in self.env_alternate_dirs_snapshot().iter() {
            if self.freshen_object_in_objects_dir(alt_dir, oid) {
                return true;
            }
        }

        if let Ok(guard) = self.submodule_alternate_dirs.lock() {
            for alt_dir in guard.iter() {
                if self.freshen_object_in_objects_dir(alt_dir, oid) {
                    return true;
                }
            }
        }

        false
    }

    fn freshen_object_in_objects_dir(&self, objects_dir: &Path, oid: &ObjectId) -> bool {
        let loose = objects_dir
            .join(oid.loose_prefix())
            .join(oid.loose_suffix());
        if loose.is_file() {
            return self.touch_object_mtime(&loose).is_some();
        }
        self.with_pack_store_for(objects_dir, || {
            let Ok(indexes) = pack::read_local_pack_indexes_cached(objects_dir) else {
                return false;
            };
            for idx in &indexes {
                if idx.contains(oid).unwrap_or(false) {
                    return self.freshen_pack_once(&idx.pack_path, idx.is_cruft);
                }
            }
            false
        })
    }

    /// Touch `pack_path`'s mtime at most once for this [`Odb`], refreshing the pack cache signature.
    ///
    /// The path is recorded in [`Self::freshened_packs`] only after a successful touch, matching
    /// Git's `packed_git->freshened` assignment after `utime` succeeds.
    fn freshen_pack_once(&self, pack_path: &Path, is_cruft: bool) -> bool {
        let Ok(mut guard) = self.freshened_packs.lock() else {
            return false;
        };
        if guard.contains(pack_path) {
            return true;
        }
        if is_cruft {
            return false;
        }
        let Some(touched_at) = self.touch_object_mtime(pack_path) else {
            return false;
        };
        pack::refresh_pack_bytes_signature(pack_path, touched_at);
        guard.insert(pack_path.to_path_buf());
        true
    }

    /// When `oid` is already reachable, freshen it and return its id. Returns `None` when the
    /// object lives only in a pack and freshening failed so the caller can materialize a loose copy.
    fn try_freshen_existing(&self, oid: &ObjectId) -> Option<ObjectId> {
        if let Ok(primary) = self.primary() {
            if let Ok(Some(existing)) =
                primary.try_finish_duplicate_write(oid, WriteOptions::default())
            {
                return Some(existing);
            }
        }
        if !self.exists(oid) {
            return None;
        }
        if self.freshen_object(oid) {
            return Some(*oid);
        }
        None
    }

    fn try_freshen_existing_local(&self, oid: &ObjectId) -> Option<ObjectId> {
        let primary = self.primary().ok()?;
        primary
            .try_finish_duplicate_write(
                oid,
                WriteOptions {
                    assume_loose_only_existence: true,
                    ..WriteOptions::default()
                },
            )
            .ok()
            .flatten()
    }

    /// Whether an object is already stored for [`Self::write_with_options`] deduplication.
    ///
    /// Like [`Self::exists`], but the well-known empty tree counts as present only when
    /// materialized on disk (see [`Self::write_local`]).
    fn object_exists_for_write_skip(&self, oid: &ObjectId) -> bool {
        if self.is_empty_tree(oid) {
            self.exists_materialized_local(oid)
        } else {
            self.exists(oid)
        }
    }

    /// Read a loose object file at `path`, verifying the uncompressed payload hashes to `expected_oid`.
    ///
    /// Git stores loose objects under paths derived from the OID; if the file contents hash to a
    /// different id (for example after a mistaken `mv`), this returns [`Error::LooseHashMismatch`].
    ///
    /// # Errors
    ///
    /// - [`Error::Zlib`] — decompression failed.
    /// - [`Error::CorruptObject`] — header is malformed.
    /// - [`Error::LooseHashMismatch`] — payload OID does not match `expected_oid`.
    pub fn read_loose_verify_oid(path: &Path, expected_oid: &ObjectId) -> Result<Object> {
        LooseStore::read_verify_oid(path, expected_oid)
    }

    fn local_loose(&self) -> Result<LooseStore> {
        self.require_files_primary("loose_write")?;
        Ok(LooseStore::new(
            self.objects_dir.clone(),
            self.hash_algo(),
            self.loose_compression()?,
        ))
    }

    fn loose_at(&self, objects_dir: &Path) -> Result<LooseStore> {
        Ok(LooseStore::new(
            objects_dir.to_path_buf(),
            self.hash_algo(),
            self.loose_compression()?,
        ))
    }

    /// Read and decompress an object from the loose store.
    ///
    /// # Errors
    ///
    /// - [`Error::ObjectNotFound`] — no file at the expected path.
    /// - [`Error::Zlib`] — decompression failed.
    /// - [`Error::CorruptObject`] — header is malformed.
    pub fn read(&self, oid: &ObjectId) -> Result<Object> {
        self.read_impl(oid, true)
    }

    /// `remember` keeps a copy in the session loose-read cache; callers that cache a
    /// parsed form themselves skip it.
    fn read_impl(&self, oid: &ObjectId, remember: bool) -> Result<Object> {
        if self.is_empty_tree(oid) {
            return Ok(Object {
                kind: ObjectKind::Tree,
                data: Vec::new(),
            });
        }

        if let Some(obj) = self.session_cached_loose_read(oid) {
            return Ok(obj);
        }

        if let Some(obj) = self.overlay_read(oid) {
            return Ok(obj);
        }
        if let Some(obj) = self.try_read_packed_in_batch_context(oid)? {
            return Ok(obj);
        }

        let use_midx = self.multi_pack_index_reads_enabled();
        if use_midx {
            self.ensure_midx_prepared();
        }

        self.sync_delta_base_cache_limit();

        let primary = self.primary_writable()?;
        let files_primary = self.primary().ok();
        let mut unreadable_local_loose = None;
        let read_primary = || primary.read(oid);
        match if PackStore::read_context_matches_objects_dir(&self.objects_dir) {
            read_primary()
        } else {
            self.with_pack_store_for(&self.objects_dir, read_primary)
        } {
            Ok(Some(obj)) => return self.finish_session_loose_read(oid, obj, remember),
            Ok(None) => {}
            Err(err)
                if files_primary
                    .as_ref()
                    .is_some_and(|p| p.local_loose_exists_but_unreadable(oid, &err)) =>
            {
                unreadable_local_loose = Some(err);
            }
            Err(err) => return Err(err),
        }

        if let Some(obj) = self.try_read_extra_sources(oid)? {
            return self.finish_session_loose_read(oid, obj, remember);
        }

        if self.alternates_enabled {
            match self.sources()?.read(oid) {
                Ok(Some(obj)) => return self.finish_session_loose_read(oid, obj, remember),
                Ok(None) => {}
                Err(err) => return Err(err),
            }
        }

        if let Some(err) = unreadable_local_loose {
            return Err(err);
        }

        self.read_after_promisor_miss(oid)
    }

    #[cold]
    #[inline(never)]
    fn read_after_promisor_miss(&self, oid: &ObjectId) -> Result<Object> {
        let mut lazy_retries = 0u32;
        loop {
            let Some(res) = self.resolve_promisor_miss(oid, lazy_retries)? else {
                return Err(Error::ObjectNotFound(oid.to_hex()));
            };
            match res {
                crate::promisor_fetch::PromisorMissResolution::RetryAfterFetch => {
                    lazy_retries += 1;
                    if let Some(obj) = self.try_read_once(oid)? {
                        return Ok(obj);
                    }
                }
                crate::promisor_fetch::PromisorMissResolution::NotPromisorRepo => {
                    return Err(Error::ObjectNotFound(oid.to_hex()));
                }
                crate::promisor_fetch::PromisorMissResolution::PromisedMissing(err) => {
                    return Err(err);
                }
            }
        }
    }

    /// Read available storage without consulting the promisor fetcher.
    /// Missing objects return `None`; corrupt storage still returns an error.
    pub(crate) fn try_read_once(&self, oid: &ObjectId) -> Result<Option<Object>> {
        if oid.is_well_known_empty_tree() {
            return Ok(Some(Object {
                kind: ObjectKind::Tree,
                data: Vec::new(),
            }));
        }

        if let Some(obj) = self.session_cached_loose_read(oid) {
            return Ok(Some(obj));
        }

        if let Some(obj) = self.overlay_read(oid) {
            return Ok(Some(obj));
        }
        if let Some(obj) = self.try_read_packed_in_batch_context(oid)? {
            return Ok(Some(self.finish_session_loose_read(oid, obj, true)?));
        }

        let use_midx = self.multi_pack_index_reads_enabled();
        if use_midx {
            self.ensure_midx_prepared();
        }

        self.sync_delta_base_cache_limit();

        let primary = self.primary_writable()?;
        let files_primary = self.primary().ok();
        let mut unreadable_local_loose = None;
        let read_primary = || primary.read(oid);
        match if PackStore::read_context_matches_objects_dir(&self.objects_dir) {
            read_primary()
        } else {
            self.with_pack_store_for(&self.objects_dir, read_primary)
        } {
            Ok(Some(obj)) => return Ok(Some(self.finish_session_loose_read(oid, obj, true)?)),
            Ok(None) => {}
            Err(err)
                if files_primary
                    .as_ref()
                    .is_some_and(|p| p.local_loose_exists_but_unreadable(oid, &err)) =>
            {
                unreadable_local_loose = Some(err);
            }
            Err(err) => return Err(err),
        }

        if let Some(obj) = self.try_read_extra_sources(oid)? {
            return Ok(Some(self.finish_session_loose_read(oid, obj, true)?));
        }

        if self.alternates_enabled {
            match self.sources()?.read(oid) {
                Ok(Some(obj)) => return Ok(Some(self.finish_session_loose_read(oid, obj, true)?)),
                Ok(None) => {}
                Err(err) => return Err(err),
            }
        }

        if let Some(err) = unreadable_local_loose {
            return Err(err);
        }

        Ok(None)
    }

    fn resolve_promisor_miss(
        &self,
        oid: &ObjectId,
        lazy_retries: u32,
    ) -> Result<Option<crate::promisor_fetch::PromisorMissResolution>> {
        let (Some(caches), Some(env)) = (&self.shared_config_state, &self.shared_environment)
        else {
            return Ok(None);
        };
        Ok(Some(crate::promisor_fetch::resolve_promisor_read_miss(
            caches,
            self,
            oid,
            env.as_ref(),
            lazy_retries,
        )?))
    }

    /// Read and decompress an object into `buf`, reusing its capacity across calls.
    ///
    /// Resolution order matches [`Self::read`]. Returns a borrowed view of the payload
    /// stored in `buf`.
    ///
    /// # Errors
    ///
    /// Same as [`Self::read`].
    pub fn read_into(&self, oid: &ObjectId, buf: &mut Vec<u8>) -> Result<ObjectKind> {
        if self.is_empty_tree(oid) {
            buf.clear();
            return Ok(ObjectKind::Tree);
        }

        if let Some(obj) = self.session_cached_loose_read(oid) {
            buf.clear();
            buf.extend_from_slice(&obj.data);
            return Ok(obj.kind);
        }

        if let Some(obj) = self.overlay_read(oid) {
            buf.clear();
            buf.extend_from_slice(&obj.data);
            return Ok(obj.kind);
        }

        if let Some(kind) = self.try_read_packed_in_batch_context_into(oid, false, buf)? {
            return Ok(kind);
        }

        let use_midx = self.multi_pack_index_reads_enabled();
        if use_midx {
            self.ensure_midx_prepared();
        }

        self.sync_delta_base_cache_limit();

        let primary = self.primary_writable()?;
        let files_primary = self.primary().ok();
        let mut unreadable_local_loose = None;
        let mut read_primary = || primary.read_into(oid, buf);
        match if PackStore::read_context_matches_objects_dir(&self.objects_dir) {
            read_primary()
        } else {
            self.with_pack_store_for(&self.objects_dir, read_primary)
        } {
            Ok(Some(kind)) => return Ok(kind),
            Ok(None) => {}
            Err(err)
                if files_primary
                    .as_ref()
                    .is_some_and(|p| p.local_loose_exists_but_unreadable(oid, &err)) =>
            {
                unreadable_local_loose = Some(err);
            }
            Err(err) => return Err(err),
        }

        if let Some(kind) = self.try_read_into_extra_sources(oid, buf)? {
            return Ok(kind);
        }

        if self.alternates_enabled {
            match self.sources()?.read_into(oid, buf) {
                Ok(Some(kind)) => return Ok(kind),
                Ok(None) => {}
                Err(err) => return Err(err),
            }
        }

        if let Some(err) = unreadable_local_loose {
            return Err(err);
        }

        self.read_into_after_promisor_miss(oid, buf)
    }

    /// Like [`Self::read_into`], but keep delta bases resolved from local packs in the
    /// delta-base cache, as [`Self::read`] does.
    ///
    /// For walks that read many objects sharing delta chains (successive root trees) and do
    /// not want the per-session object copy [`Self::read`] makes. Objects outside the local
    /// object directory (overlay, alternates, promisor fetches) and any local read failure go
    /// through [`Self::read`], so errors and fallbacks match it.
    ///
    /// # Errors
    ///
    /// Same as [`Self::read`].
    pub(crate) fn read_into_caching_delta_bases(
        &self,
        oid: &ObjectId,
        buf: &mut Vec<u8>,
    ) -> Result<ObjectKind> {
        if let Some(kind) = self.try_read_packed_in_batch_context_into(oid, true, buf)? {
            return Ok(kind);
        }
        if self.custom_primary.is_none() && !self.is_empty_tree(oid) {
            if let Ok(files) = self.primary() {
                self.ensure_midx_prepared();
                self.sync_delta_base_cache_limit();
                let mut read = || files.read_into_with_delta_policy(oid, true, buf);
                let local = if PackStore::read_context_matches_objects_dir(&self.objects_dir) {
                    read()
                } else {
                    self.with_pack_store_for(&self.objects_dir, read)
                };
                if let Ok(Some(kind)) = local {
                    return Ok(kind);
                }
            }
        }
        let object = self.read(oid)?;
        buf.clear();
        buf.extend_from_slice(&object.data);
        Ok(object.kind)
    }

    #[cold]
    #[inline(never)]
    fn read_into_after_promisor_miss(
        &self,
        oid: &ObjectId,
        buf: &mut Vec<u8>,
    ) -> Result<ObjectKind> {
        let mut lazy_retries = 0u32;
        loop {
            let Some(res) = self.resolve_promisor_miss(oid, lazy_retries)? else {
                return Err(Error::ObjectNotFound(oid.to_hex()));
            };
            match res {
                crate::promisor_fetch::PromisorMissResolution::RetryAfterFetch => {
                    lazy_retries += 1;
                    if let Some(kind) = self.try_read_into_once(oid, buf)? {
                        return Ok(kind);
                    }
                }
                crate::promisor_fetch::PromisorMissResolution::NotPromisorRepo => {
                    return Err(Error::ObjectNotFound(oid.to_hex()));
                }
                crate::promisor_fetch::PromisorMissResolution::PromisedMissing(err) => {
                    return Err(err);
                }
            }
        }
    }

    fn try_read_into_once(&self, oid: &ObjectId, buf: &mut Vec<u8>) -> Result<Option<ObjectKind>> {
        if oid.is_well_known_empty_tree() {
            buf.clear();
            return Ok(Some(ObjectKind::Tree));
        }

        if let Some(obj) = self.session_cached_loose_read(oid) {
            buf.clear();
            buf.extend_from_slice(&obj.data);
            return Ok(Some(obj.kind));
        }

        if let Some(obj) = self.overlay_read(oid) {
            buf.clear();
            buf.extend_from_slice(&obj.data);
            return Ok(Some(obj.kind));
        }

        if let Some(kind) = self.try_read_packed_in_batch_context_into(oid, false, buf)? {
            return Ok(Some(kind));
        }

        let use_midx = self.multi_pack_index_reads_enabled();
        if use_midx {
            self.ensure_midx_prepared();
        }

        self.sync_delta_base_cache_limit();

        let primary = self.primary_writable()?;
        let files_primary = self.primary().ok();
        let mut unreadable_local_loose = None;
        let mut read_primary = || primary.read_into(oid, buf);
        match if PackStore::read_context_matches_objects_dir(&self.objects_dir) {
            read_primary()
        } else {
            self.with_pack_store_for(&self.objects_dir, read_primary)
        } {
            Ok(Some(kind)) => return Ok(Some(kind)),
            Ok(None) => {}
            Err(err)
                if files_primary
                    .as_ref()
                    .is_some_and(|p| p.local_loose_exists_but_unreadable(oid, &err)) =>
            {
                unreadable_local_loose = Some(err);
            }
            Err(err) => return Err(err),
        }

        if let Some(kind) = self.try_read_into_extra_sources(oid, buf)? {
            return Ok(Some(kind));
        }

        if self.alternates_enabled {
            match self.sources()?.read_into(oid, buf) {
                Ok(Some(kind)) => return Ok(Some(kind)),
                Ok(None) => {}
                Err(err) => return Err(err),
            }
        }

        if let Some(err) = unreadable_local_loose {
            return Err(err);
        }

