grit_lib/

unpack_objects.rs

//! `unpack-objects`: unpack a pack stream into individual objects.
//!
//! Reads a pack-format byte stream, validates the trailing checksum, and
//! writes each object to the object database ([`unpack_objects`]) or to any
//! [`WritableObjectStore`] ([`ingest_pack_into`]).  Delta objects (both
//! `OFS_DELTA` and `REF_DELTA`) are resolved against already-unpacked objects
//! or objects already present in the target.
//!
//! Large blobs stream to the ODB without retaining payload bytes; delta bases use a
//! bounded byte LRU so multi-object unpack stays flat in memory (streaming read).

use std::borrow::Cow;
use std::collections::{HashMap, HashSet};
use std::io::{self, Read};

use crate::error::{Error, Result};
use crate::gitmodules::{self, DotFsckTracker};
use crate::hash::{ChecksumHasher, ObjectPayloadHasher};
use crate::index::MODE_GITLINK;
use crate::index_pack::ObjectKindCounts;
use crate::objects::{parse_commit, parse_tag, parse_tree, HashAlgo, Object, ObjectId, ObjectKind};
use crate::odb::store::{ObjectStore, WritableObjectStore};
use crate::odb::{Odb, WriteOptions};
use crate::progress::{poll_cancel, Cancel, NullProgress, ProgressPhase, ProgressSink};
use flate2::read::ZlibDecoder;
use flate2::{Decompress, FlushDecompress, Status};

/// Options controlling `unpack-objects` behaviour.
#[derive(Debug, Default)]
pub struct UnpackOptions {
    /// Validate and decompress objects but do not write them to the ODB.
    pub dry_run: bool,
    /// Suppress informational output.
    pub quiet: bool,
    /// Reject packs whose commits/trees/tags reference missing objects.
    pub strict: bool,
    /// Object IDs that strict connectivity may treat as promised by a configured promisor remote.
    pub allowed_missing: HashSet<ObjectId>,
    /// Whether strict connectivity may tolerate references to missing objects in a promisor repo.
    pub allow_promisor_missing_references: bool,
    /// Maximum number of raw pack bytes that may be consumed (including the 20-byte trailer).
    ///
    /// Matches Git's `unpack-objects --max-input-size` / `receive.maxInputSize`: counts every
    /// byte read from the pack stream after crossing the limit. `None` means no limit.
    pub max_input_bytes: Option<u64>,
    /// Commit OIDs that are shallow boundaries (grafts): their parents are intentionally absent and
    /// must not be required during the `--strict` connectivity walk.
    ///
    /// Mirrors `unpack-objects --shallow-file <file>` in upstream `receive-pack`, where the shallow
    /// file lists the commits whose parent objects were deliberately not transferred.
    pub shallow_boundaries: HashSet<ObjectId>,
}

/// A delta that could not yet be resolved because its base was not yet known.
struct PendingDelta {
    /// Byte offset of this object in the pack stream (used to anchor
    /// `OFS_DELTA` back-references from later objects).
    offset: usize,
    /// For `REF_DELTA`: SHA-1 of the base object.
    base_oid: Option<ObjectId>,
    /// For `OFS_DELTA`: absolute byte offset of the base object.
    base_offset: Option<usize>,
    /// Decompressed delta data.
    delta_data: Vec<u8>,
}

/// Default cap on unpacked delta-base bytes retained in RAM (`core.deltaBaseCacheLimit`).
const UNPACK_DELTA_BASE_BYTE_LIMIT: usize = 96 * 1024 * 1024;

/// Byte-bounded LRU of recently resolved object payloads keyed by pack offset (delta bases only).
struct UnpackBaseByteCache {
    byte_limit: usize,
    bytes_used: usize,
    entries: HashMap<usize, (ObjectKind, Vec<u8>)>,
    head: Option<usize>,
    tail: Option<usize>,
    links: HashMap<usize, (Option<usize>, Option<usize>)>,
}

impl UnpackBaseByteCache {
    fn new(byte_limit: usize) -> Self {
        Self {
            byte_limit,
            bytes_used: 0,
            entries: HashMap::new(),
            head: None,
            tail: None,
            links: HashMap::new(),
        }
    }

    fn unlink(&mut self, key: usize) {
        let Some((prev, next)) = self.links.remove(&key) else {
            return;
        };
        match prev {
            Some(p) => {
                if let Some(link) = self.links.get_mut(&p) {
                    link.1 = next;
                }
            }
            None => self.head = next,
        }
        match next {
            Some(n) => {
                if let Some(link) = self.links.get_mut(&n) {
                    link.0 = prev;
                }
            }
            None => self.tail = prev,
        }
    }

    fn append_tail(&mut self, key: usize) {
        let prev = self.tail;
        self.links.insert(key, (prev, None));
        if let Some(p) = prev {
            if let Some(link) = self.links.get_mut(&p) {
                link.1 = Some(key);
            }
        } else {
            self.head = Some(key);
        }
        self.tail = Some(key);
    }

    fn touch(&mut self, key: usize) {
        self.unlink(key);
        self.append_tail(key);
    }

    fn evict_key(&mut self, key: usize) {
        self.unlink(key);
        if let Some((_, data)) = self.entries.remove(&key) {
            self.bytes_used = self.bytes_used.saturating_sub(data.len());
        }
    }