        Ok(None)
    }

    fn try_read_into_extra_sources(
        &self,
        oid: &ObjectId,
        buf: &mut Vec<u8>,
    ) -> Result<Option<ObjectKind>> {
        for src in &self.extra_read_sources {
            if let Some(kind) = src.read_into(oid, buf)? {
                return Ok(Some(kind));
            }
        }
        Ok(None)
    }

    fn try_read_packed_in_batch_context_into(
        &self,
        oid: &ObjectId,
        populate_delta_lru: bool,
        buf: &mut Vec<u8>,
    ) -> Result<Option<ObjectKind>> {
        if self.custom_primary.is_some()
            || !PackStore::read_context_matches_objects_dir(&self.objects_dir)
            || self.multi_pack_index_reads_enabled()
        {
            return Ok(None);
        }
        self.sync_delta_base_cache_limit();
        match pack::try_read_object_from_packs_with_options_into(
            &self.objects_dir,
            oid,
            pack::PackLookupOptions::ALL_LOCAL_PACKS,
            populate_delta_lru,
            buf,
        ) {
            Ok(kind) => Ok(Some(kind)),
            Err(Error::ObjectNotFound(_)) => Ok(None),
            Err(err) => Err(err),
        }
    }

    /// Return the kind and uncompressed size of `oid` without loading the full object body.
    ///
    /// Resolution order matches [`Self::read`]: in-memory overlay, multi-pack-index, local packs
    /// (MRU order, skipping MIDX-covered indexes when MIDX is enabled), loose objects, a pack
    /// directory re-prepare and pack retry, then alternates.
    ///
    /// # Errors
    ///
    /// - [`Error::ObjectNotFound`] — no object with this id in any consulted store.
    /// - [`Error::Zlib`] — decompression failed while reading a loose or pack header.
    /// - [`Error::CorruptObject`] — malformed object or pack headers.
    pub fn read_info(&self, oid: &ObjectId) -> Result<ObjectInfo> {
        if self.is_empty_tree(oid) {
            return Ok(ObjectInfo {
                kind: ObjectKind::Tree,
                size: 0,
            });
        }

        if let Some(obj) = self.overlay_read(oid) {
            return Ok(ObjectInfo {
                kind: obj.kind,
                size: u64::try_from(obj.data.len())
                    .map_err(|_| Error::CorruptObject("object size overflow".to_owned()))?,
            });
        }

        if let Some(info) = self.try_read_info_packed_in_batch_context(oid)? {
            return Ok(info);
        }

        let use_midx = self.multi_pack_index_reads_enabled();
        if use_midx {
            self.ensure_midx_prepared();
        }

        let primary = self.primary_writable()?;
        let files_primary = self.primary().ok();
        let mut unreadable_local_loose = None;
        match primary.read_info(oid) {
            Ok(Some(info)) => return Ok(info),
            Ok(None) => {}
            Err(err)
                if files_primary
                    .as_ref()
                    .is_some_and(|p| p.local_loose_exists_but_unreadable(oid, &err)) =>
            {
                unreadable_local_loose = Some(err);
            }
            Err(err) => return Err(err),
        }

        if let Some(info) = self.try_read_info_extra_sources(oid)? {
            return Ok(info);
        }

        if self.alternates_enabled {
            match self.sources()?.read_info(oid) {
                Ok(Some(info)) => return Ok(info),
                Ok(None) => {}
                Err(err) => return Err(err),
            }
        }

        if let Some(err) = unreadable_local_loose {
            return Err(err);
        }

        self.read_info_after_promisor_miss(oid)
    }

    /// Object metadata for local-only probes, serving, and maintenance.
    ///
    /// Like [`Self::read_info`], but does not resolve promisor misses by fetching
    /// from a remote; missing local objects return [`Error::ObjectNotFound`].
    pub(crate) fn read_info_without_fetch(&self, oid: &ObjectId) -> Result<ObjectInfo> {
        self.try_read_info_once(oid)?
            .ok_or_else(|| Error::ObjectNotFound(oid.to_hex()))
    }

    #[cold]
    #[inline(never)]
    fn read_info_after_promisor_miss(&self, oid: &ObjectId) -> Result<ObjectInfo> {
        let mut lazy_retries = 0u32;
        loop {
            let Some(res) = self.resolve_promisor_miss(oid, lazy_retries)? else {
                return Err(Error::ObjectNotFound(oid.to_hex()));
            };
            match res {
                crate::promisor_fetch::PromisorMissResolution::PromisedMissing(err) => {
                    return Err(err);
                }
                crate::promisor_fetch::PromisorMissResolution::NotPromisorRepo => {
                    return Err(Error::ObjectNotFound(oid.to_hex()));
                }
                crate::promisor_fetch::PromisorMissResolution::RetryAfterFetch => {
                    lazy_retries += 1;
                    if let Some(info) = self.try_read_info_once(oid)? {
                        return Ok(info);
                    }
                }
            }
        }
    }

    fn try_read_info_once(&self, oid: &ObjectId) -> Result<Option<ObjectInfo>> {
        if oid.is_well_known_empty_tree() {
            return Ok(Some(ObjectInfo {
                kind: ObjectKind::Tree,
                size: 0,
            }));
        }

        if let Some(obj) = self.overlay_read(oid) {
            return Ok(Some(ObjectInfo {
                kind: obj.kind,
                size: u64::try_from(obj.data.len())
                    .map_err(|_| Error::CorruptObject("object size overflow".to_owned()))?,
            }));
        }

        if let Some(info) = self.try_read_info_packed_in_batch_context(oid)? {
            return Ok(Some(info));
        }

        let use_midx = self.multi_pack_index_reads_enabled();
        if use_midx {
            self.ensure_midx_prepared();
        }

        let primary = self.primary_writable()?;
        let files_primary = self.primary().ok();
        let mut unreadable_local_loose = None;
        match primary.read_info(oid) {
            Ok(Some(info)) => return Ok(Some(info)),
            Ok(None) => {}
            Err(err)
                if files_primary
                    .as_ref()
                    .is_some_and(|p| p.local_loose_exists_but_unreadable(oid, &err)) =>
            {
                unreadable_local_loose = Some(err);
            }
            Err(err) => return Err(err),
        }

        if let Some(info) = self.try_read_info_extra_sources(oid)? {
            return Ok(Some(info));
        }

        if self.alternates_enabled {
            match self.sources()?.read_info(oid) {
                Ok(Some(info)) => return Ok(Some(info)),
                Ok(None) => {}
                Err(err) => return Err(err),
            }
        }

        if let Some(err) = unreadable_local_loose {
            return Err(err);
        }

        Ok(None)
    }

    /// Read `oid` unless its payload is larger than `limit` bytes, in which case
    /// [`BoundedRead::Larger`] is returned (for example so the caller can stream it with
    /// [`Self::with_stream`] instead).
    ///
    /// Costs about one [`Self::read`]: a loose object is opened once and inflation stops at
    /// its header when it is too large, and an object stored whole in a pack is sized from its
    /// entry header before it is read. A pack delta is always read whole, since reconstructing
    /// it needs the full object in memory anyway.
    ///
    /// `cache_small` reuses and retains small loose objects for bulk checkout; callers
    /// doing isolated writes can bypass that bookkeeping.
    ///
    /// # Errors
    ///
    /// Same as [`Self::read`], including [`Error::ObjectNotFound`].
    pub(crate) fn read_unless_larger(
        &self,
        oid: &ObjectId,
        limit: u64,
        cache_small: bool,
    ) -> Result<BoundedRead> {
        if cache_small {
            if let Some(read) = self.session_loose_reads.lock().ok().and_then(|cache| {
                cache.get(oid).map(|obj| {
                    if obj.data.len() as u64 > limit {
                        BoundedRead::Larger
                    } else {
                        BoundedRead::Object(obj.clone())
                    }
                })
            }) {
                return Ok(read);
            }
        }
        if self.custom_primary.is_none() && !self.multi_pack_index_reads_enabled() {
            if let Some(read) =
                pack::pack_cache::try_read_object_via_single_pack_reader_unless_whole_larger(
                    &self.objects_dir,
                    oid,
                    limit,
                )?
            {
                return Ok(read);
            }
        }
        if let Ok(files) = self.primary() {
            if self.multi_pack_index_reads_enabled() {
                self.ensure_midx_prepared();
            }
            match files.read_unless_larger(oid, limit)? {
                LocalSizedRead::Packed(Some(size)) if size > limit => {
                    return Ok(BoundedRead::Larger);
                }
                LocalSizedRead::Packed(_) => return self.read(oid).map(BoundedRead::Object),
                LocalSizedRead::Loose(BoundedRead::Object(obj)) => {
                    if cache_small && obj.data.len() <= SESSION_BOUNDED_LOOSE_CACHE_MAX {
                        self.remember_session_loose_read(oid, &obj);
                    }
                    return Ok(BoundedRead::Object(obj));
                }
                LocalSizedRead::Loose(read) => return Ok(read),
                LocalSizedRead::Missing => {}
            }
        }
        let info = self.read_info(oid)?;
        if info.size > limit {
            return Ok(BoundedRead::Larger);
        }
        self.read(oid).map(BoundedRead::Object)
    }

    /// Open a stream over `oid`'s uncompressed payload (overlay, primary, then alternates).
    ///
    /// The returned stream owns its payload: it is read whole into memory before returning.
    /// Use [`Self::with_stream`] to read a large object in bounded memory.
    ///
    /// # Errors
    ///
    /// Propagates backend failures other than a missing object (`Ok(None)`).
    pub fn open_stream(&self, oid: &ObjectId) -> Result<Option<ObjectStream<'static>>> {
        self.with_stream(oid, materialize_stream)
    }

    /// Run `f` on a stream over `oid`'s uncompressed payload (overlay, primary, then alternates).
    ///
    /// Unlike [`Self::open_stream`] the payload is not buffered: loose objects and objects
    /// stored whole in a pack are inflated as `f` reads, so memory stays bounded however large
    /// the object is. Objects stored as pack deltas are reconstructed in memory first.
    ///
    /// # Parameters
    ///
    /// - `oid` — the object to stream.
    /// - `f` — consumes the stream; its result is returned as `Ok(Some(_))`.
    ///
    /// # Errors
    ///
    /// Propagates backend failures other than a missing object (`Ok(None)`, and `f` is not
    /// called), and any error returned by `f`. A corrupt or truncated object surfaces as an
    /// [`std::io::Error`] from the stream's reader.
    pub fn with_stream<R>(
        &self,
        oid: &ObjectId,
        f: impl FnOnce(ObjectStream<'_>) -> Result<R>,
    ) -> Result<Option<R>> {
        use std::io::Cursor;
        if let Some(obj) = self.overlay_read(oid) {
            let size = u64::try_from(obj.data.len()).map_err(|_| {
                Error::CorruptObject(format!("object size overflow for {}", oid.to_hex()))
            })?;
            return f(ObjectStream {
                kind: obj.kind,
                size,
                reader: Box::new(Cursor::new(obj.data)),
            })
            .map(Some);
        }
        if self.multi_pack_index_reads_enabled() {
            self.ensure_midx_prepared();
        }
        self.sync_delta_base_cache_limit();
        let primary = self.primary_writable()?;
        let files_primary = self.primary().ok();
        let mut unreadable_local_loose = None;
        match primary.open_stream(oid) {
            Ok(Some(stream)) => return f(stream).map(Some),
            Ok(None) => {}
            Err(err)
                if files_primary
                    .as_ref()
                    .is_some_and(|p| p.local_loose_exists_but_unreadable(oid, &err)) =>
            {
                unreadable_local_loose = Some(err);
            }
            Err(err) => return Err(err),
        }
        if let Some(stream) = self.try_open_stream_extra_sources(oid)? {
            return f(stream).map(Some);
        }
        if self.alternates_enabled {
            let sources = self.sources()?;
            let stream = sources.open_stream(oid)?;
            if let Some(stream) = stream {
                return f(stream).map(Some);
            }
        }
        if let Some(err) = unreadable_local_loose {
            return Err(err);
        }
        Ok(None)
    }

    /// Invoke `f` for every object id reachable from this database until `f` returns [`ControlFlow::Break`].
    ///
    /// # Errors
    ///
    /// Propagates enumeration failures from any consulted backend.
    pub fn for_each_object(&self, f: &mut dyn FnMut(&ObjectId) -> ControlFlow<()>) -> Result<()> {
        let mut seen = oid_hash_set();
        let mut visit = |store: &dyn ObjectStore| -> Result<bool> {
            store::for_each_propagate_break(store, &mut |oid| {
                if !seen.insert(*oid) {
                    return ControlFlow::Continue(());
                }
                f(oid)
            })
        };
        if let Some(overlay) = self.overlay_store() {
            if visit(overlay.as_ref())? {
                return Ok(());
            }
        }
        if visit(self.primary_writable()?.as_ref())? {
            return Ok(());
        }
        for src in &self.extra_read_sources {
            if visit(src.as_ref())? {
                return Ok(());
            }
        }
        if self.alternates_enabled {
            let _ = visit(self.sources()?.as_ref())?;
        }
        Ok(())
    }

    /// Fill `out` with ids whose hex starts with `prefix` (deduplicated across overlay, primary, alternates).
    ///
    /// # Errors
    ///
    /// Returns [`Error::InvalidObjectId`] when `prefix` is not valid hex.
    pub fn lookup_prefix(&self, prefix: &str, limit: usize, out: &mut Vec<ObjectId>) -> Result<()> {
        let mut seen: OidHashSet = out.iter().copied().collect();
        if let Some(overlay) = self.overlay_store() {
            append_lookup_prefix_layer(overlay.as_ref(), prefix, limit, out, &mut seen)?;
            if limit != 0 && out.len() >= limit {
                return Ok(());
            }
        }
        append_lookup_prefix_layer(
            self.primary_writable()?.as_ref(),
            prefix,
            limit,
            out,
            &mut seen,
        )?;
        if limit != 0 && out.len() >= limit {
            return Ok(());
        }
        for src in &self.extra_read_sources {
            append_lookup_prefix_layer(src.as_ref(), prefix, limit, out, &mut seen)?;
            if limit != 0 && out.len() >= limit {
                return Ok(());
            }
        }
        if self.alternates_enabled {
            append_lookup_prefix_layer(self.sources()?.as_ref(), prefix, limit, out, &mut seen)?;
        }
        Ok(())
    }

    /// Enumerate loose objects in this database's primary `objects/` directory as `(oid, path)`.
    ///
    /// Pack files and alternate object directories are not scanned.
    ///
    /// # Errors
    ///
    /// Propagates failures while opening the primary files source or walking loose storage.
    pub fn enumerate_local_loose_objects(&self) -> Result<Vec<(ObjectId, PathBuf)>> {
        let primary = self.primary()?;
        let loose = primary.loose_store();
        let mut out = Vec::new();
        loose.for_each_object(&mut |oid| {
            out.push((*oid, loose.object_path(oid)));
            ControlFlow::Continue(())
        })?;
        Ok(out)
    }