    fn get(&mut self, offset: usize) -> Option<(ObjectKind, Vec<u8>)> {
        let (kind, data) = self.entries.get(&offset).map(|(k, d)| (*k, d.clone()))?;
        self.touch(offset);
        Some((kind, data))
    }

    fn put(&mut self, offset: usize, kind: ObjectKind, data: Vec<u8>) {
        if self.byte_limit == 0 {
            return;
        }
        let sz = data.len();
        if sz > self.byte_limit {
            return;
        }
        use std::collections::hash_map::Entry;
        match self.entries.entry(offset) {
            Entry::Occupied(mut slot) => {
                let old_len = slot.get().1.len();
                slot.insert((kind, data));
                self.bytes_used = self.bytes_used.saturating_sub(old_len).saturating_add(sz);
                self.touch(offset);
            }
            Entry::Vacant(slot) => {
                slot.insert((kind, data));
                self.bytes_used = self.bytes_used.saturating_add(sz);
                self.append_tail(offset);
            }
        }
        while self.bytes_used > self.byte_limit {
            let Some(evict) = self.head else {
                break;
            };
            self.evict_key(evict);
        }
        test_note_retained_bytes(self.bytes_used);
    }

    fn bytes_used(&self) -> usize {
        self.bytes_used
    }
}

#[cfg(test)]
static TEST_UNPACK_CACHE_BYTE_LIMIT: std::sync::atomic::AtomicUsize =
    std::sync::atomic::AtomicUsize::new(0);
#[cfg(test)]
static TEST_UNPACK_RETAINED_PEAK: std::sync::atomic::AtomicUsize =
    std::sync::atomic::AtomicUsize::new(0);
#[cfg(test)]
static TEST_UNPACK_RETAIN_LOCK: std::sync::Mutex<()> = std::sync::Mutex::new(());

fn unpack_base_cache_byte_limit() -> usize {
    #[cfg(test)]
    {
        let override_limit =
            TEST_UNPACK_CACHE_BYTE_LIMIT.load(std::sync::atomic::Ordering::Relaxed);
        if override_limit != 0 {
            return override_limit;
        }
    }
    UNPACK_DELTA_BASE_BYTE_LIMIT
}

#[cfg(test)]
pub(crate) fn test_set_unpack_base_cache_byte_limit(bytes: usize) {
    TEST_UNPACK_CACHE_BYTE_LIMIT.store(bytes, std::sync::atomic::Ordering::Relaxed);
    TEST_UNPACK_RETAINED_PEAK.store(0, std::sync::atomic::Ordering::Relaxed);
}

#[cfg(test)]
pub(crate) fn test_peak_unpack_retained_bytes() -> usize {
    TEST_UNPACK_RETAINED_PEAK.load(std::sync::atomic::Ordering::Relaxed)
}

fn test_note_retained_bytes(bytes: usize) {
    #[cfg(test)]
    {
        let mut cur = TEST_UNPACK_RETAINED_PEAK.load(std::sync::atomic::Ordering::Relaxed);
        while cur < bytes {
            match TEST_UNPACK_RETAINED_PEAK.compare_exchange_weak(
                cur,
                bytes,
                std::sync::atomic::Ordering::Relaxed,
                std::sync::atomic::Ordering::Relaxed,
            ) {
                Ok(_) => break,
                Err(v) => cur = v,
            }
        }
    }
    #[cfg(not(test))]
    let _ = bytes;
}

fn test_note_pending_delta_bytes(pending: &[PendingDelta], cache_bytes: usize) {
    #[cfg(test)]
    {
        let pending_bytes: usize = pending.iter().map(|d| d.delta_data.len()).sum();
        test_note_retained_bytes(cache_bytes.saturating_add(pending_bytes));
    }
    let _ = (pending, cache_bytes);
}

/// Objects a pack stream delivered into an object store, from [`ingest_pack_into`].
#[derive(Debug, Clone, Default)]
pub struct IngestedObjects {
    /// Id of every object the pack held (delta objects by their resolved id).
    pub object_ids: HashSet<ObjectId>,
    /// How many objects of each type the pack held.
    pub object_counts: ObjectKindCounts,
    /// Bytes of pack stream consumed, trailer included.
    pub pack_bytes: u64,
}

/// Where [`unpack_into`] stores resolved objects and finds objects the pack did not carry.
trait UnpackSink {
    fn hash_algo(&self) -> HashAlgo;

    /// Store `data`, or only hash it when `dry_run` is set.
    fn write(&self, kind: ObjectKind, data: &[u8], dry_run: bool) -> Result<ObjectId>;

    /// When true, [`Self::write_inflated_from_pack`] streams large inflated payloads.
    fn supports_streaming_inflate(&self) -> bool {
        false
    }

    /// Inflate one pack object from `rd` and store it without holding the full payload in RAM.
    fn write_inflated_from_pack(
        &self,
        kind: ObjectKind,
        size: usize,
        rd: &mut StreamingPackReader<'_>,
        dry_run: bool,
    ) -> Result<ObjectId> {
        let _ = (kind, size, rd, dry_run);
        Err(Error::UnsupportedObjectStore {
            operation: "write_inflated_from_pack",
        })
    }

    /// Read an object stored before or during this unpack (thin-pack bases, large blobs).
    fn read_stored(&self, oid: &ObjectId) -> Result<Option<Object>>;

    /// Whether `oid` is already stored (strict connectivity).
    fn contains(&self, oid: &ObjectId) -> Result<bool>;
}

impl UnpackSink for Odb {
    fn hash_algo(&self) -> HashAlgo {
        Odb::hash_algo(self)
    }

    fn write(&self, kind: ObjectKind, data: &[u8], dry_run: bool) -> Result<ObjectId> {
        if dry_run {
            return self.hash(kind, data);
        }
        if !self.uses_files_primary() {
            return Odb::write(self, kind, data);
        }
        // Always materialize into this ODB: objects reachable only via alternates must still be
        // written locally (matches git unpack-objects; t5519-push-alternates).
        self.write_local(kind, data)
    }

    fn read_stored(&self, oid: &ObjectId) -> Result<Option<Object>> {
        match self.read(oid) {
            Ok(obj) => Ok(Some(obj)),
            Err(Error::ObjectNotFound(_)) => Ok(None),
            Err(e) => Err(e),
        }
    }

    fn contains(&self, oid: &ObjectId) -> Result<bool> {
        Odb::contains(self, oid)
    }

    fn supports_streaming_inflate(&self) -> bool {
        self.uses_files_primary()
    }

    fn write_inflated_from_pack(
        &self,
        kind: ObjectKind,
        size: usize,
        rd: &mut StreamingPackReader<'_>,
        dry_run: bool,
    ) -> Result<ObjectId> {
        let size_u64 = u64::try_from(size).map_err(|_| {
            Error::CorruptObject(format!("object size {size} exceeds address space"))
        })?;
        if dry_run {
            let mut hasher = ObjectPayloadHasher::new(self.hash_algo(), kind, size_u64);
            rd.decompress_into(size, |chunk| {
                hasher.update(chunk);
                Ok(())
            })?;
            return hasher.finish();
        }
        if !self.uses_files_primary() {
            let data = rd.decompress(size)?;
            return self.write_with_options(kind, &data, WriteOptions::default());
        }
        self.write_local_stream_with(kind, size_u64, |chunk_sink| {
            rd.decompress_into(size, chunk_sink)
        })
    }
}

/// A [`WritableObjectStore`] target with a separate view for objects it already holds.
struct StoreSink<'a> {
    store: &'a dyn WritableObjectStore,
    bases: &'a dyn ObjectStore,
}

impl UnpackSink for StoreSink<'_> {
    fn hash_algo(&self) -> HashAlgo {
        self.store.hash_algo()
    }

    fn write(&self, kind: ObjectKind, data: &[u8], dry_run: bool) -> Result<ObjectId> {
        if dry_run {
            return crate::hash::hash_object(self.store.hash_algo(), kind, data);
        }
        self.store.write(kind, data, WriteOptions::default())
    }

    fn read_stored(&self, oid: &ObjectId) -> Result<Option<Object>> {
        if let Some(obj) = self.store.read(oid)? {
            return Ok(Some(obj));
        }
        self.bases.read(oid)
    }

    fn contains(&self, oid: &ObjectId) -> Result<bool> {
        Ok(self.store.contains(oid)? || self.bases.contains(oid)?)
    }
}

/// Unpack a pack stream from `reader` into `odb`.
///
/// Reads the complete pack from `reader`, validates the trailing SHA-1
/// checksum, unpacks all objects (including full delta-chain resolution), and —
/// unless [`UnpackOptions::dry_run`] is set — writes each object to `odb`.
///
/// With a files-backed primary every object is written as a loose object in this
/// object directory, even when an alternate already has it. With a custom
/// [`OdbBuilder::primary`](crate::odb::OdbBuilder::primary) objects go to that store
/// through [`Odb::write`].
///
/// Returns the total number of objects processed.
///
/// # Errors
///
/// - [`Error::CorruptObject`] — invalid pack format or checksum mismatch.
/// - [`Error::UnresolvedDeltas`] — deltas whose bases are in neither the pack nor `odb`.
/// - [`Error::Io`] — I/O failure reading from `reader`.
/// - [`Error::Zlib`] — decompression failure.
pub fn unpack_objects(reader: &mut dyn Read, odb: &Odb, opts: &UnpackOptions) -> Result<usize> {
    unpack_into(reader, odb, opts, None, None, None).map(|unpacked| unpacked.count)
}

/// Asks whether a reference the pack does not resolve is promised.
///
/// Called with the missing object and the object naming it; `Ok(true)` lets the
/// strict check pass it.
pub(crate) type PromisedReference<'a> = &'a dyn Fn(&ObjectId, &ObjectId) -> Result<bool>;

/// [`unpack_objects`] for a repository with promisor remotes: under
/// [`UnpackOptions::strict`], a reference that resolves neither in the pack nor in
/// `odb` passes when `promised` says a promisor remote has it, as Git's
/// connectivity check skips promisor objects.
///
/// # Errors
///
/// As [`unpack_objects`], plus whatever `promised` returns.
pub(crate) fn unpack_objects_with_promises(
    reader: &mut dyn Read,
    odb: &Odb,
    opts: &UnpackOptions,
    promised: PromisedReference<'_>,
) -> Result<usize> {
    unpack_into(reader, odb, opts, None, None, Some(promised)).map(|unpacked| unpacked.count)
}