    /// Hash raw content of a given kind using this repository's hash algorithm.
    ///
    /// This does **not** write anything to disk.
    ///
    /// # Errors
    ///
    /// Returns [`Error::HashCollision`] when sha1dc detects a chosen-prefix collision.
    pub fn hash(&self, kind: ObjectKind, data: &[u8]) -> Result<ObjectId> {
        hash_object_data_with(self.hash_algo(), kind, data)
    }

    /// Hash `size` payload bytes of `kind` read from `reader`, without storing anything.
    ///
    /// Memory use does not depend on `size`: the payload is read in fixed-size chunks. Exactly
    /// `size` bytes are consumed; anything after them is left unread.
    ///
    /// # Errors
    ///
    /// - [`Error::ObjectStreamTruncated`] — `reader` ended before `size` bytes.
    /// - [`Error::Io`] — reading failed.
    /// - [`Error::HashCollision`] — sha1dc detected a chosen-prefix collision.
    pub fn hash_stream(
        &self,
        kind: ObjectKind,
        size: u64,
        reader: &mut dyn std::io::Read,
    ) -> Result<ObjectId> {
        #[cfg(test)]
        self.hot_path_metrics().record_stream_hash();
        stream::hash_stream(self.hash_algo(), kind, size, reader)
    }

    /// Store `size` payload bytes of `kind` read from `reader` and return the object id.
    ///
    /// Objects smaller than [`Self::big_file_threshold`] are read into memory and written like
    /// [`Self::write_with_options`]. Bigger ones are hashed and deflated in fixed-size chunks
    /// straight into a new single-object pack (at the [`ConfigSet::pack_objects_zlib_level`]
    /// level), as Git does for big files, so memory use does not depend on `size`. If the
    /// object already exists the new pack is discarded and the existing copy is freshened
    /// unless `options.silent` is set.
    ///
    /// A custom primary receives the stream through [`WritableObjectStore::write_stream`];
    /// while the in-memory overlay is active the object is buffered into the overlay.
    ///
    /// Exactly `size` bytes are consumed from `reader`.
    ///
    /// # Errors
    ///
    /// - [`Error::ObjectStreamTruncated`] — `reader` ended before `size` bytes.
    /// - [`Error::Io`] / [`Error::Zlib`] — reading, compressing, or writing failed.
    /// - [`Error::HashCollision`] — sha1dc detected a chosen-prefix collision.
    /// - Config errors from resolving the threshold or compression level.
    pub fn write_stream(
        &self,
        kind: ObjectKind,
        size: u64,
        reader: &mut dyn std::io::Read,
        options: WriteOptions,
    ) -> Result<ObjectId> {
        #[cfg(test)]
        self.hot_path_metrics().record_stream_write();
        if let Some(primary) = &self.custom_primary {
            if !self.overlay_active() {
                let oid = primary.write_stream(kind, size, reader, options)?;
                self.note_commit_object_written(kind);
                return Ok(oid);
            }
        }
        if self.overlay_active() || size < self.big_file_threshold()? {
            let len = usize::try_from(size).map_err(|_| {
                Error::CorruptObject(format!("object size {size} exceeds address space"))
            })?;
            let mut data = Vec::with_capacity(len);
            let read =
                std::io::Read::read_to_end(&mut std::io::Read::take(reader, size), &mut data)?;
            if read != len {
                return Err(Error::ObjectStreamTruncated {
                    expected: size,
                    actual: read as u64,
                });
            }
            return self.write_with_options(kind, &data, options);
        }
        let staged = stream::StagedPack::write(
            &self.objects_dir.join("pack"),
            self.hash_algo(),
            kind,
            size,
            reader,
            self.pack_compression()?,
        )?;
        let oid = staged.oid();
        if self.object_exists_for_write_skip(&oid) {
            if !options.silent {
                let _ = self.freshen_object(&oid);
            }
            return Ok(oid);
        }
        staged.install()?;
        self.invalidate_packs();
        self.note_commit_object_written(kind);
        Ok(oid)
    }

    /// Object id of the empty tree for this store's hash algorithm.
    ///
    /// Hashes the canonical empty tree payload only; nothing is written to disk.
    ///
    /// # Errors
    ///
    /// Returns [`Error::HashCollision`] when sha1dc detects a chosen-prefix collision.
    pub fn empty_tree_oid(&self) -> Result<ObjectId> {
        self.hash(ObjectKind::Tree, b"")
    }

    /// Whether `oid` is this repository's empty tree (virtually present when not on disk).
    #[must_use]
    pub fn is_empty_tree(&self, oid: &ObjectId) -> bool {
        empty_tree_oid_matches(self.hash_algo(), oid)
    }

    /// Write an object to the loose store and return its [`ObjectId`].
    ///
    /// If the object already exists it is not overwritten (Git behaviour).
    ///
    /// The payload is stored as given, without fsck checks; callers that build tree,
    /// commit or tag bytes themselves should go through
    /// [`Objects::write`](crate::objects::Objects::write) instead.
    ///
    /// # Errors
    ///
    /// - [`Error::Io`] — could not create the directory or write the file.
    /// - [`Error::Zlib`] — compression failed.
    pub fn write(&self, kind: ObjectKind, data: &[u8]) -> Result<ObjectId> {
        self.write_with_options(kind, data, WriteOptions::default())
    }

    /// Create all 256 loose-object prefix directories (`objects/xx/`).
    ///
    /// Bulk `git add`-style staging calls this once before parallel loose writes. A no-op for
    /// custom primaries, which receive those writes instead of `objects/`.
    ///
    /// # Errors
    ///
    /// Returns [`Error::Io`] if a directory cannot be created.
    pub fn ensure_all_loose_prefix_dirs(&self) -> Result<()> {
        if self.custom_primary.is_some() {
            return Ok(());
        }
        self.local_loose()?.ensure_all_loose_prefix_dirs()
    }

    /// Test-only: filesystem path for a loose object (see `grit-lib/tests/odb_abstraction_guard.rs`).
    #[cfg(test)]
    #[must_use]
    pub fn test_loose_object_path(&self, oid: &ObjectId) -> PathBuf {
        self.object_path(oid)
    }

    /// Test-only: delete a loose object file if present.
    #[cfg(test)]
    pub fn test_remove_loose_object(&self, oid: &ObjectId) {
        let _ = fs::remove_file(self.object_path(oid));
    }

    /// Count loose object files under this repository's `objects/` directory.
    ///
    /// # Errors
    ///
    /// Propagates directory enumeration failures from the loose store walker.
    pub fn count_loose_objects(&self) -> Result<usize> {
        let mut count = 0usize;
        for_each_loose_object_id(&self.objects_dir, self.hash_algo(), &mut |_| {
            count += 1;
            ControlFlow::Continue(())
        })?;
        Ok(count)
    }

    /// Approximate object count for Git-style automatic `core.abbrev` sizing.
    ///
    /// Sums object counts from cached pack indexes (no per-object scan) plus loose files under
    /// this database's `objects/` directory. Matches Git's `ODB_COUNT_OBJECTS_APPROXIMATE` well
    /// enough for abbreviation heuristics on typical repositories.
    ///
    /// # Errors
    ///
    /// Propagates pack index listing and loose enumeration failures.
    pub fn approximate_object_count(&self) -> Result<usize> {
        let pack_objects: Result<usize> = self.with_pack_read_context(|| {
            let indexes = pack::read_local_pack_indexes_cached(self.objects_dir())?;
            Ok(indexes.iter().map(|idx| idx.len()).sum())
        });
        let mut count = pack_objects?;
        count += self.count_loose_objects()?;
        Ok(count)
    }

    /// Zlib-compress canonical loose store bytes using the repository's loose level.
    ///
    /// # Errors
    ///
    /// Returns [`Error::Zlib`] when compression fails.
    pub fn zlib_compress_loose_store(&self, store_bytes: &[u8]) -> Result<Vec<u8>> {
        self.local_loose()?.zlib_compress_store_bytes(store_bytes)
    }

    /// Write a loose object from precomputed id and zlib-compressed store bytes.
    ///
    /// The caller must supply bytes that the internal zlib loose reader would expand to
    /// valid canonical store form (`"<kind> <len>\\0<payload>"`). Hashing and
    /// compression are skipped; use after parallel batch preparation.
    ///
    /// With a custom primary the bytes are inflated and stored through
    /// [`WritableObjectStore::write`] instead, so staging works on any backend.
    ///
    /// # Errors
    ///
    /// Same as [`Self::write`]; [`Error::Zlib`] or [`Error::CorruptObject`] when a custom
    /// primary receives bytes that do not inflate to a valid object.
    pub fn write_loose_zlib_prehashed(
        &self,
        oid: &ObjectId,
        zlib_store: &[u8],
        options: WriteOptions,
    ) -> Result<ObjectId> {
        if self.custom_primary.is_some() {
            let raw = decompress_zlib_loose_bytes(zlib_store)?;
            let obj = parse_object_bytes(&raw)?;
            return self.write_with_options(obj.kind, &obj.data, options);
        }
        let primary = self.primary()?;
        if let Some(existing) = primary.try_finish_duplicate_write(oid, options)? {
            return Ok(existing);
        }

        let already_exists = !(options.trust_new_loose || options.assume_loose_only_existence)
            && self.object_exists_for_write_skip(oid);

        if self.overlay_active() && !already_exists {
            if let Ok(raw) = decompress_zlib_loose_bytes(zlib_store) {
                if let Ok(obj) = parse_object_bytes(&raw) {
                    if self.overlay_store_object(*oid, obj.kind, &obj.data) {
                        return Ok(*oid);
                    }
                }
            }
        }

        if already_exists {
            if !options.silent {
                let _ = self.freshen_object(oid);
            }
            return Ok(*oid);
        }

        primary.write_zlib_prehashed(oid, zlib_store, options)
    }

    /// Write blob `data` whose id `oid` the caller already computed.
    ///
    /// On a files primary the content is deflated straight into the loose object file, so
    /// no compressed copy is held in memory; a custom primary gets
    /// [`Self::write_with_options`].
    pub(crate) fn write_blob_prehashed(
        &self,
        oid: &ObjectId,
        data: &[u8],
        options: WriteOptions,
    ) -> Result<ObjectId> {
        if self.custom_primary.is_some() {
            return self.write_with_options(ObjectKind::Blob, data, options);
        }
        let primary = self.primary()?;
        if let Some(existing) = primary.try_finish_duplicate_write(oid, options)? {
            return Ok(existing);
        }
        let already_exists = !(options.trust_new_loose || options.assume_loose_only_existence)
            && self.object_exists_for_write_skip(oid);
        if self.overlay_active()
            && !already_exists
            && self.overlay_store_object(*oid, ObjectKind::Blob, data)
        {
            return Ok(*oid);
        }
        if already_exists {
            if !options.silent {
                let _ = self.freshen_object(oid);
            }
            return Ok(*oid);
        }
        primary
            .loose_store()
            .write_prehashed(oid, ObjectKind::Blob, data, options)
    }
    /// Like [`Self::write`], with control over freshen behaviour on existing objects.
    pub fn write_with_options(
        &self,
        kind: ObjectKind,
        data: &[u8],
        options: WriteOptions,
    ) -> Result<ObjectId> {
        let oid = hash_object_data_with(self.hash_algo(), kind, data)?;

        if let Some(primary) = &self.custom_primary {
            let already_exists = if options.trust_new_loose {
                false
            } else if options.assume_loose_only_existence {
                primary.contains(&oid)?
            } else {
                self.object_exists_for_write_skip(&oid)
            };
            if self.overlay_active()
                && !already_exists
                && self.overlay_store_object(oid, kind, data)
            {
                return Ok(oid);
            }
            if already_exists {
                return Ok(oid);
            }
            let oid = WritableObjectStore::write(primary.as_ref(), kind, data, options)?;
            self.note_commit_object_written(kind);
            return Ok(oid);
        }

        let primary = self.primary()?;
        if let Some(existing) = primary.try_finish_duplicate_write(&oid, options)? {
            return Ok(existing);
        }

        let already_exists = !(options.trust_new_loose || options.assume_loose_only_existence)
            && self.object_exists_for_write_skip(&oid);

        // When the in-memory overlay is active, keep the object in memory only (unless it is
        // already present on disk, in which case nothing new needs to be written anyway).
        if self.overlay_active() && !already_exists && self.overlay_store_object(oid, kind, data) {
            return Ok(oid);
        }

        if already_exists {
            if !options.silent {
                let _ = self.freshen_object(&oid);
            }
            return Ok(oid);
        }

        let oid = WritableObjectStore::write(primary.as_ref(), kind, data, options)?;
        self.note_commit_object_written(kind);
        Ok(oid)
    }

    /// Write an object as a loose file in this object directory only.
    ///
    /// Unlike [`Self::write`], this ignores `info/alternates` and
    /// `GIT_ALTERNATE_OBJECT_DIRECTORIES`: if the object exists only in an
    /// alternate store, it is still written here. That matches how Git's
    /// `unpack-objects` materializes every packed object into the receiving
    /// repository even when the same OID is already reachable via alternates
    /// (see `t5519-push-alternates`).
    ///
    /// The well-known empty tree is still written when no loose file exists yet,
    /// even though [`Self::exists_local`] treats it as virtually present.
    ///
    /// # Errors
    ///
    /// Same as [`Self::write`].
    pub fn write_local(&self, kind: ObjectKind, data: &[u8]) -> Result<ObjectId> {
        if self.custom_primary.is_some() {
            return Err(Error::UnsupportedObjectStore {
                operation: "write_local",
            });
        }
        let oid = hash_object_data_with(self.hash_algo(), kind, data)?;

        if let Some(existing) = self.try_freshen_existing_local(&oid) {
            return Ok(existing);
        }

        let oid = self
            .local_loose()?
            .write(kind, data, WriteOptions::default())?;
        self.note_commit_object_written(kind);
        Ok(oid)
    }

    /// Like [`Self::write_local`], but stores payload bytes supplied chunk-by-chunk (pack unpack).
    pub(crate) fn write_local_stream_with(
        &self,
        kind: ObjectKind,
        size: u64,
        supply: impl FnMut(&mut dyn FnMut(&[u8]) -> Result<()>) -> Result<()>,
    ) -> Result<ObjectId> {
        if self.custom_primary.is_some() {
            return Err(Error::UnsupportedObjectStore {
                operation: "write_local_stream",
            });
        }
        let loose = self.local_loose()?;
        let oid = loose.write_payload_stream_with(kind, size, WriteOptions::default(), supply)?;
        self.note_commit_object_written(kind);
        Ok(oid)
    }

    /// Write a loose object file when it is missing, even if [`Self::exists`] is true because
    /// the object lives only in a pack.
    ///
    /// Used when materializing a partial-clone layout: objects must be duplicated as loose files
    /// before local packs are removed. Unlike [`Self::write_local`], objects present only in a
    /// promisor pack are still written because [`Self::exists_local`] treats those as absent.
    pub fn write_loose_materialize(&self, kind: ObjectKind, data: &[u8]) -> Result<ObjectId> {
        if self.custom_primary.is_some() {
            return Err(Error::UnsupportedObjectStore {
                operation: "write_loose_materialize",
            });
        }
        let oid = hash_object_data_with(self.hash_algo(), kind, data)?;
        if let Ok(primary) = self.primary() {
            if let Ok(Some(existing)) = primary
                .loose_store()
                .try_finish_if_loose_present(&oid, WriteOptions::default())
            {
                return Ok(existing);
            }
        }

        self.local_loose()?
            .write(kind, data, WriteOptions::default())
    }