/// Apply a pack stream to any [`WritableObjectStore`], resolving every delta.
///
/// This is how a pack received by fetch, clone, push or a bundle reaches a custom
/// backend (SQLite, key-value, remote blob storage): each object is inflated, its
/// `OFS_DELTA`/`REF_DELTA` chain resolved, and the result stored with
/// [`WritableObjectStore::write`]. No `objects/` directory is involved.
///
/// - `reader` yields the raw pack (`PACK` header through the trailing checksum). It is
///   read in small pieces, so wrap files and sockets in a [`std::io::BufReader`].
/// - `store` receives every object in the pack.
/// - `bases` resolves `REF_DELTA` bases a *thin* pack leaves out, and answers
///   [`UnpackOptions::strict`] connectivity checks. `store` is consulted first, so pass
///   `store` again when it is the only source, or a repository-wide view (a repository's
///   [`Odb::read_store`], or a [`CompositeStore`](crate::odb::store::CompositeStore)) when
///   bases may live elsewhere.
///
/// The trailing checksum is verified before any delta is applied. Objects without a
/// delta are written as they are read, so a failed ingest can leave some objects in
/// `store`; Git stores tolerate unreferenced objects and later garbage collection
/// removes them. Delta bases are bounded by a byte LRU (default 96 MiB, like
/// `core.deltaBaseCacheLimit`); larger blobs and strict connectivity re-read from `store` on demand.
///
/// Only [`UnpackOptions::dry_run`], [`UnpackOptions::strict`] and its allowances, and
/// [`UnpackOptions::max_input_bytes`] apply; [`UnpackOptions::quiet`] has no effect.
///
/// # Examples
///
/// ```
/// use std::io::Cursor;
/// use std::sync::Arc;
///
/// use grit_lib::objects::odb::store::{MemoryStore, ObjectStore};
/// use grit_lib::objects::odb::OdbBuilder;
/// use grit_lib::objects::pack_objects::{build_pack, PackBuildOptions};
/// use grit_lib::objects::unpack_objects::{ingest_pack_into, UnpackOptions};
/// use grit_lib::objects::{HashAlgo, ObjectKind};
///
/// # fn main() -> grit_lib::error::Result<()> {
/// // Any object database can produce the pack; here another in-memory store.
/// let source = OdbBuilder::files("/nonexistent/objects")
///     .primary(Arc::new(MemoryStore::new(HashAlgo::Sha1)))
///     .alternates(false)
///     .build();
/// let blob = source.write(ObjectKind::Blob, b"hello\n")?;
/// let pack = build_pack(&source, &[blob], &[], &PackBuildOptions::default())?;
///
/// let target = MemoryStore::new(HashAlgo::Sha1);
/// let ingested = ingest_pack_into(
///     &mut Cursor::new(pack),
///     &target,
///     &target,
///     &UnpackOptions::default(),
/// )?;
/// assert!(ingested.object_ids.contains(&blob));
/// assert!(target.contains(&blob)?);
/// # Ok(())
/// # }
/// ```
///
/// # Errors
///
/// - [`Error::CorruptObject`] — invalid pack format, checksum mismatch, or (with
///   [`UnpackOptions::strict`]) a reference that resolves nowhere.
/// - [`Error::UnresolvedDeltas`] — deltas whose bases are in neither the pack nor `bases`.
/// - [`Error::Io`] — failure reading from `reader`.
/// - [`Error::Zlib`] — decompression failure.
/// - Any error `store` or `bases` return.
pub fn ingest_pack_into(
    reader: &mut dyn Read,
    store: &dyn WritableObjectStore,
    bases: &dyn ObjectStore,
    opts: &UnpackOptions,
) -> Result<IngestedObjects> {
    ingest_pack_into_with_progress(reader, store, bases, opts, None, None)
}

/// Like [`ingest_pack_into`], reporting [`ProgressPhase::IndexingObjects`] and
/// [`ProgressPhase::ResolvingDeltas`] on `progress` and stopping with
/// [`Error::Cancelled`] once `cancel` fires.
///
/// # Errors
///
/// Same as [`ingest_pack_into`], plus [`Error::Cancelled`].
pub fn ingest_pack_into_with_progress(
    reader: &mut dyn Read,
    store: &dyn WritableObjectStore,
    bases: &dyn ObjectStore,
    opts: &UnpackOptions,
    progress: Option<&mut dyn ProgressSink>,
    cancel: Option<&dyn Cancel>,
) -> Result<IngestedObjects> {
    match unpack_into(
        reader,
        &StoreSink { store, bases },
        opts,
        progress,
        cancel,
        None,
    ) {
        Ok(unpacked) => {
            store.flush_for_publish()?;
            Ok(unpacked.into_ingested())
        }
        Err(err) => {
            let _ = store.rollback();
            Err(err)
        }
    }
}

/// Like [`ingest_pack_into_with_progress`], writing into `odb` the way [`unpack_objects`]
/// does (loose objects for a files primary, the custom primary otherwise).
pub(crate) fn ingest_pack_into_odb(
    reader: &mut dyn Read,
    odb: &Odb,
    opts: &UnpackOptions,
    progress: Option<&mut dyn ProgressSink>,
    cancel: Option<&dyn Cancel>,
) -> Result<IngestedObjects> {
    match unpack_into(reader, odb, opts, progress, cancel, None) {
        Ok(unpacked) => {
            odb.flush_for_publish()?;
            Ok(unpacked.into_ingested())
        }
        Err(err) => {
            let _ = odb.rollback();
            Err(err)
        }
    }
}

/// Everything [`unpack_into`] resolved.
struct Unpacked {
    count: usize,
    by_oid: HashMap<ObjectId, PackedObjectEntry>,
    object_counts: ObjectKindCounts,
    pack_bytes: u64,
}

impl Unpacked {
    fn into_ingested(self) -> IngestedObjects {
        IngestedObjects {
            object_ids: self.by_oid.into_keys().collect(),
            object_counts: self.object_counts,
            pack_bytes: self.pack_bytes,
        }
    }
}

fn poll_unpack_cancel(
    progress: &mut Option<&mut dyn ProgressSink>,
    cancel: Option<&dyn Cancel>,
) -> Result<()> {
    let Some(cancel) = cancel else {
        return Ok(());
    };
    match progress {
        Some(p) => poll_cancel(&mut **p, cancel),
        None => poll_cancel(&mut NullProgress, cancel),
    }
}

fn unpack_into<S: UnpackSink + ?Sized>(
    reader: &mut dyn Read,
    sink: &S,
    opts: &UnpackOptions,
    mut progress: Option<&mut dyn ProgressSink>,
    cancel: Option<&dyn Cancel>,
    promised: Option<PromisedReference<'_>>,
) -> Result<Unpacked> {
    /// Blobs larger than this may stream straight to disk without caching payload bytes.
    const MAX_STREAM_BLOB_BYTES: usize = 1024 * 1024;

    let algo = sink.hash_algo();
    let mut rd = StreamingPackReader::new(reader, opts.max_input_bytes, algo);
    let mut base_cache = UnpackBaseByteCache::new(unpack_base_cache_byte_limit());

    // Validate magic and version.
    let sig = rd.read_exact_n(4)?;
    if sig != b"PACK" {
        return Err(Error::CorruptObject(
            "not a pack stream: invalid signature".to_owned(),
        ));
    }
    let version = rd.read_u32_be()?;
    if version != 2 && version != 3 {
        return Err(Error::CorruptObject(format!(
            "unsupported pack version {version}"
        )));
    }
    let nr_objects = rd.read_u32_be()? as usize;

    let mut by_offset: HashMap<usize, PackedObjectEntry> = HashMap::new();
    let mut by_oid: HashMap<ObjectId, PackedObjectEntry> = HashMap::new();

    let mut pending: Vec<PendingDelta> = Vec::new();
    let mut count = 0usize;
    let mut object_counts = ObjectKindCounts::default();

    if let Some(p) = progress.as_mut() {
        p.start(ProgressPhase::IndexingObjects, Some(nr_objects as u64));
    }
    for _ in 0..nr_objects {
        poll_unpack_cancel(&mut progress, cancel)?;
        let obj_offset = rd.stream_pos();
        let (type_code, size) = rd.read_type_size()?;

        match type_code {
            1..=4 => {
                let kind = type_code_to_kind(type_code)?;
                if !opts.dry_run
                    && size > MAX_STREAM_BLOB_BYTES
                    && kind == ObjectKind::Blob
                    && sink.supports_streaming_inflate()
                {
                    let oid = sink.write_inflated_from_pack(kind, size, &mut rd, false)?;
                    let entry = PackedObjectEntry::Stored { kind, oid };
                    by_offset.insert(obj_offset, entry.clone());
                    by_oid.insert(oid, entry);
                } else {
                    let data = rd.decompress(size)?;
                    let oid = sink.write(kind, &data, opts.dry_run)?;
                    record_resolved_object(
                        &mut by_offset,
                        &mut by_oid,
                        obj_offset,
                        oid,
                        kind,
                        data,
                        opts,
                        MAX_STREAM_BLOB_BYTES,
                        &mut base_cache,
                    )?;
                }
                object_counts.add(kind);
                count += 1;
            }
            6 => {
                let neg = rd.read_ofs_neg_offset()?;
                let base_offset = obj_offset.checked_sub(neg).ok_or_else(|| {
                    Error::CorruptObject("ofs-delta base offset underflow".to_owned())
                })?;
                let delta_data = rd.decompress(size)?;
                let delta = PendingDelta {
                    offset: obj_offset,
                    base_oid: None,
                    base_offset: Some(base_offset),
                    delta_data,
                };
                test_note_pending_delta_bytes(&pending, base_cache.bytes_used());
                if let Some((oid, kind, data)) = resolve_pending_delta(
                    &delta,
                    &by_offset,
                    &by_oid,
                    sink,
                    opts,
                    MAX_STREAM_BLOB_BYTES,
                    &mut base_cache,
                )? {
                    record_resolved_object(
                        &mut by_offset,
                        &mut by_oid,
                        delta.offset,
                        oid,
                        kind,
                        data,
                        opts,
                        MAX_STREAM_BLOB_BYTES,
                        &mut base_cache,
                    )?;
                    object_counts.add(kind);
                    count += 1;
                } else {
                    pending.push(delta);
                }
            }
            7 => {
                let base_bytes = rd.read_exact_n(algo.len())?;
                let base_oid = ObjectId::from_bytes(&base_bytes)?;
                let delta_data = rd.decompress(size)?;
                let delta = PendingDelta {
                    offset: obj_offset,
                    base_oid: Some(base_oid),
                    base_offset: None,
                    delta_data,
                };
                test_note_pending_delta_bytes(&pending, base_cache.bytes_used());
                if let Some((oid, kind, data)) = resolve_pending_delta(
                    &delta,
                    &by_offset,
                    &by_oid,
                    sink,
                    opts,
                    MAX_STREAM_BLOB_BYTES,
                    &mut base_cache,
                )? {
                    record_resolved_object(
                        &mut by_offset,
                        &mut by_oid,
                        delta.offset,
                        oid,
                        kind,
                        data,
                        opts,
                        MAX_STREAM_BLOB_BYTES,
                        &mut base_cache,
                    )?;
                    object_counts.add(kind);
                    count += 1;
                } else {
                    pending.push(delta);
                }
            }
            other => {
                return Err(Error::CorruptObject(format!(
                    "unknown packed-object type {other}"
                )))
            }
        }
        if let Some(p) = progress.as_mut() {
            p.inc(1);
        }
    }
    if let Some(p) = progress.as_mut() {
        p.finish();
    }