    /// Write an already-serialized object (header + data) to the loose store.
    ///
    /// Useful when the caller has the full store bytes (e.g. from stdin with
    /// `--literally`).
    ///
    /// # Errors
    ///
    /// - [`Error::CorruptObject`] — the provided bytes don't form a valid header.
    /// - [`Error::Io`] / [`Error::Zlib`] — storage errors.
    pub fn write_raw(&self, store_bytes: &[u8]) -> Result<ObjectId> {
        let obj = parse_object_bytes(store_bytes)?;
        let oid = hash_object_data_with(self.hash_algo(), obj.kind, &obj.data)?;

        if let Some(primary) = &self.custom_primary {
            if self.exists(&oid) {
                return Ok(oid);
            }
            if self.overlay_active() && self.overlay_store_object(oid, obj.kind, &obj.data) {
                return Ok(oid);
            }
            return WritableObjectStore::write(
                primary.as_ref(),
                obj.kind,
                &obj.data,
                WriteOptions::default(),
            );
        }

        if let Some(existing) = self.try_freshen_existing(&oid) {
            return Ok(existing);
        }

        self.local_loose()?
            .write_store_prehashed(&oid, store_bytes, WriteOptions::default())
    }

    /// Like [`Self::write_raw`] but only consults this object directory, not alternates.
    ///
    /// See [`Self::write_local`].
    ///
    /// # Errors
    ///
    /// Same as [`Self::write_raw`].
    pub fn write_raw_local(&self, store_bytes: &[u8]) -> Result<ObjectId> {
        if self.custom_primary.is_some() {
            return Err(Error::UnsupportedObjectStore {
                operation: "write_raw_local",
            });
        }
        let obj = parse_object_bytes(store_bytes)?;
        let oid = hash_object_data_with(self.hash_algo(), obj.kind, &obj.data)?;
        if let Some(existing) = self.try_freshen_existing_local(&oid) {
            return Ok(existing);
        }

        self.local_loose()?
            .write_store_prehashed(&oid, store_bytes, WriteOptions::default())
    }

    /// Update `path`'s mtime to "now" (Git `utime(path, NULL)`), returning whether it succeeded.
    fn touch_object_mtime(&self, path: &Path) -> Option<std::time::SystemTime> {
        #[cfg(test)]
        self.hot_path_test_metrics.record_freshen();
        // `utime(path, NULL)` sets both atime and mtime to the current time.
        let now = filetime::FileTime::now();
        filetime::set_file_times(path, now, now).ok()?;
        Some(
            std::time::UNIX_EPOCH
                + std::time::Duration::new(now.unix_seconds() as u64, now.nanoseconds()),
        )
    }
}

/// Hash the canonical store bytes of an object (`"<kind> <len>\0<data>"`) with
/// the given hash algorithm.
fn hash_object_data_with(algo: HashAlgo, kind: ObjectKind, data: &[u8]) -> Result<ObjectId> {
    hash::hash_object(algo, kind, data)
}

/// Whether `oid` is Git's empty tree for `algo` (including the legacy SHA-1 typo OID).
#[must_use]
pub(crate) fn empty_tree_oid_matches(algo: HashAlgo, oid: &ObjectId) -> bool {
    oid.is_empty_tree_for(algo)
}

/// Zlib-compress blob `data` in loose-object form (`"blob <len>\\0<payload>"`) without first
/// copying it behind the header.
pub(crate) fn zlib_compress_blob_loose(
    data: &[u8],
    compression: flate2::Compression,
) -> Result<Vec<u8>> {
    use std::io::Write as _;
    let mut encoder = flate2::write::ZlibEncoder::new(Vec::new(), compression);
    let zlib_err = |e: std::io::Error| Error::Zlib(e.to_string());
    write!(encoder, "{} {}\0", ObjectKind::Blob, data.len()).map_err(zlib_err)?;
    encoder.write_all(data).map_err(zlib_err)?;
    encoder.finish().map_err(zlib_err)
}

fn append_lookup_prefix_layer(
    store: &dyn ObjectStore,
    prefix: &str,
    limit: usize,
    out: &mut Vec<ObjectId>,
    seen: &mut OidHashSet,
) -> Result<()> {
    let mut scratch = Vec::new();
    store.lookup_prefix(prefix, 0, &mut scratch)?;
    for oid in scratch {
        if seen.insert(oid) {
            out.push(oid);
            if limit != 0 && out.len() >= limit {
                return Ok(());
            }
        }
    }
    Ok(())
}

fn materialize_stream(stream: ObjectStream<'_>) -> Result<ObjectStream<'static>> {
    use std::io::{Cursor, Read};
    let ObjectStream {
        kind,
        size,
        mut reader,
    } = stream;
    let mut payload = Vec::new();
    reader.read_to_end(&mut payload).map_err(Error::Io)?;
    Ok(ObjectStream {
        kind,
        size,
        reader: Box::new(Cursor::new(payload)),
    })
}

/// Parse a colon-separated alternates string, handling double-quoted entries
/// with octal escape sequences.
fn parse_alternate_env(val: &str) -> Vec<PathBuf> {
    let mut result = Vec::new();
    let mut chars = val.chars().peekable();
    while chars.peek().is_some() {
        if chars.peek() == Some(&':') {
            chars.next();
            continue;
        }
        if chars.peek() == Some(&'"') {
            // Try quoted parsing; if EOF is hit without closing quote,
            // fall back to treating the whole segment as a raw path.
            chars.next(); // consume the opening '"'
            let saved: Vec<char> = chars.clone().collect();
            let mut path = String::new();
            let mut properly_closed = false;
            loop {
                match chars.next() {
                    None => break,
                    Some('"') => {
                        properly_closed = true;
                        break;
                    }
                    Some('\\') => match chars.peek() {
                        Some(c) if c.is_ascii_digit() => {
                            let mut oct = String::new();
                            for _ in 0..3 {
                                if let Some(&c) = chars.peek() {
                                    if c.is_ascii_digit() {
                                        oct.push(c);
                                        chars.next();
                                    } else {
                                        break;
                                    }
                                } else {
                                    break;
                                }
                            }
                            if let Ok(byte) = u8::from_str_radix(&oct, 8) {
                                path.push(byte as char);
                            }
                        }
                        Some(_) => {
                            if let Some(c) = chars.next() {
                                match c {
                                    'n' => path.push('\n'),
                                    't' => path.push('\t'),
                                    'r' => path.push('\r'),
                                    _ => path.push(c),
                                }
                            }
                        }
                        None => {}
                    },
                    Some(c) => path.push(c),
                }
            }
            if !properly_closed {
                // Broken quoting: fall back to treating raw value (with leading ")
                // as a literal path.
                let raw: String = std::iter::once('"').chain(saved).collect();
                // Extract up to ':' or end
                let raw_path = raw.split(':').next().unwrap_or(&raw);
                if !raw_path.is_empty() {
                    result.push(PathBuf::from(raw_path));
                }
                // Advance past the ':' in the original chars (we consumed the saved copy)
                // Since chars is now at EOF, we need to handle remaining items.
                // Actually, we consumed chars fully. Let's reconstruct from raw.
                let remainder = &raw[raw_path.len()..];
                if let Some(rest) = remainder.strip_prefix(':') {
                    // Parse remaining entries
                    result.extend(parse_alternate_env(rest));
                }
                return result;
            } else if !path.is_empty() {
                result.push(PathBuf::from(path));
            }
        } else {
            let mut path = String::new();
            while let Some(&c) = chars.peek() {
                if c == ':' {
                    break;
                }
                path.push(c);
                chars.next();
            }
            if !path.is_empty() {
                result.push(PathBuf::from(path));
            }
        }
    }
    result
}

fn system_time_to_file_time(t: std::time::SystemTime) -> FileTime {
    let dur = t
        .duration_since(std::time::SystemTime::UNIX_EPOCH)
        .unwrap_or_default();
    FileTime::from_unix_time(
        i64::try_from(dur.as_secs()).unwrap_or(i64::MAX),
        dur.subsec_nanos(),
    )
}

fn superseded_pack_indexes(
    pack_dir: &Path,
    keep_pack: &Path,
    retained_packs: &HashSet<PathBuf>,
) -> Result<Vec<(PathBuf, std::time::SystemTime)>> {
    let keep_stem = keep_pack
        .file_stem()
        .and_then(|s| s.to_str())
        .ok_or_else(|| {
            Error::Io(std::io::Error::new(
                std::io::ErrorKind::InvalidInput,
                "bad pack path",
            ))
        })?;
    if !pack_dir.is_dir() {
        return Ok(Vec::new());
    }
    let mut out = Vec::new();
    for entry in fs::read_dir(pack_dir).map_err(Error::Io)? {
        let entry = entry.map_err(Error::Io)?;
        let name = entry.file_name();
        let name_str = name.to_string_lossy();
        if name_str == keep_stem || name_str.starts_with(&format!("{keep_stem}.")) {
            continue;
        }
        if !name_str.ends_with(".idx") {
            continue;
        }
        let idx_path = entry.path();
        let pack_path = idx_path.with_extension("pack");
        if !pack_path.is_file() {
            continue;
        }
        if superseded_pack_has_keep_marker(pack_dir, &pack_path)
            || pack_path.with_extension("promisor").is_file()
            || retained_packs.contains(&pack_path)
        {
            continue;
        }
        let pack_mtime = fs::metadata(&pack_path)
            .and_then(|m| m.modified())
            .unwrap_or(std::time::SystemTime::UNIX_EPOCH);
        out.push((idx_path, pack_mtime));
    }
    Ok(out)
}

fn superseded_pack_has_keep_marker(pack_dir: &Path, pack_path: &Path) -> bool {
    let Some(stem) = pack_path.file_stem().and_then(|s| s.to_str()) else {
        return false;
    };
    pack_dir.join(format!("{stem}.keep")).is_file()
}

fn oids_in_marker_kept_packs(pack_dir: &Path, keep_pack: &Path) -> Result<OidHashSet> {
    let keep_stem = keep_pack.file_stem().and_then(|s| s.to_str()).unwrap_or("");
    let mut oids = oid_hash_set();
    if !pack_dir.is_dir() {
        return Ok(oids);
    }
    for entry in fs::read_dir(pack_dir).map_err(Error::Io)? {
        let entry = entry.map_err(Error::Io)?;
        let pack_path = entry.path();
        if pack_path.extension().is_none_or(|e| e != "pack") {
            continue;
        }
        let Some(stem) = pack_path.file_stem().and_then(|s| s.to_str()) else {
            continue;
        };
        if stem == keep_stem {
            continue;
        }
        if !superseded_pack_has_keep_marker(pack_dir, &pack_path) {
            continue;
        }
        let idx_path = pack_path.with_extension("idx");
        if idx_path.is_file() {
            oids.extend(PackedObjects::object_ids_from_index_path(&idx_path)?);
        }
    }
    Ok(oids)
}

#[cfg(test)]
mod tests {
    #![allow(clippy::expect_used, clippy::unwrap_used)]

    use super::*;
    use tempfile::TempDir;

    #[test]
    fn round_trip_blob() {
        let dir = TempDir::new().unwrap();
        let odb = Odb::new(dir.path());
        let data = b"hello world";
        let oid = odb.write(ObjectKind::Blob, data).unwrap();
        let obj = odb.read(&oid).unwrap();
        assert_eq!(obj.kind, ObjectKind::Blob);
        assert_eq!(obj.data, data);
    }

    #[test]
    fn known_blob_hash() {
        // Verified: echo -n "hello" | git hash-object --stdin
        //        => b6fc4c620b67d95f953a5c1c1230aaab5db5a1b0
        let oid = HashAlgo::Sha1
            .hash_object(ObjectKind::Blob, b"hello")
            .expect("hash");
        assert_eq!(oid.to_hex(), "b6fc4c620b67d95f953a5c1c1230aaab5db5a1b0");
    }

    /// Pack `oids` into one new pack and drop their loose copies.
    fn pack_objects(objects: &Path, oids: &[ObjectId]) {
        let odb = Odb::new(objects);
        let pack = crate::pack_objects::build_pack_from_send_list(
            &odb,
            oids,
            &HashSet::new(),
            &crate::pack_objects::PackBuildOptions::default(),
            &HashMap::new(),
        )
        .unwrap();
        for oid in oids {
            fs::remove_file(oid.loose_path_in(objects)).unwrap();
        }
        crate::index_pack::install_pack_bytes(
            pack,
            &odb,
            &crate::index_pack::IngestPackOptions::default(),
        )
        .unwrap();
    }

    #[test]
    fn general_reads_cache_trees_and_tiny_blobs_but_not_larger_blobs() {
        let dir = TempDir::new().unwrap();
        let odb = Odb::new(dir.path());
        let blob = odb
            .write(
                ObjectKind::Blob,
                &vec![b'x'; SESSION_READ_BLOB_CACHE_MAX + 1],
            )
            .unwrap();
        let tiny = odb.write(ObjectKind::Blob, b"tiny\n").unwrap();
        let tree = odb
            .write(
                ObjectKind::Tree,
                &crate::objects::serialize_tree(&[crate::objects::TreeEntry {
                    mode: 0o100644,
                    name: b"f".to_vec(),
                    oid: blob,
                }]),
            )
            .unwrap();
        assert_eq!(
            odb.read(&blob).unwrap().data.len(),
            SESSION_READ_BLOB_CACHE_MAX + 1
        );
        assert!(odb.session_loose_reads.lock().unwrap().is_empty());
        let tree_bytes = odb.read(&tree).unwrap().data;
        assert_eq!(odb.read(&tiny).unwrap().data, b"tiny\n");
        for oid in [blob, tree, tiny] {
            let hex = oid.to_hex();
            fs::remove_file(dir.path().join(&hex[..2]).join(&hex[2..])).unwrap();
        }
        assert_eq!(odb.read(&tree).unwrap().data, tree_bytes, "tree reused");
        assert_eq!(odb.read(&tiny).unwrap().data, b"tiny\n", "tiny blob reused");
        assert!(odb.read(&blob).is_err(), "blob was not retained");
    }

    #[test]
    fn session_read_cache_is_bounded_by_bytes_and_entries() {
        let mut cache = SessionLooseReads::default();
        let chunk = SESSION_LOOSE_CACHE_MAX_BYTES / 4;
        let object = |len: usize| Object {
            kind: ObjectKind::Tree,
            data: vec![0; len],
        };
        let oid = |n: u32| {
            let mut raw = [0u8; 20];
            raw[..4].copy_from_slice(&n.to_be_bytes());
            ObjectId::from_bytes(&raw).unwrap()
        };
        for n in 0..4 {
            cache.insert(oid(n), &object(chunk));
        }
        assert_eq!(cache.bytes, SESSION_LOOSE_CACHE_MAX_BYTES);
        cache.insert(oid(4), &object(1));
        assert_eq!(cache.bytes, 1, "exceeding the byte budget starts over");
        assert!(cache.get(&oid(0)).is_none());
        cache.insert(oid(4), &object(10));
        assert_eq!(cache.bytes, 10, "replacing an entry drops its old size");
        cache.insert(oid(5), &object(SESSION_LOOSE_CACHE_MAX_BYTES + 1));
        assert!(
            cache.get(&oid(5)).is_none(),
            "oversized objects are never kept"
        );
        assert!(cache.get(&oid(4)).is_some());
        for n in 10..(10 + SESSION_LOOSE_CACHE_MAX_ENTRIES as u32 + 2) {
            cache.insert(oid(n), &object(1));
        }
        assert!(cache.objects.len() <= SESSION_LOOSE_CACHE_MAX_ENTRIES + 1);
        assert!(cache.bytes <= SESSION_LOOSE_CACHE_MAX_BYTES);
    }

    fn bounded_read_fixture() -> (TempDir, PathBuf, ObjectId, ObjectId) {
        let dir = TempDir::new().unwrap();
        let objects = dir.path().to_path_buf();
        let odb = Odb::new(&objects);
        let small = odb.write(ObjectKind::Blob, b"small blob\n").unwrap();
        let big: Vec<u8> = (0..300_000u32).flat_map(|i| i.to_le_bytes()).collect();
        let big = odb.write(ObjectKind::Blob, &big).unwrap();
        (dir, objects, small, big)
    }

    fn assert_bounded_reads(odb: &Odb, small: &ObjectId, big: &ObjectId) {
        match odb.read_unless_larger(small, 1000, false).unwrap() {
            BoundedRead::Object(obj) => assert_eq!(obj.data, b"small blob\n"),
            BoundedRead::Larger => panic!("small blob is within the limit"),
        }
        assert!(matches!(
            odb.read_unless_larger(big, 1000, false).unwrap(),
            BoundedRead::Larger
        ));
        match odb.read_unless_larger(big, 1_200_000, false).unwrap() {
            BoundedRead::Object(obj) => assert_eq!(obj.data.len(), 1_200_000),
            BoundedRead::Larger => panic!("limit equal to the size reads the object"),
        }
        let missing = ObjectId::from_hex("1234567890123456789012345678901234567890").unwrap();
        assert!(matches!(
            odb.read_unless_larger(&missing, 1000, false),
            Err(Error::ObjectNotFound(_))
        ));
    }