    let digest = rd.finalize_hasher();
    let trailing = rd.read_trailer()?;
    if digest != trailing {
        return Err(Error::CorruptObject(
            "pack trailing checksum mismatch".to_owned(),
        ));
    }
    let pack_bytes = rd.stream_pos() as u64;

    let mut remaining = pending;
    if let (Some(p), false) = (progress.as_mut(), remaining.is_empty()) {
        p.start(ProgressPhase::ResolvingDeltas, Some(remaining.len() as u64));
    }
    let resolving = !remaining.is_empty();
    loop {
        if remaining.is_empty() {
            break;
        }
        poll_unpack_cancel(&mut progress, cancel)?;
        test_note_pending_delta_bytes(&remaining, base_cache.bytes_used());
        let before = remaining.len();
        let mut still_pending: Vec<PendingDelta> = Vec::new();

        for delta in remaining {
            if let Some((oid, kind, data)) = resolve_pending_delta(
                &delta,
                &by_offset,
                &by_oid,
                sink,
                opts,
                MAX_STREAM_BLOB_BYTES,
                &mut base_cache,
            )? {
                record_resolved_object(
                    &mut by_offset,
                    &mut by_oid,
                    delta.offset,
                    oid,
                    kind,
                    data,
                    opts,
                    MAX_STREAM_BLOB_BYTES,
                    &mut base_cache,
                )?;
                object_counts.add(kind);
                count += 1;
                if let Some(p) = progress.as_mut() {
                    p.inc(1);
                }
            } else {
                still_pending.push(delta);
            }
        }

        remaining = still_pending;
        if remaining.len() == before {
            return Err(Error::UnresolvedDeltas {
                count: u64::try_from(remaining.len()).unwrap_or(u64::MAX),
            });
        }
    }
    if let (Some(p), true) = (progress.as_mut(), resolving) {
        p.finish();
    }

    if opts.strict {
        verify_packed_dot_special_entries(&by_oid, sink)?;
        strict_verify_packed_references_map(
            sink,
            &by_oid,
            &opts.allowed_missing,
            opts.allow_promisor_missing_references,
            &opts.shallow_boundaries,
            promised,
        )?;
    }

    Ok(Unpacked {
        count,
        by_oid,
        object_counts,
        pack_bytes,
    })
}

/// Slot in the in-pack index: dry-run payloads or a stored object id.
#[derive(Debug, Clone)]
enum PackedObjectEntry {
    DryRun { kind: ObjectKind, data: Vec<u8> },
    Stored { kind: ObjectKind, oid: ObjectId },
}

fn packed_entry_kind(entry: &PackedObjectEntry) -> ObjectKind {
    match entry {
        PackedObjectEntry::DryRun { kind, .. } | PackedObjectEntry::Stored { kind, .. } => *kind,
    }
}

#[allow(clippy::too_many_arguments)]
fn record_resolved_object(
    by_offset: &mut HashMap<usize, PackedObjectEntry>,
    by_oid: &mut HashMap<ObjectId, PackedObjectEntry>,
    pack_offset: usize,
    oid: ObjectId,
    kind: ObjectKind,
    data: Vec<u8>,
    opts: &UnpackOptions,
    max_stream_blob: usize,
    cache: &mut UnpackBaseByteCache,
) -> Result<()> {
    let entry =
        packed_entry_after_write(kind, data, oid, opts, max_stream_blob, pack_offset, cache);
    by_offset.insert(pack_offset, entry.clone());
    by_oid.insert(oid, entry);
    Ok(())
}

fn resolve_pending_delta<S: UnpackSink + ?Sized>(
    delta: &PendingDelta,
    by_offset: &HashMap<usize, PackedObjectEntry>,
    by_oid: &HashMap<ObjectId, PackedObjectEntry>,
    sink: &S,
    opts: &UnpackOptions,
    max_stream_blob: usize,
    cache: &mut UnpackBaseByteCache,
) -> Result<Option<(ObjectId, ObjectKind, Vec<u8>)>> {
    let Some((base_kind, base_data)) =
        delta_base_bytes(delta, by_offset, by_oid, sink, opts, cache)?
    else {
        return Ok(None);
    };
    let result = apply_delta(base_data.as_ref(), &delta.delta_data)?;
    let oid = sink.write(base_kind, &result, opts.dry_run)?;
    if !opts.dry_run && (base_kind != ObjectKind::Blob || result.len() <= max_stream_blob) {
        cache.put(delta.offset, base_kind, result.clone());
    }
    Ok(Some((oid, base_kind, result)))
}