    #[test]
    fn read_unless_larger_sizes_loose_objects_from_their_header() {
        let (_dir, objects, small, big) = bounded_read_fixture();
        assert_bounded_reads(&Odb::new(&objects), &small, &big);
    }

    #[test]
    fn read_unless_larger_sizes_packed_objects_in_and_out_of_read_context() {
        let (_dir, objects, small, big) = bounded_read_fixture();
        pack_objects(&objects, &[small, big]);
        let odb = Odb::new(&objects);
        assert_bounded_reads(&odb, &small, &big);
        odb.with_pack_read_context(|| assert_bounded_reads(&odb, &small, &big));
    }

    #[test]
    fn parsed_tree_reads_keep_no_raw_copy_in_the_session_cache() {
        let (_dir, objects, small, _big) = bounded_read_fixture();
        let odb = Odb::new(&objects);
        let entries = [crate::objects::TreeEntry {
            mode: 0o100644,
            name: b"small".to_vec(),
            oid: small,
        }];
        let tree = odb
            .write(ObjectKind::Tree, &crate::objects::serialize_tree(&entries))
            .unwrap();
        for packed in [false, true] {
            if packed {
                pack_objects(&objects, &[tree]);
            }
            let odb = Odb::new(&objects);
            assert_eq!(*odb.cached_tree_entries(&tree).unwrap(), entries);
            assert!(odb.session_cached_loose_read(&tree).is_none());
            assert!(Arc::ptr_eq(
                &odb.cached_tree_entries(&tree).unwrap(),
                &odb.cached_tree_entries(&tree).unwrap()
            ));
            assert_eq!(odb.read(&tree).unwrap().kind, ObjectKind::Tree);
            assert!(
                odb.session_cached_loose_read(&tree).is_some(),
                "plain reads still use the session cache (packed: {packed})"
            );
        }
    }

    #[test]
    fn read_unless_larger_reuses_small_loose_reads_but_never_caches_big_ones() {
        let (_dir, objects, small, big) = bounded_read_fixture();
        let odb = Odb::new(&objects);
        assert_bounded_reads(&odb, &small, &big);
        assert!(odb.session_loose_reads.lock().unwrap().is_empty());
        assert!(matches!(
            odb.read_unless_larger(&small, 1000, true).unwrap(),
            BoundedRead::Object(_)
        ));
        assert!(matches!(
            odb.read_unless_larger(&big, 1_200_000, true).unwrap(),
            BoundedRead::Object(_)
        ));
        for oid in [small, big] {
            let hex = oid.to_hex();
            fs::remove_file(objects.join(&hex[..2]).join(&hex[2..])).unwrap();
        }
        match odb.read_unless_larger(&small, 1000, true).unwrap() {
            BoundedRead::Object(obj) => assert_eq!(obj.data, b"small blob\n"),
            BoundedRead::Larger => panic!("small blob is within the limit"),
        }
        assert!(matches!(
            odb.read_unless_larger(&small, 4, true),
            Ok(BoundedRead::Larger)
        ));
        assert!(matches!(
            odb.read_unless_larger(&big, 1_200_000, true),
            Err(Error::ObjectNotFound(_))
        ));
    }

    #[test]
    fn read_unless_larger_finds_objects_across_several_packs() {
        let (_dir, objects, small, big) = bounded_read_fixture();
        pack_objects(&objects, &[small]);
        pack_objects(&objects, &[big]);
        let odb = Odb::new(&objects);
        odb.with_pack_read_context(|| assert_bounded_reads(&odb, &small, &big));
    }

    #[test]
    fn parse_object_header_prefix_unknown_kind_is_typed_error() {
        let err = crate::odb::store::loose::parse_object_header_prefix(b"not-a-git-kind 0\0")
            .unwrap_err();
        assert!(matches!(err, Error::UnknownObjectType(_)));
    }

    fn git_dir_with_config(config: &str) -> (TempDir, PathBuf, PathBuf) {
        let dir = TempDir::new().unwrap();
        let git_dir = dir.path().join(".git");
        let objects = git_dir.join("objects");
        fs::create_dir_all(&objects).unwrap();
        fs::write(git_dir.join("config"), config).unwrap();
        (dir, git_dir, objects)
    }

    /// Zlib FLEVEL flag (upper two bits of the second header byte).
    fn zlib_flevel(compressed: &[u8]) -> u8 {
        assert!(compressed.len() >= 2);
        (compressed[1] >> 6) & 3
    }

    fn tmp_loose_object_paths(objects: &Path) -> Vec<PathBuf> {
        let mut tmp = Vec::new();
        let Ok(shards) = fs::read_dir(objects) else {
            return tmp;
        };
        for shard in shards.flatten() {
            if !shard.file_type().is_ok_and(|t| t.is_dir()) {
                continue;
            }
            let Ok(files) = fs::read_dir(shard.path()) else {
                continue;
            };
            for file in files.flatten() {
                let name = file.file_name();
                if name.to_string_lossy().starts_with("tmp_") {
                    tmp.push(file.path());
                }
            }
        }
        tmp
    }

    #[test]
    fn loose_write_honours_level() {
        let payload: Vec<u8> = (0..4096).map(|i| (i % 251) as u8).collect();

        let (_dir, git_dir, objects) = git_dir_with_config("");
        let odb = Odb::new(&objects).with_config_git_dir(git_dir.clone());
        let oid_default = odb.write(ObjectKind::Blob, &payload).unwrap();
        let bytes_default = fs::read(odb.object_path(&oid_default)).unwrap();
        assert!(zlib_flevel(&bytes_default) <= 1);
        assert_eq!(odb.read(&oid_default).unwrap().data, payload);

        for level in -1i32..=9 {
            let config = format!("[core]\n\tloosecompression = {level}\n");
            let (_dir, git_dir, objects) = git_dir_with_config(&config);
            let odb = Odb::new(&objects).with_config_git_dir(git_dir);
            let byte = u8::try_from(level.rem_euclid(256)).unwrap_or(0);
            let bytes: Vec<u8> = vec![byte; 512];
            let oid = odb.write(ObjectKind::Blob, &bytes).unwrap();
            let on_disk = fs::read(odb.object_path(&oid)).unwrap();
            assert!(on_disk.len() >= 2, "level {level}");
            assert_eq!(odb.read(&oid).unwrap().data, bytes.as_slice());
        }

        let config = "[core]\n\tcompression = 9\n";
        let (_dir, git_dir, objects) = git_dir_with_config(config);
        let odb = Odb::new(&objects).with_config_git_dir(git_dir);
        let oid = odb.write(ObjectKind::Blob, &payload).unwrap();
        assert_eq!(zlib_flevel(&fs::read(odb.object_path(&oid)).unwrap()), 3);

        let config = "[core]\n\tcompression = 0\n\tloosecompression = 1\n";
        let (_dir, git_dir, objects) = git_dir_with_config(config);
        let odb = Odb::new(&objects).with_config_git_dir(git_dir);
        let oid = odb.write(ObjectKind::Blob, &payload).unwrap();
        assert_eq!(zlib_flevel(&fs::read(odb.object_path(&oid)).unwrap()), 0);

        let config = "[core]\n\tloosecompression = 99\n";
        let (_dir, git_dir, objects) = git_dir_with_config(config);
        let odb = Odb::new(&objects).with_config_git_dir(git_dir);
        assert!(odb.write(ObjectKind::Blob, b"x").is_err());
        assert!(tmp_loose_object_paths(&objects).is_empty());
    }

    fn git_run(dir: &Path, args: &[&str]) -> std::process::Output {
        std::process::Command::new("git") // hygiene: git fixture comparison in cfg(test) module
            .current_dir(dir)
            .args(args)
            .env("GIT_CONFIG_GLOBAL", "/dev/null")
            .env("GIT_CONFIG_SYSTEM", "/dev/null")
            .env("GIT_AUTHOR_NAME", "T")
            .env("GIT_AUTHOR_EMAIL", "t@example.com")
            .env("GIT_COMMITTER_NAME", "T")
            .env("GIT_COMMITTER_EMAIL", "t@example.com")
            .output()
            .unwrap()
    }

    fn git_in(dir: &Path, args: &[&str]) {
        let out = git_run(dir, args);
        assert!(
            out.status.success(),
            "git {:?}: {}",
            args,
            String::from_utf8_lossy(&out.stderr)
        );
    }

    fn git_rev_parse_oid(dir: &Path, spec: &str) -> ObjectId {
        let out = git_run(dir, &["rev-parse", spec]);
        assert!(
            out.status.success(),
            "git rev-parse {spec}: {}",
            String::from_utf8_lossy(&out.stderr)
        );
        ObjectId::from_hex(std::str::from_utf8(&out.stdout).unwrap().trim()).unwrap()
    }

    #[test]
    fn write_loose_zlib_prehashed_matches_write() {
        let dir = TempDir::new().unwrap();
        let objects = dir.path().join("objects");
        let odb = Odb::new(&objects);
        let data = b"parallel prehash path";
        let oid_normal = odb.write(ObjectKind::Blob, data).unwrap();

        let dir2 = TempDir::new().unwrap();
        let objects2 = dir2.path().join("objects");
        let odb2 = Odb::new(&objects2);
        let oid = hash::hash_object(odb2.hash_algo(), ObjectKind::Blob, data).expect("hash");
        assert_eq!(oid, oid_normal);
        let zlib =
            super::zlib_compress_blob_loose(data, odb2.loose_compression().unwrap()).unwrap();
        let oid2 = odb2
            .write_loose_zlib_prehashed(
                &oid,
                &zlib,
                WriteOptions {
                    assume_loose_only_existence: true,
                    ..WriteOptions::default()
                },
            )
            .unwrap();
        assert_eq!(oid2, oid_normal);
        assert_eq!(
            fs::read(odb2.object_path(&oid2)).unwrap(),
            fs::read(odb.object_path(&oid_normal)).unwrap()
        );
    }

    #[test]
    fn write_blob_prehashed_matches_write_and_skips_existing_objects() {
        let dir = TempDir::new().unwrap();
        let odb = Odb::new(&dir.path().join("objects"));
        let data = vec![7u8; 100_000];
        let oid_normal = odb.write(ObjectKind::Blob, &data).unwrap();

        let dir2 = TempDir::new().unwrap();
        let odb2 = Odb::new(&dir2.path().join("objects"));
        let oid = hash::hash_object(odb2.hash_algo(), ObjectKind::Blob, &data).unwrap();
        let options = WriteOptions {
            assume_loose_only_existence: true,
            ..WriteOptions::default()
        };
        assert_eq!(
            odb2.write_blob_prehashed(&oid, &data, options).unwrap(),
            oid
        );
        assert_eq!(oid, oid_normal);
        assert!(odb2.object_path(&oid).is_file());
        let obj = odb2.read(&oid).unwrap();
        assert_eq!(
            (obj.kind, obj.data.as_slice()),
            (ObjectKind::Blob, data.as_slice())
        );

        // An existing object is kept as is, even when handed different bytes for its id.
        assert_eq!(
            odb2.write_blob_prehashed(&oid, b"other", options).unwrap(),
            oid
        );
        assert_eq!(odb2.read(&oid).unwrap().data, data);
    }

    #[test]
    fn write_loose_zlib_prehashed_routes_to_custom_primary() {
        use crate::objects::HashAlgo;
        use std::sync::Arc;

        let dir = TempDir::new().unwrap();
        let objects = dir.path().join("objects");
        fs::create_dir_all(&objects).unwrap();
        let store = Arc::new(MemoryStore::new(HashAlgo::Sha1));
        let odb = OdbBuilder::files(&objects)
            .primary(store.clone())
            .alternates(false)
            .build();
        let data = b"staged through a custom primary";
        let oid = hash::hash_object(HashAlgo::Sha1, ObjectKind::Blob, data).unwrap();
        let zlib = super::zlib_compress_blob_loose(data, Compression::fast()).unwrap();

        odb.ensure_all_loose_prefix_dirs().unwrap();
        let written = odb
            .write_loose_zlib_prehashed(&oid, &zlib, WriteOptions::default())
            .unwrap();
        assert_eq!(written, oid);
        assert!(store.contains(&oid).unwrap());
        assert_eq!(odb.read(&oid).unwrap().data, data);
        assert_eq!(
            fs::read_dir(&objects).unwrap().count(),
            0,
            "no loose shard directories or files beside a custom primary"
        );

        assert!(odb
            .write_loose_zlib_prehashed(&oid, b"not zlib", WriteOptions::default())
            .is_err());
    }

    /// Re-writing an object that already exists as a loose file must not replace it; Git
    /// only touches mtime via [`Self::freshen_object`].
    #[test]
    fn write_existing_loose_does_not_rewrite() {
        let dir = TempDir::new().unwrap();
        let objects = dir.path().join("objects");
        fs::create_dir_all(&objects).unwrap();
        let odb = Odb::new(&objects);
        let payload = b"already loose";
        let oid = odb.write(ObjectKind::Blob, payload).unwrap();
        let path = odb.object_path(&oid);
        let before_bytes = fs::read(&path).unwrap();
        let meta_before = fs::metadata(&path).unwrap();
        let before_mtime = filetime::FileTime::from_last_modification_time(&meta_before);
        #[cfg(unix)]
        let ino_before = {
            use std::os::unix::fs::MetadataExt;
            meta_before.ino()
        };
        std::thread::sleep(std::time::Duration::from_millis(20));
        let oid2 = odb.write(ObjectKind::Blob, payload).unwrap();
        assert_eq!(oid, oid2);
        assert_eq!(fs::read(&path).unwrap(), before_bytes);
        let meta_after = fs::metadata(&path).unwrap();
        let after_mtime = filetime::FileTime::from_last_modification_time(&meta_after);
        assert!(
            after_mtime >= before_mtime,
            "expected freshen to bump or preserve mtime"
        );
        #[cfg(unix)]
        {
            use std::os::unix::fs::MetadataExt;
            assert_eq!(
                meta_after.ino(),
                ino_before,
                "rewrite must not replace the loose object file"
            );
        }
    }

    /// Objects present only in a pack must not be duplicated as loose files on [`Self::write`].
    #[cfg_attr(
        not(feature = "full-tests"),
        ignore = "starts system git or is heavy; run with --features full-tests"
    )]
    #[test]
    fn write_skips_loose_when_object_in_pack_only() {
        let dir = TempDir::new().unwrap();
        git_in(dir.path(), &["init", "-q"]);
        git_in(dir.path(), &["config", "user.email", "t@example.com"]);
        git_in(dir.path(), &["config", "user.name", "T"]);
        fs::write(dir.path().join("f"), b"pack-only payload").unwrap();
        git_in(dir.path(), &["add", "f"]);
        git_in(dir.path(), &["commit", "-m", "c"]);
        git_in(dir.path(), &["repack", "-ad"]);
        let objects = dir.path().join(".git").join("objects");
        let odb = Odb::new(&objects);
        let oid = odb.write(ObjectKind::Blob, b"pack-only payload").unwrap();
        assert!(!odb.object_path(&oid).exists());
        assert!(odb.exists(&oid));
    }

    /// Same as pack-only: alternates satisfy [`Self::exists`] without a local loose copy.
    #[cfg_attr(
        not(feature = "full-tests"),
        ignore = "starts system git or is heavy; run with --features full-tests"
    )]
    #[test]
    fn write_freshens_loose_object_in_alternate() {
        use filetime::FileTime;
        use std::thread;
        use std::time::Duration;

        let alt = TempDir::new().unwrap();
        git_in(alt.path(), &["init", "-q", "--bare"]);
        git_in(alt.path(), &["config", "user.email", "t@example.com"]);
        let alt_objects = alt.path().join("objects");
        let alt_odb = Odb::new(&alt_objects);
        let payload = b"alternate-loose";
        let oid = alt_odb.write(ObjectKind::Blob, payload).unwrap();
        let alt_loose = alt_odb.object_path(&oid);

        let primary = TempDir::new().unwrap();
        let primary_objects = primary.path().join("objects");
        fs::create_dir_all(primary_objects.join("info")).unwrap();
        fs::write(
            primary_objects.join("info").join("alternates"),
            format!("{}\n", alt_objects.display()),
        )
        .unwrap();
        let odb = Odb::new(&primary_objects);
        thread::sleep(Duration::from_millis(50));
        let before = FileTime::from_last_modification_time(&fs::metadata(&alt_loose).unwrap());
        filetime::set_file_mtime(&alt_loose, before).unwrap();

        let oid2 = odb.write(ObjectKind::Blob, payload).unwrap();
        assert_eq!(oid, oid2);
        assert!(!odb.object_path(&oid).exists());
        let after = FileTime::from_last_modification_time(&fs::metadata(&alt_loose).unwrap());
        assert!(after > before, "write must freshen alternate loose object");
    }

    #[cfg_attr(
        not(feature = "full-tests"),
        ignore = "starts system git or is heavy; run with --features full-tests"
    )]
    #[test]
    fn write_freshens_packed_object_in_alternate() {
        use filetime::FileTime;
        use std::thread;
        use std::time::Duration;

        let alt = TempDir::new().unwrap();
        git_in(alt.path(), &["init", "-q"]);
        git_in(alt.path(), &["config", "user.email", "t@example.com"]);
        git_in(alt.path(), &["config", "user.name", "T"]);
        let payload = b"alternate-pack";
        fs::write(alt.path().join("f"), payload).unwrap();
        git_in(alt.path(), &["add", "f"]);
        git_in(alt.path(), &["commit", "-m", "c"]);
        git_in(alt.path(), &["repack", "-ad"]);
        let alt_objects = alt.path().join(".git").join("objects");
        let alt_odb = Odb::new(&alt_objects);
        let oid = alt_odb.write(ObjectKind::Blob, payload).unwrap();
        let pack_path = fs::read_dir(alt_objects.join("pack"))
            .unwrap()
            .flatten()
            .find(|e| e.path().extension().is_some_and(|x| x == "pack"))
            .unwrap()
            .path();
        thread::sleep(Duration::from_millis(50));
        let before = FileTime::from_last_modification_time(&fs::metadata(&pack_path).unwrap());

        let primary = TempDir::new().unwrap();
        let primary_objects = primary.path().join("objects");
        fs::create_dir_all(primary_objects.join("info")).unwrap();
        fs::write(
            primary_objects.join("info").join("alternates"),
            format!("{}\n", alt_objects.display()),
        )
        .unwrap();
        pack::clear_pack_cache();
        let odb = Odb::new(&primary_objects);
        assert!(odb.exists(&oid));
        let oid2 = odb.write(ObjectKind::Blob, payload).unwrap();
        assert_eq!(oid, oid2);
        assert!(!odb.object_path(&oid).exists());
        let after = FileTime::from_last_modification_time(&fs::metadata(&pack_path).unwrap());
        assert!(after >= before, "write must freshen alternate pack");
    }

    #[cfg_attr(
        not(feature = "full-tests"),
        ignore = "starts system git or is heavy; run with --features full-tests"
    )]
    #[test]
    fn write_skips_loose_when_object_only_in_alternate() {
        let alt = TempDir::new().unwrap();
        git_in(alt.path(), &["init", "-q", "--bare"]);
        git_in(alt.path(), &["config", "user.email", "t@example.com"]);
        let alt_objects = alt.path().join("objects");
        let alt_odb = Odb::new(&alt_objects);
        let payload = b"alternate-only";
        let oid = alt_odb.write(ObjectKind::Blob, payload).unwrap();

        let primary = TempDir::new().unwrap();
        let primary_objects = primary.path().join("objects");
        fs::create_dir_all(primary.path().join("objects").join("info")).unwrap();
        fs::write(
            primary
                .path()
                .join("objects")
                .join("info")
                .join("alternates"),
            format!("{}\n", alt_objects.display()),
        )
        .unwrap();
        let odb = Odb::new(&primary_objects);
        assert!(odb.exists(&oid));
        let oid2 = odb.write(ObjectKind::Blob, payload).unwrap();
        assert_eq!(oid, oid2);
        assert!(!odb.object_path(&oid).exists());
    }

    #[cfg_attr(
        not(feature = "full-tests"),
        ignore = "starts system git or is heavy; run with --features full-tests"
    )]
    #[test]
    fn write_with_mem_overlay_stays_in_memory() {
        let dir = TempDir::new().unwrap();
        let objects = dir.path().join("objects");
        fs::create_dir_all(&objects).unwrap();
        let odb = Odb::new(&objects);
        odb.enable_mem_overlay();
        let payload = b"overlay blob";
        let oid = odb.write(ObjectKind::Blob, payload).unwrap();
        assert!(!odb.object_path(&oid).exists());
        assert_eq!(odb.read(&oid).unwrap().data, payload);
        odb.disable_mem_overlay();
        assert!(odb.read(&oid).is_err());
    }

    #[test]
    fn read_alternates_relative_path_and_symlinked_objects_dir() {
        use crate::pack::read_alternates_recursive;

        let layout = TempDir::new().unwrap();
        let alt_objects = layout.path().join("alt-store").join("objects");
        fs::create_dir_all(&alt_objects).unwrap();
        let primary_objects = layout.path().join("repo").join("objects");
        fs::create_dir_all(primary_objects.join("info")).unwrap();
        fs::write(
            primary_objects.join("info").join("alternates"),
            "../../alt-store/objects\n",
        )
        .unwrap();
        let resolved = read_alternates_recursive(&primary_objects).unwrap();
        let alt_canonical = alt_objects
            .canonicalize_simplified()
            .unwrap_or(alt_objects.clone());
        assert!(
            resolved.iter().any(|p| {
                p.canonicalize_simplified().unwrap_or_else(|_| p.clone()) == alt_canonical
            }),
            "relative alternate entry should resolve: {resolved:?}"
        );

        #[cfg(unix)]
        {
            let link_parent = TempDir::new().unwrap();
            std::os::unix::fs::symlink(&primary_objects, link_parent.path().join("objects-link"))
                .unwrap();
            let via_link =
                read_alternates_recursive(&link_parent.path().join("objects-link")).unwrap();
            assert!(
                !via_link.is_empty(),
                "alternates through symlinked objects dir: {via_link:?}"
            );
        }
    }
    #[test]
    fn empty_tree_exists_without_hex() {
        let odb = Odb::new(std::path::Path::new("/nonexistent/objects"));
        let canon = ObjectId::from_hex("4b825dc642cb6eb9a060e54bf8d69288fbee4904").unwrap();
        let legacy = ObjectId::from_hex("4b825dc642cb6eb9a060e54bf899d69f7c6948d4").unwrap();
        assert!(odb.exists(&canon));
        assert!(odb.exists(&legacy));
        assert!(odb.exists_local(&canon));
        assert!(!odb.freshen_object(&canon));
    }

    #[test]
    fn sha256_empty_tree_virtual_read_without_loose_file() {
        use crate::objects::HashAlgo;
        use std::sync::Arc;

        let dir = TempDir::new().unwrap();
        let objects = dir.path().join("objects");
        fs::create_dir_all(&objects).unwrap();
        let store = Arc::new(MemoryStore::new(HashAlgo::Sha256));
        let odb = OdbBuilder::files(&objects)
            .primary(store)
            .alternates(false)
            .build();
        let empty = odb.empty_tree_oid().expect("hash empty tree");
        assert!(!odb.exists_materialized_local(&empty));
        assert!(odb.exists_local(&empty));
        assert!(odb.is_empty_tree(&empty));
        let obj = odb.read(&empty).expect("virtual read empty tree");
        assert_eq!(obj.kind, ObjectKind::Tree);
        assert!(obj.data.is_empty());
    }

    #[test]
    fn empty_tree_oid_and_object_id_empty_tree_match_sha1() {
        let odb = Odb::new(std::path::Path::new("/nonexistent/objects"));
        let from_odb = odb.empty_tree_oid().expect("empty tree oid");
        assert_eq!(from_odb, ObjectId::empty_tree());
        assert!(odb.is_empty_tree(&ObjectId::empty_tree()));
    }

    #[test]
    fn empty_tree_virtual_presence_skips_materialized_local() {
        let dir = TempDir::new().unwrap();
        let objects = dir.path().join("objects");
        fs::create_dir_all(&objects).unwrap();
        let odb = Odb::new(&objects);
        let canon = ObjectId::from_hex("4b825dc642cb6eb9a060e54bf8d69288fbee4904").unwrap();
        assert!(odb.exists_local(&canon));
        assert!(!odb.exists_materialized_local(&canon));
    }

    #[test]
    fn write_materializes_well_known_empty_tree() {
        let dir = TempDir::new().unwrap();
        let objects = dir.path().join("objects");
        fs::create_dir_all(&objects).unwrap();
        let odb = Odb::new(&objects);
        let canon = ObjectId::from_hex("4b825dc642cb6eb9a060e54bf8d69288fbee4904").unwrap();
        assert!(!odb.exists_materialized_local(&canon));
        let oid = odb
            .write(crate::objects::ObjectKind::Tree, &[])
            .expect("write empty tree");
        assert_eq!(oid, canon);
        assert!(odb.exists_materialized_local(&canon));
    }

    #[test]
    fn append_alternate_objects_line_errors_on_unreadable_existing_file() {
        let dir = TempDir::new().unwrap();
        let objects = dir.path().join("objects");
        fs::create_dir_all(objects.join("info")).unwrap();
        let alt_path = objects.join("info/alternates");
        fs::write(&alt_path, b"not valid utf-8 \xff\n").unwrap();
        let err = Odb::append_alternate_objects_line(&objects, Path::new("/tmp/other/objects"))
            .unwrap_err();
        assert!(matches!(err, Error::Io(_)));
        assert_eq!(fs::read(&alt_path).unwrap(), b"not valid utf-8 \xff\n");
    }

    #[test]
    fn append_file_alternate_refreshes_cached_snapshot() {
        let primary = TempDir::new().unwrap();
        let alternate = TempDir::new().unwrap();
        let alt_objects = alternate.path().join("objects");
        std::fs::create_dir_all(&alt_objects).unwrap();
        let alt_odb = Odb::new(&alt_objects);
        let oid = alt_odb.write(ObjectKind::Blob, b"via-alternate").unwrap();

        let primary_objects = primary.path().join("objects");
        std::fs::create_dir_all(primary_objects.join("info")).unwrap();
        let primary_odb = Odb::new(&primary_objects);
        assert!(!primary_odb.exists(&oid));
        let _ = primary_odb.file_alternate_dirs_snapshot();
        primary_odb.append_file_alternate(&alt_objects).unwrap();
        assert!(primary_odb.exists(&oid));
    }

    #[test]
    fn exists_via_env_alternate_dirs_at_construction() {
        let primary = TempDir::new().unwrap();
        let alternate = TempDir::new().unwrap();
        let alt_objects = alternate.path().join("objects");
        std::fs::create_dir_all(&alt_objects).unwrap();
        let alt_odb = Odb::new(&alt_objects);
        let oid = alt_odb.write(ObjectKind::Blob, b"env-alt").unwrap();

        let primary_objects = primary.path().join("objects");
        std::fs::create_dir_all(&primary_objects).unwrap();
        let primary_odb = Odb::new(&primary_objects).with_env_alternate_dirs(vec![alt_objects]);
        assert!(primary_odb.exists(&oid));
    }

    fn flaky_odb(objects: &Path) -> (Odb, Arc<store::flaky::FlakyStore>) {
        let flaky = store::flaky::FlakyStore::new();
        let odb = OdbBuilder::files(objects)
            .primary(flaky.clone())
            .alternates(false)
            .build();
        (odb, flaky)
    }

    #[test]
    fn contains_reports_primary_failure_instead_of_missing() {
        let dir = TempDir::new().unwrap();
        let (odb, flaky) = flaky_odb(dir.path());
        let oid = odb.write(ObjectKind::Blob, b"stored").unwrap();
        assert!(odb.contains(&oid).unwrap());

        flaky.fail_all(true);
        assert!(store::flaky::is_injected(&odb.read(&oid).unwrap_err()));
        assert!(store::flaky::is_injected(&odb.contains(&oid).unwrap_err()));
        assert!(
            !odb.exists(&oid),
            "exists() keeps treating failures as absent"
        );
        assert!(odb.contains(&odb.empty_tree_oid().unwrap()).unwrap());

        let try_local = odb.try_exists_local(&oid).unwrap_err();
        assert!(store::flaky::is_injected(&try_local));
        assert!(!odb.exists_local(&oid));
        let try_stored = odb.try_exists_stored(&oid).unwrap_err();
        assert!(store::flaky::is_injected(&try_stored));
        assert!(!odb.exists_stored(&oid));
    }

    #[test]
    fn contains_reports_extra_read_source_failure() {
        let dir = TempDir::new().unwrap();
        let flaky = store::flaky::FlakyStore::new();
        let odb = OdbBuilder::files(dir.path())
            .push_read_source(flaky.clone())
            .alternates(false)
            .build();
        let local = odb.write(ObjectKind::Blob, b"in primary").unwrap();
        let absent = HashAlgo::Sha1
            .hash_object(ObjectKind::Blob, b"nowhere")
            .unwrap();
        assert!(!odb.contains(&absent).unwrap());

        flaky.fail_contains(true);
        assert!(
            odb.contains(&local).unwrap(),
            "primary hit wins over a failing source"
        );
        assert!(store::flaky::is_injected(
            &odb.contains(&absent).unwrap_err()
        ));
        assert!(!odb.exists(&absent));
    }

    #[test]
    fn contains_reports_primary_failure_even_when_later_source_has_object() {
        let dir = TempDir::new().unwrap();
        let extra = Arc::new(store::MemoryStore::new(HashAlgo::Sha1));
        let flaky = store::flaky::FlakyStore::new();
        let odb = OdbBuilder::files(dir.path())
            .primary(flaky.clone())
            .push_read_source(extra.clone())
            .alternates(false)
            .build();
        let oid = extra
            .write(ObjectKind::Blob, b"extra", WriteOptions::default())
            .unwrap();
        assert!(odb.contains(&oid).unwrap());

        flaky.fail_all(true);
        assert!(store::flaky::is_injected(&odb.read(&oid).unwrap_err()));
        assert!(
            store::flaky::is_injected(&odb.contains(&oid).unwrap_err()),
            "contains must agree with read"
        );
        assert!(odb.exists(&oid), "exists() skips the failing primary");
    }

    #[test]
    fn contains_reports_alternate_failure() {
        let primary = TempDir::new().unwrap();
        let alternate = TempDir::new().unwrap();
        let alt_objects = alternate.path().join("objects");
        std::fs::create_dir_all(&alt_objects).unwrap();
        let primary_objects = primary.path().join("objects");
        std::fs::create_dir_all(&primary_objects).unwrap();
        let odb = Odb::new(&primary_objects).with_env_alternate_dirs(vec![alt_objects]);
        let local = odb.write(ObjectKind::Blob, b"local").unwrap();
        let absent = HashAlgo::Sha1
            .hash_object(ObjectKind::Blob, b"nowhere")
            .unwrap();