fn delta_base_bytes<'a, S: UnpackSink + ?Sized>(
    delta: &PendingDelta,
    by_offset: &'a HashMap<usize, PackedObjectEntry>,
    by_oid: &'a HashMap<ObjectId, PackedObjectEntry>,
    sink: &S,
    opts: &UnpackOptions,
    cache: &mut UnpackBaseByteCache,
) -> Result<Option<(ObjectKind, Cow<'a, [u8]>)>> {
    if let Some(base_off) = delta.base_offset {
        if let Some((kind, data)) = cache.get(base_off) {
            return Ok(Some((kind, Cow::Owned(data))));
        }
        let Some(entry) = by_offset.get(&base_off) else {
            return Ok(None);
        };
        return entry_object_bytes(entry, sink, Some(base_off), cache).map(Some);
    }
    if let Some(ref base_id) = delta.base_oid {
        if let Some(entry) = by_oid.get(base_id) {
            return entry_object_bytes(entry, sink, None, cache).map(Some);
        }
        if !opts.dry_run {
            if let Some(obj) = sink.read_stored(base_id)? {
                return Ok(Some((obj.kind, Cow::Owned(obj.data))));
            }
        }
    }
    Ok(None)
}

fn verify_packed_dot_special_entries<S: UnpackSink + ?Sized>(
    by_oid: &HashMap<ObjectId, PackedObjectEntry>,
    sink: &S,
) -> Result<()> {
    let mut tracker = DotFsckTracker::default();
    let mut keys: Vec<ObjectId> = by_oid.keys().copied().collect();
    keys.sort();
    for oid in keys {
        let entry = &by_oid[&oid];
        if matches!(
            entry,
            PackedObjectEntry::Stored {
                kind: ObjectKind::Blob,
                ..
            } | PackedObjectEntry::DryRun {
                kind: ObjectKind::Blob,
                ..
            }
        ) {
            continue;
        }
        let (kind, data) = entry_payload_for_strict(entry, sink, &oid)?;
        if kind == ObjectKind::Tree {
            for di in tracker.on_tree(&oid, &data)? {
                if di.is_error_severity() {
                    return Err(Error::CorruptObject(di.format_line()));
                }
            }
        }
        for di in tracker.on_object(&oid, kind, &data) {
            if di.is_error_severity() {
                return Err(Error::CorruptObject(di.format_line()));
            }
        }
    }
    let pending_gm: Vec<ObjectId> = tracker
        .gitmodules_found
        .difference(&tracker.gitmodules_done)
        .copied()
        .collect();
    let pending_ga: Vec<ObjectId> = tracker
        .gitattributes_found
        .difference(&tracker.gitattributes_done)
        .copied()
        .collect();
    for oid in pending_gm {
        tracker.gitmodules_done.insert(oid);
        let Some(entry) = by_oid.get(&oid) else {
            continue;
        };
        let (kind, data) = dot_special_blob_bytes(entry, sink, &oid)?;
        for di in gitmodules::fsck_dot_special_object(
            &oid,
            kind,
            &data,
            &tracker.gitmodules_found,
            &tracker.gitattributes_found,
        ) {
            if di.is_error_severity() {
                return Err(Error::CorruptObject(di.format_line()));
            }
        }
    }
    for oid in pending_ga {
        if tracker.gitattributes_done.contains(&oid) {
            continue;
        }
        tracker.gitattributes_done.insert(oid);
        let Some(entry) = by_oid.get(&oid) else {
            continue;
        };
        let (kind, data) = dot_special_blob_bytes(entry, sink, &oid)?;
        for di in gitmodules::fsck_dot_special_object(
            &oid,
            kind,
            &data,
            &tracker.gitmodules_found,
            &tracker.gitattributes_found,
        ) {
            if di.is_error_severity() {
                return Err(Error::CorruptObject(di.format_line()));
            }
        }
    }
    Ok(())
}

fn dot_special_blob_bytes<S: UnpackSink + ?Sized>(
    entry: &PackedObjectEntry,
    sink: &S,
    id: &ObjectId,
) -> Result<(ObjectKind, Vec<u8>)> {
    entry_payload_for_strict(entry, sink, id)
}

fn entry_payload_for_strict<S: UnpackSink + ?Sized>(
    entry: &PackedObjectEntry,
    sink: &S,
    id: &ObjectId,
) -> Result<(ObjectKind, Vec<u8>)> {
    match entry {
        PackedObjectEntry::DryRun { kind, data } => Ok((*kind, data.clone())),
        PackedObjectEntry::Stored { kind, oid } => {
            debug_assert_eq!(oid, id);
            let Some(obj) = sink.read_stored(id)? else {
                return Err(Error::CorruptObject(format!("missing object {id}")));
            };
            Ok((*kind, obj.data))
        }
    }
}

fn packed_entry_after_write(
    kind: ObjectKind,
    data: Vec<u8>,
    oid: ObjectId,
    opts: &UnpackOptions,
    max_stream_blob: usize,
    _pack_offset: usize,
    _cache: &mut UnpackBaseByteCache,
) -> PackedObjectEntry {
    let _ = max_stream_blob;
    if opts.dry_run {
        return PackedObjectEntry::DryRun { kind, data };
    }
    PackedObjectEntry::Stored { kind, oid }
}