        // Files alternates skip unreadable packs, so stand a failing store in for the
        // composite the alternate directories resolve to.
        let flaky = store::flaky::FlakyStore::new();
        odb.sources().unwrap();
        odb.alternate_sources_cache.write().unwrap().sources = Some(Arc::new(CompositeStore::new(
            vec![flaky.clone()],
            HashAlgo::Sha1,
        )));
        assert!(!odb.contains(&absent).unwrap());

        flaky.fail_contains(true);
        assert!(odb.contains(&local).unwrap());
        assert!(store::flaky::is_injected(
            &odb.contains(&absent).unwrap_err()
        ));
        assert!(!odb.exists(&absent));
    }

    #[test]
    fn contains_on_files_backend_covers_present_absent_empty_tree_and_alternate() {
        let primary = TempDir::new().unwrap();
        let alternate = TempDir::new().unwrap();
        let alt_objects = alternate.path().join("objects");
        std::fs::create_dir_all(&alt_objects).unwrap();
        let alt_oid = Odb::new(&alt_objects)
            .write(ObjectKind::Blob, b"alternate")
            .unwrap();
        let primary_objects = primary.path().join("objects");
        std::fs::create_dir_all(&primary_objects).unwrap();
        let odb = Odb::new(&primary_objects).with_env_alternate_dirs(vec![alt_objects]);
        let present = odb.write(ObjectKind::Blob, b"present").unwrap();
        let absent = HashAlgo::Sha1
            .hash_object(ObjectKind::Blob, b"absent")
            .unwrap();

        assert!(odb.contains(&present).unwrap());
        assert!(odb.contains(&odb.empty_tree_oid().unwrap()).unwrap());
        assert!(odb.contains(&alt_oid).unwrap());
        assert!(!odb.contains(&absent).unwrap());
        let empty_tree = odb.empty_tree_oid().unwrap();
        assert_eq!(
            odb.contains_many(&[present, absent, alt_oid, empty_tree])
                .unwrap(),
            vec![true, false, true, true]
        );
        assert!(odb.contains_many(&[]).unwrap().is_empty());
        assert_eq!(
            odb.present_oids([present, absent, alt_oid]).unwrap(),
            vec![present, alt_oid]
        );
        assert_eq!(
            odb.missing_oids([present, absent, alt_oid]).unwrap(),
            vec![absent]
        );
        assert_eq!(
            odb.missing_stored_oids([present, absent, alt_oid]).unwrap(),
            vec![absent, alt_oid],
            "stored ignores alternates"
        );
    }

    #[test]
    fn oid_filters_propagate_backend_failure() {
        let dir = TempDir::new().unwrap();
        let (odb, flaky) = flaky_odb(dir.path());
        let oid = odb.write(ObjectKind::Blob, b"x").unwrap();
        assert_eq!(odb.contains_many(&[oid]).unwrap(), vec![true]);
        flaky.fail_contains(true);
        assert!(store::flaky::is_injected(
            &odb.contains_many(&[oid]).unwrap_err()
        ));
        assert!(store::flaky::is_injected(
            &odb.present_oids([oid]).unwrap_err()
        ));
        assert!(store::flaky::is_injected(
            &odb.missing_oids([oid]).unwrap_err()
        ));
        assert!(store::flaky::is_injected(
            &odb.missing_stored_oids([oid]).unwrap_err()
        ));
    }

    #[test]
    fn write_existing_loose_uses_path_stat_not_exists() {
        let dir = TempDir::new().unwrap();
        let odb = Odb::new(dir.path());
        let oid = odb.write(ObjectKind::Blob, b"once").unwrap();
        let path = odb.object_path(&oid);
        assert!(path.is_file());
        odb.reset_exists_probe();
        let oid2 = odb.write(ObjectKind::Blob, b"once").unwrap();
        assert_eq!(oid, oid2);
        assert_eq!(
            odb.exists_probe_count(),
            0,
            "loose fast path must not call exists()"
        );
    }

    #[cfg_attr(
        not(feature = "full-tests"),
        ignore = "starts system git or is heavy; run with --features full-tests"
    )]
    #[test]
    fn exists_and_write_after_invalidate_packs_use_odb_store() {
        let dir = TempDir::new().unwrap();
        git_in(dir.path(), &["init", "-q"]);
        git_in(dir.path(), &["config", "user.email", "t@example.com"]);
        git_in(dir.path(), &["config", "user.name", "T"]);
        let payload = b"invalidate-pack-exists";
        fs::write(dir.path().join("f"), payload).unwrap();
        git_in(dir.path(), &["add", "f"]);
        git_in(dir.path(), &["commit", "-m", "c"]);
        git_in(dir.path(), &["repack", "-ad"]);
        let objects = dir.path().join(".git").join("objects");
        let odb = Odb::new(&objects);
        let oid = odb.hash(ObjectKind::Blob, payload).expect("hash");
        assert!(odb.exists(&oid), "blob must live in pack after repack");
        let pack_dir = objects.join("pack");
        for entry in fs::read_dir(&pack_dir).unwrap().flatten() {
            let path = entry.path();
            if path.extension().is_some_and(|e| e == "pack" || e == "idx") {
                fs::remove_file(path).unwrap();
            }
        }
        odb.invalidate_packs();
        assert!(
            !odb.exists(&oid),
            "exists must not read stale legacy pack listing after invalidate_packs"
        );
        let oid2 = odb.write(ObjectKind::Blob, payload).unwrap();
        assert_eq!(oid, oid2);
        assert!(
            odb.object_path(&oid).is_file(),
            "write must materialize loose object when pack copy is gone"
        );
        odb.read(&oid).expect("read after write");
    }

    #[cfg_attr(
        not(feature = "full-tests"),
        ignore = "starts system git or is heavy; run with --features full-tests"
    )]
    #[test]
    fn write_uses_single_exists_probe_when_not_loose() {
        let dir = TempDir::new().unwrap();
        git_in(dir.path(), &["init", "-q"]);
        git_in(dir.path(), &["config", "user.email", "t@example.com"]);
        git_in(dir.path(), &["config", "user.name", "T"]);
        fs::write(dir.path().join("f"), b"probe-once").unwrap();
        git_in(dir.path(), &["add", "f"]);
        git_in(dir.path(), &["commit", "-m", "c"]);
        git_in(dir.path(), &["repack", "-ad"]);
        let objects = dir.path().join(".git").join("objects");
        let odb = Odb::new(&objects);
        let oid = odb.write(ObjectKind::Blob, b"probe-once").unwrap();
        assert!(!odb.object_path(&oid).exists());
        odb.reset_exists_probe();
        let oid2 = odb.write(ObjectKind::Blob, b"probe-once").unwrap();
        assert_eq!(oid, oid2);
        assert_eq!(
            odb.exists_probe_count(),
            1,
            "pack/alternate write path must call exists() exactly once"
        );
    }
    #[test]
    fn write_existing_object_freshens() {
        use filetime::FileTime;
        use std::fs;
        use std::thread;
        use std::time::Duration;

        let dir = TempDir::new().unwrap();
        let odb = Odb::new(dir.path());
        let oid = odb.write(ObjectKind::Blob, b"hello").unwrap();
        let path = odb.object_path(&oid);
        thread::sleep(Duration::from_millis(50));
        let before = FileTime::from_last_modification_time(&fs::metadata(&path).unwrap());
        filetime::set_file_mtime(&path, before).unwrap();

        let _ = odb.write(ObjectKind::Blob, b"hello").unwrap();
        let after = FileTime::from_last_modification_time(&fs::metadata(&path).unwrap());
        assert!(
            after > before,
            "rewriting an existing loose object must freshen its mtime"
        );
    }

    #[test]
    fn freshen_pack_once_per_odb() {
        use filetime::FileTime;
        use flate2::write::ZlibEncoder;
        use flate2::Compression;
        use std::fs;
        use std::io::Write;
        use std::thread;
        use std::time::Duration;

        fn append_pack_object(buf: &mut Vec<u8>, type_code: u8, data: &[u8]) {
            let mut size = data.len();
            let first = ((type_code & 0x7) << 4) | (size & 0x0f) as u8;
            size >>= 4;
            if size > 0 {
                buf.push(first | 0x80);
                while size > 0 {
                    let b = (size & 0x7f) as u8;
                    size >>= 7;
                    buf.push(if size > 0 { b | 0x80 } else { b });
                }
            } else {
                buf.push(first);
            }
            let mut enc = ZlibEncoder::new(Vec::new(), Compression::default());
            enc.write_all(data).unwrap();
            buf.extend_from_slice(&enc.finish().unwrap());
        }

        fn write_v2_idx(
            idx_path: &Path,
            pack_path: &Path,
            entries: &[(ObjectId, u64)],
        ) -> Result<()> {
            let mut sorted = entries.to_vec();
            sorted.sort_by(|a, b| a.0.as_bytes().cmp(b.0.as_bytes()));
            let n = sorted.len();
            let mut fanout = [0u32; 256];
            for byte in 0u32..256 {
                let count = sorted
                    .iter()
                    .filter(|(oid, _)| u32::from(oid.as_bytes()[0]) <= byte)
                    .count();
                fanout[byte as usize] = u32::try_from(count).unwrap_or(u32::MAX);
            }
            let mut buf = Vec::new();
            buf.extend_from_slice(b"\xfftOc");
            buf.extend_from_slice(&2u32.to_be_bytes());
            for f in fanout {
                buf.extend_from_slice(&f.to_be_bytes());
            }
            for (oid, _) in &sorted {
                buf.extend_from_slice(oid.as_bytes());
            }
            for _ in 0..n {
                buf.extend_from_slice(&0u32.to_be_bytes());
            }
            for (_, off) in &sorted {
                buf.extend_from_slice(&u32::try_from(*off).unwrap_or(0x8000_0000).to_be_bytes());
            }
            let pack_bytes = fs::read(pack_path)?;
            buf.extend_from_slice(&pack_bytes[pack_bytes.len() - 20..]);
            buf.extend_from_slice(&HashAlgo::Sha1.checksum_digest(&buf));
            fs::write(idx_path, buf)?;
            Ok(())
        }

        let dir = TempDir::new().unwrap();
        let objects_dir = dir.path();
        let odb = Odb::new(objects_dir);

        let oid_a = odb.write(ObjectKind::Blob, b"alpha").unwrap();
        let oid_b = odb.write(ObjectKind::Blob, b"beta").unwrap();
        fs::remove_file(odb.object_path(&oid_a)).unwrap();
        fs::remove_file(odb.object_path(&oid_b)).unwrap();

        let mut pack = Vec::new();
        pack.extend_from_slice(b"PACK");
        pack.extend_from_slice(&2u32.to_be_bytes());
        pack.extend_from_slice(&2u32.to_be_bytes());
        let off_a = pack.len() as u64;
        append_pack_object(&mut pack, 3, b"alpha");
        let off_b = pack.len() as u64;
        append_pack_object(&mut pack, 3, b"beta");
        pack.extend_from_slice(&HashAlgo::Sha1.checksum_digest(&pack));

        let pack_dir = objects_dir.join("pack");
        fs::create_dir_all(&pack_dir).unwrap();
        let pack_path = pack_dir.join("test-177.pack");
        let idx_path = pack_dir.join("test-177.idx");
        fs::write(&pack_path, &pack).unwrap();
        write_v2_idx(&idx_path, &pack_path, &[(oid_a, off_a), (oid_b, off_b)]).unwrap();
        odb.invalidate_packs();

        thread::sleep(Duration::from_millis(50));
        let before = FileTime::from_last_modification_time(&fs::metadata(&pack_path).unwrap());

        assert!(odb.freshen_object(&oid_a));
        let after_first = FileTime::from_last_modification_time(&fs::metadata(&pack_path).unwrap());
        assert!(after_first > before);

        assert!(odb.freshen_object(&oid_b));
        let after_second =
            FileTime::from_last_modification_time(&fs::metadata(&pack_path).unwrap());
        assert_eq!(
            after_second, after_first,
            "second freshen of another object in the same pack must not touch the pack again"
        );
    }

    #[test]
    fn write_existing_packed_object_freshens_pack() {
        use filetime::FileTime;
        use flate2::write::ZlibEncoder;
        use flate2::Compression;
        use std::fs;
        use std::io::Write;
        use std::thread;
        use std::time::Duration;

        let dir = TempDir::new().unwrap();
        let objects_dir = dir.path();
        let odb = Odb::new(objects_dir);
        let oid = odb.write(ObjectKind::Blob, b"packed-only").unwrap();
        fs::remove_file(odb.object_path(&oid)).unwrap();

        let mut pack = Vec::new();
        pack.extend_from_slice(b"PACK");
        pack.extend_from_slice(&2u32.to_be_bytes());
        pack.extend_from_slice(&1u32.to_be_bytes());
        let off = pack.len() as u64;
        let mut enc = ZlibEncoder::new(Vec::new(), Compression::default());
        enc.write_all(b"packed-only").unwrap();
        let compressed = enc.finish().unwrap();
        pack.push(0x30 | 0x03); // blob len 11
        pack.extend_from_slice(&compressed);
        pack.extend_from_slice(&HashAlgo::Sha1.checksum_digest(&pack));

        let pack_dir = objects_dir.join("pack");
        fs::create_dir_all(&pack_dir).unwrap();
        let pack_path = pack_dir.join("single-177.pack");
        fs::write(&pack_path, &pack).unwrap();
        // Minimal idx: reuse logic from freshen_pack_once_per_odb via inline write
        let idx_path = pack_dir.join("single-177.idx");
        let mut sorted = [(oid, off)];
        sorted.sort_by(|a, b| a.0.as_bytes().cmp(b.0.as_bytes()));
        let mut fanout = [0u32; 256];
        for byte in 0u32..256 {
            fanout[byte as usize] = if u32::from(oid.as_bytes()[0]) <= byte {
                1
            } else {
                0
            };
        }
        let mut buf = Vec::new();
        buf.extend_from_slice(b"\xfftOc");
        buf.extend_from_slice(&2u32.to_be_bytes());
        for f in fanout {
            buf.extend_from_slice(&f.to_be_bytes());
        }
        buf.extend_from_slice(oid.as_bytes());
        buf.extend_from_slice(&0u32.to_be_bytes());
        buf.extend_from_slice(&(off as u32).to_be_bytes());
        buf.extend_from_slice(&pack[pack.len() - 20..]);
        buf.extend_from_slice(&HashAlgo::Sha1.checksum_digest(&buf));
        fs::write(&idx_path, buf).unwrap();
        odb.invalidate_packs();

        thread::sleep(Duration::from_millis(50));
        let before = FileTime::from_last_modification_time(&fs::metadata(&pack_path).unwrap());

        odb.write(ObjectKind::Blob, b"packed-only").unwrap();

        let after = FileTime::from_last_modification_time(&fs::metadata(&pack_path).unwrap());
        assert!(
            after > before,
            "write of an existing packed object must freshen the pack mtime"
        );
    }

    /// When a local pack is indexed but its `.pack` path cannot be utime'd, [`Odb::write`]
    /// materializes a loose copy (Git `write_object_file` fallback). Replacing the pack file
    /// with a directory simulates a failed freshen without requiring a root-owned pack.
    #[cfg_attr(
        not(feature = "full-tests"),
        ignore = "starts system git or is heavy; run with --features full-tests"
    )]
    #[test]
    fn write_materializes_loose_when_pack_freshen_fails() {
        let dir = TempDir::new().unwrap();
        git_in(dir.path(), &["init", "-q"]);
        git_in(dir.path(), &["config", "user.email", "t@example.com"]);
        git_in(dir.path(), &["config", "user.name", "T"]);
        let payload = b"packed-fallback";
        fs::write(dir.path().join("f"), payload).unwrap();
        git_in(dir.path(), &["add", "f"]);
        git_in(dir.path(), &["commit", "-m", "c"]);
        git_in(dir.path(), &["repack", "-ad"]);
        let objects = dir.path().join(".git").join("objects");
        let odb = Odb::new(&objects);
        let oid = odb.write(ObjectKind::Blob, payload).unwrap();
        assert!(!odb.object_path(&oid).exists());
        assert!(odb.exists(&oid));

        let pack_path = fs::read_dir(objects.join("pack"))
            .unwrap()
            .flatten()
            .find(|e| e.path().extension().is_some_and(|x| x == "pack"))
            .unwrap()
            .path();
        fs::remove_file(&pack_path).unwrap();
        fs::create_dir(&pack_path).unwrap();
        odb.invalidate_packs();
        assert!(!odb.freshen_object(&oid));

        odb.write(ObjectKind::Blob, payload).unwrap();
        assert!(odb.object_path(&oid).exists());
    }
    #[test]
    fn write_with_options_silent_skips_freshen_on_existing_loose_object() {
        let dir = TempDir::new().unwrap();
        let odb = Odb::new(dir.path());
        let oid = odb.write(ObjectKind::Blob, b"stable").unwrap();
        let path = odb.object_path(&oid);
        let before = std::fs::metadata(&path).unwrap().modified().unwrap();
        std::thread::sleep(std::time::Duration::from_millis(20));
        let again = odb
            .write_with_options(
                ObjectKind::Blob,
                b"stable",
                WriteOptions {
                    silent: true,
                    ..WriteOptions::default()
                },
            )
            .unwrap();
        assert_eq!(oid, again);
        let after = std::fs::metadata(&path).unwrap().modified().unwrap();
        assert_eq!(before, after);

        let touched = odb
            .write_with_options(
                ObjectKind::Blob,
                b"stable",
                WriteOptions {
                    silent: false,
                    ..WriteOptions::default()
                },
            )
            .unwrap();
        assert_eq!(oid, touched);
        let freshened = std::fs::metadata(&path).unwrap().modified().unwrap();
        assert!(freshened >= before);
    }

    #[cfg_attr(
        not(feature = "full-tests"),
        ignore = "starts system git or is heavy; run with --features full-tests"
    )]
    #[test]
    fn promisor_marker_checked_once_per_prepare() {
        use crate::pack::{
            clear_pack_cache, pack_cache_test_guard, test_pack_marker_stat_count,
            test_reset_pack_marker_stat_count,
        };

        let _guard = pack_cache_test_guard();
        let dir = TempDir::new().unwrap();
        git_in(dir.path(), &["init", "-q", "-b", "main"]);
        git_in(dir.path(), &["config", "user.email", "t@example.com"]);
        git_in(dir.path(), &["config", "user.name", "T"]);
        for i in 0..64 {
            fs::write(dir.path().join(format!("f{i}.txt")), format!("body {i}")).unwrap();
            git_in(dir.path(), &["add", "."]);
            git_in(dir.path(), &["commit", "-m", &format!("c{i}")]);
        }
        git_in(dir.path(), &["repack", "-a", "-d"]);
        let objects = dir.path().join(".git/objects");
        let oid = git_rev_parse_oid(dir.path(), "HEAD");

        clear_pack_cache();
        test_reset_pack_marker_stat_count();
        let pack_count = fs::read_dir(objects.join("pack"))
            .unwrap()
            .filter_map(|e| e.ok())
            .filter(|e| e.path().extension().is_some_and(|x| x == "pack"))
            .count();
        let expected_prepare_stats = u64::try_from(pack_count * 2).unwrap();
        let odb = Odb::new(&objects);
        let before_prepare = test_pack_marker_stat_count();
        assert!(odb.exists_local(&oid));
        assert_eq!(
            test_pack_marker_stat_count().saturating_sub(before_prepare),
            expected_prepare_stats,
            "prepare should stat promisor and mtimes once per pack"
        );

        test_reset_pack_marker_stat_count();
        for _ in 0..10_000 {
            let before_probe = test_pack_marker_stat_count();
            assert!(odb.exists_local(&oid));
            assert_eq!(
                test_pack_marker_stat_count(),
                before_probe,
                "cached sidecar flags must not stat markers on each exists_local probe"
            );
        }
    }

    #[cfg_attr(
        not(feature = "full-tests"),
        ignore = "starts system git or is heavy; run with --features full-tests"
    )]
    #[test]
    fn promisor_marker_added_after_prepare_is_seen_after_reprepare() {
        use crate::pack::{
            clear_pack_cache, pack_cache_test_guard, reprepare_pack_directory_on_miss,
        };

        let _guard = pack_cache_test_guard();
        let dir = TempDir::new().unwrap();
        git_in(dir.path(), &["init", "-q", "-b", "main"]);
        git_in(dir.path(), &["config", "user.email", "t@example.com"]);
        git_in(dir.path(), &["config", "user.name", "T"]);
        fs::write(dir.path().join("one.txt"), b"x").unwrap();
        git_in(dir.path(), &["add", "one.txt"]);
        git_in(dir.path(), &["commit", "-m", "one"]);
        git_in(dir.path(), &["repack", "-a", "-d"]);
        let objects = dir.path().join(".git/objects");
        let oid = git_rev_parse_oid(dir.path(), "HEAD");

        clear_pack_cache();
        let odb = Odb::new(&objects);
        assert!(odb.exists_local(&oid));

        let pack_path = fs::read_dir(objects.join("pack"))
            .unwrap()
            .filter_map(|e| e.ok())
            .map(|e| e.path())
            .find(|p| p.extension().is_some_and(|x| x == "pack"))
            .unwrap();
        fs::write(pack_path.with_extension("promisor"), b"").unwrap();
        filetime::set_file_mtime(objects.join("pack"), filetime::FileTime::now()).unwrap();

        assert!(
            odb.with_pack_store_for(&objects, || {
                reprepare_pack_directory_on_miss(&objects).unwrap()
            }),
            "pack directory change must require reprepare"
        );
        assert!(
            !odb.exists_local(&oid),
            "promisor marker must hide pack objects after reprepare refreshed sidecar flags"
        );
    }

    #[cfg_attr(
        not(feature = "full-tests"),
        ignore = "starts system git or is heavy; run with --features full-tests"
    )]
    #[test]
    fn git_promisor_marker_pack_not_materialized_for_exists_local() {
        use crate::pack::clear_pack_cache;

        let dir = TempDir::new().unwrap();
        git_in(dir.path(), &["init", "-q", "-b", "main"]);
        git_in(dir.path(), &["config", "user.email", "t@example.com"]);
        git_in(dir.path(), &["config", "user.name", "T"]);
        fs::write(dir.path().join("blob.txt"), b"git promisor compat").unwrap();
        git_in(dir.path(), &["add", "blob.txt"]);
        git_in(dir.path(), &["commit", "-m", "seed"]);
        git_in(dir.path(), &["repack", "-a", "-d"]);
        let objects = dir.path().join(".git/objects");
        let oid = git_rev_parse_oid(dir.path(), "HEAD");
        let pack_path = fs::read_dir(objects.join("pack"))
            .unwrap()
            .filter_map(|e| e.ok())
            .map(|e| e.path())
            .find(|p| p.extension().is_some_and(|x| x == "pack"))
            .unwrap();
        fs::write(pack_path.with_extension("promisor"), b"").unwrap();

        clear_pack_cache();
        let odb = Odb::new(&objects);
        assert!(
            !odb.exists_local(&oid),
            "git-written pack with .promisor must not count as local materialization"
        );

        let cat = git_run(dir.path(), &["cat-file", "-e", &oid.to_hex()]);
        assert!(
            cat.status.success(),
            "git still resolves the object in a promisor-marked pack"
        );
    }

    #[cfg_attr(
        not(feature = "full-tests"),
        ignore = "starts system git or is heavy; run with --features full-tests"
    )]
    #[test]
    fn packed_object_read_avoids_loose_path_open() {
        use crate::hot_path_test_metrics::HotPathMetricsScope;
        use crate::pack::clear_pack_cache;

        let dir = TempDir::new().unwrap();
        git_in(dir.path(), &["init", "-q", "-b", "main"]);
        git_in(dir.path(), &["config", "user.email", "t@example.com"]);
        git_in(dir.path(), &["config", "user.name", "T"]);
        fs::write(dir.path().join("packed.txt"), b"packed-only payload").unwrap();
        git_in(dir.path(), &["add", "packed.txt"]);
        git_in(dir.path(), &["commit", "-m", "pack"]);
        git_in(dir.path(), &["repack", "-a", "-d"]);
        let objects = dir.path().join(".git/objects");
        let oid = git_rev_parse_oid(dir.path(), "HEAD");

        clear_pack_cache();
        let odb = Odb::new(&objects);
        odb.hot_path_metrics().set_loose_open_counting(true);
        odb.hot_path_metrics().reset_loose_path_open_attempts();
        let _scope = HotPathMetricsScope::install(odb.hot_path_metrics_arc());

        let obj = odb.read(&oid).expect("packed read");
        assert_eq!(obj.kind, ObjectKind::Commit);
        assert_eq!(
            odb.hot_path_metrics().loose_path_open_attempts(),
            0,
            "packed objects must not attempt a loose-path open"
        );
    }

    #[test]
    fn loose_only_read_records_loose_path_open() {
        use crate::hot_path_test_metrics::HotPathMetricsScope;

        let dir = TempDir::new().unwrap();
        let objects = dir.path().join("objects");
        fs::create_dir_all(&objects).unwrap();
        let odb = Odb::new(&objects);
        let oid = odb
            .write(ObjectKind::Blob, b"loose-only")
            .expect("write loose");
        odb.hot_path_metrics().set_loose_open_counting(true);
        odb.hot_path_metrics().reset_loose_path_open_attempts();
        let _scope = HotPathMetricsScope::install(odb.hot_path_metrics_arc());

        odb.read(&oid).expect("loose read");
        assert_eq!(
            odb.hot_path_metrics().loose_path_open_attempts(),
            1,
            "loose fallback must record a path open attempt"
        );
    }

    #[cfg_attr(
        not(feature = "full-tests"),
        ignore = "starts system git or is heavy; run with --features full-tests"
    )]
    #[test]
    fn loose_and_packed_copies_return_identical_bytes() {
        use crate::pack::clear_pack_cache;

        let dir = TempDir::new().unwrap();
        git_in(dir.path(), &["init", "-q", "-b", "main"]);
        git_in(dir.path(), &["config", "user.email", "t@example.com"]);
        git_in(dir.path(), &["config", "user.name", "T"]);
        fs::write(dir.path().join("dup.txt"), b"duplicate loose and packed\n").unwrap();
        git_in(dir.path(), &["add", "dup.txt"]);
        git_in(dir.path(), &["commit", "-m", "seed"]);
        let objects = dir.path().join(".git/objects");
        let tree_oid = git_rev_parse_oid(dir.path(), "HEAD^{tree}");
        git_in(dir.path(), &["repack", "-a"]);
        clear_pack_cache();
        let odb = Odb::new(&objects);
        let loose_path = tree_oid.loose_path_in(&objects);
        assert!(
            loose_path.is_file(),
            "loose copy must remain after repack -a"
        );
        let loose = Odb::read_loose_verify_oid(&loose_path, &tree_oid).expect("loose tree");
        let packed = odb.read(&tree_oid).expect("packed tree");
        assert_eq!(packed.data, loose.data);
        assert_eq!(packed.kind, loose.kind);
    }

    #[cfg_attr(
        not(feature = "full-tests"),
        ignore = "starts system git or is heavy; run with --features full-tests"
    )]
    #[test]
    fn freshly_written_loose_object_visible_before_repack() {
        let dir = TempDir::new().unwrap();
        git_in(dir.path(), &["init", "-q", "-b", "main"]);
        git_in(dir.path(), &["config", "user.email", "t@example.com"]);
        git_in(dir.path(), &["config", "user.name", "T"]);
        fs::write(dir.path().join("seed.txt"), b"seed").unwrap();
        git_in(dir.path(), &["add", "seed.txt"]);
        git_in(dir.path(), &["commit", "-m", "seed"]);
        git_in(dir.path(), &["repack", "-a", "-d"]);
        let objects = dir.path().join(".git/objects");
        let odb = Odb::new(&objects);
        let fresh = b"brand-new loose blob\n";
        let oid = odb.write(ObjectKind::Blob, fresh).expect("loose write");
        let obj = odb.read(&oid).expect("immediate loose read");
        assert_eq!(obj.data.as_slice(), fresh);
    }

    #[cfg_attr(
        not(feature = "full-tests"),
        ignore = "starts system git or is heavy; run with --features full-tests"
    )]
    #[test]
    fn pack_added_after_first_read_found_via_odb_reprepare() {
        use crate::pack::{clear_pack_cache, pack_cache_test_guard};

        let _guard = pack_cache_test_guard();
        clear_pack_cache();
        let dir = TempDir::new().unwrap();
        git_in(dir.path(), &["init", "-q", "-b", "main"]);
        git_in(dir.path(), &["config", "user.email", "t@example.com"]);
        git_in(dir.path(), &["config", "user.name", "T"]);
        fs::write(dir.path().join("a.txt"), b"pack-a").unwrap();
        git_in(dir.path(), &["add", "a.txt"]);
        git_in(dir.path(), &["commit", "-m", "a"]);
        git_in(dir.path(), &["repack", "-a", "-d"]);
        let objects = dir.path().join(".git/objects");
        let pack_dir = objects.join("pack");
        let git_dir = dir.path().join(".git");

        let odb = Odb::new(&objects).with_config_git_dir(git_dir);
        let warm_oid = git_rev_parse_oid(dir.path(), "HEAD");
        odb.read(&warm_oid).expect("warm read");

        fs::write(dir.path().join("b.txt"), b"pack-b").unwrap();
        git_in(dir.path(), &["add", "b.txt"]);
        git_in(dir.path(), &["commit", "-m", "b"]);
        let new_oid = git_rev_parse_oid(dir.path(), "HEAD");

        std::thread::sleep(std::time::Duration::from_secs(1));
        filetime::set_file_mtime(&pack_dir, filetime::FileTime::now()).unwrap();

        let obj = odb.read(&new_oid).expect("new pack via reprepare");
        assert_eq!(obj.kind, ObjectKind::Commit);
    }

    #[cfg_attr(
        not(feature = "full-tests"),
        ignore = "starts system git or is heavy; run with --features full-tests"
    )]
    #[test]
    fn late_midx_after_warm_read_resolves_new_commit() {
        use crate::pack::{clear_pack_cache, pack_cache_test_guard};

        let _guard = pack_cache_test_guard();
        clear_pack_cache();
        let dir = TempDir::new().unwrap();
        git_in(dir.path(), &["init", "-q", "-b", "main"]);
        git_in(dir.path(), &["config", "user.email", "t@example.com"]);
        git_in(dir.path(), &["config", "user.name", "T"]);
        fs::write(dir.path().join("a.txt"), b"pack-a").unwrap();
        git_in(dir.path(), &["add", "a.txt"]);
        git_in(dir.path(), &["commit", "-m", "a"]);
        git_in(dir.path(), &["repack", "-a", "-d"]);
        let objects = dir.path().join(".git/objects");
        let git_dir = dir.path().join(".git");
        assert!(
            crate::midx::cached_tip_midx_path(&objects.join("pack")).is_none(),
            "fixture starts without a MIDX"
        );

        let odb = Odb::new(&objects).with_config_git_dir(git_dir);
        let warm_oid = git_rev_parse_oid(dir.path(), "HEAD");
        odb.read(&warm_oid).expect("warm read before MIDX exists");

        fs::write(dir.path().join("b.txt"), b"pack-b").unwrap();
        git_in(dir.path(), &["add", "b.txt"]);
        git_in(dir.path(), &["commit", "-m", "b"]);
        git_in(dir.path(), &["repack", "-a", "-d"]);
        git_in(dir.path(), &["multi-pack-index", "write"]);
        let new_oid = git_rev_parse_oid(dir.path(), "HEAD");

        let cat = git_run(dir.path(), &["cat-file", "-e", &new_oid.to_hex()]);
        assert!(
            cat.status.success(),
            "system git must resolve the new commit"
        );

        let obj = odb
            .read(&new_oid)
            .expect("MIDX-covered commit after late MIDX write");
        assert_eq!(obj.kind, ObjectKind::Commit);
        assert!(
            crate::midx::cached_tip_midx_path(&objects.join("pack")).is_some(),
            "successful read after late MIDX must refresh the tip cache"
        );
    }
}