fn entry_object_bytes<'a, S: UnpackSink + ?Sized>(
    entry: &'a PackedObjectEntry,
    sink: &S,
    pack_offset: Option<usize>,
    cache: &mut UnpackBaseByteCache,
) -> Result<(ObjectKind, Cow<'a, [u8]>)> {
    match entry {
        PackedObjectEntry::DryRun { kind, data } => Ok((*kind, Cow::Borrowed(data.as_slice()))),
        PackedObjectEntry::Stored { kind, oid } => {
            if let Some(off) = pack_offset {
                if let Some((k, data)) = cache.get(off) {
                    return Ok((k, Cow::Owned(data)));
                }
            }
            let obj = sink
                .read_stored(oid)?
                .ok_or_else(|| Error::ObjectNotFound(oid.to_hex()))?;
            if let Some(off) = pack_offset {
                cache.put(off, *kind, obj.data.clone());
            }
            Ok((*kind, Cow::Owned(obj.data)))
        }
    }
}

fn strict_verify_packed_references_map<S: UnpackSink + ?Sized>(
    sink: &S,
    pack: &HashMap<ObjectId, PackedObjectEntry>,
    allowed_missing: &HashSet<ObjectId>,
    allow_promisor_missing_references: bool,
    shallow_boundaries: &HashSet<ObjectId>,
    promised: Option<PromisedReference<'_>>,
) -> Result<()> {
    let resolves = |target: &ObjectId, referrer: &ObjectId| -> Result<bool> {
        if strict_ref_resolves_map(
            target,
            pack,
            sink,
            allowed_missing,
            allow_promisor_missing_references,
        )? {
            return Ok(true);
        }
        promised.map_or(Ok(false), |promised| promised(target, referrer))
    };
    for (oid, entry) in pack {
        let kind = packed_entry_kind(entry);
        if kind == ObjectKind::Blob {
            continue;
        }
        let (_, data) = entry_payload_for_strict(entry, sink, oid)?;
        match kind {
            ObjectKind::Tree => {
                for e in parse_tree(&data)? {
                    // Gitlink (submodule) entries point at commits that live
                    // in the submodule repository, not the superproject's
                    // pack/ODB. Skip them in the connectivity walk, matching
                    // upstream git (git/fsck.c:374 `if (S_ISGITLINK) continue;`).
                    if e.mode == MODE_GITLINK {
                        continue;
                    }
                    if !resolves(&e.oid, oid)? {
                        return Err(Error::CorruptObject(format!(
                            "strict: missing object {} referenced by tree",
                            e.oid.to_hex()
                        )));
                    }
                }
            }
            ObjectKind::Commit => {
                let c = parse_commit(&data)?;
                if !resolves(&c.tree, oid)? {
                    return Err(Error::CorruptObject(format!(
                        "strict: missing tree {} referenced by commit",
                        c.tree.to_hex()
                    )));
                }
                // A commit recorded as a shallow boundary (graft) has its parents intentionally
                // absent — skip parent connectivity for it, matching unpack-objects run with a
                // `--shallow-file` listing this commit.
                if shallow_boundaries.contains(oid) {
                    continue;
                }
                for p in &c.parents {
                    if !resolves(p, oid)? {
                        return Err(Error::CorruptObject(format!(
                            "strict: missing parent {} referenced by commit",
                            p.to_hex()
                        )));
                    }
                }
            }
            ObjectKind::Tag => {
                let t = parse_tag(&data)?;
                if !resolves(&t.object, oid)? {
                    return Err(Error::CorruptObject(format!(
                        "strict: missing object {} referenced by tag",
                        t.object.to_hex()
                    )));
                }
            }
            ObjectKind::Blob => {}
        }
    }
    Ok(())
}

fn strict_ref_resolves_map<S: UnpackSink + ?Sized>(
    oid: &ObjectId,
    pack: &HashMap<ObjectId, PackedObjectEntry>,
    sink: &S,
    allowed_missing: &HashSet<ObjectId>,
    allow_promisor_missing_references: bool,
) -> Result<bool> {
    if pack.contains_key(oid) || allowed_missing.contains(oid) || allow_promisor_missing_references
    {
        return Ok(true);
    }
    sink.contains(oid)
}

fn strict_ref_resolves(
    oid: &ObjectId,
    pack: &std::collections::HashMap<ObjectId, (ObjectKind, Vec<u8>)>,
    odb: Option<&Odb>,
) -> Result<bool> {
    if pack.contains_key(oid) {
        return Ok(true);
    }
    odb.map_or(Ok(false), |o| o.contains(oid))
}

/// Verifies that references from commits, trees, and tags resolve to objects present in `pack`
/// or, when `odb` is [`Some`], to loose objects in that database.
///
/// Use [`None`] for `odb` when indexing or unpacking in a context with no repository (Git allows
/// `index-pack --strict` outside a work tree when the pack is self-contained).
///
/// # Errors
///
/// [`Error::CorruptObject`] for a reference that resolves nowhere; [`Odb::contains`]
/// failures propagate unchanged.
pub fn strict_verify_packed_references(
    odb: Option<&Odb>,
    pack: &HashMap<ObjectId, (ObjectKind, Vec<u8>)>,
) -> Result<()> {
    for (kind, data) in pack.values() {
        match kind {
            ObjectKind::Tree => {
                for e in parse_tree(data)? {
                    // Gitlink (submodule) entries point at commits that live in
                    // the submodule repository, not this pack/ODB. Skip them in
                    // the connectivity walk, matching upstream git
                    // (git/fsck.c:374 `if (S_ISGITLINK) continue;`).
                    if e.mode == MODE_GITLINK {
                        continue;
                    }
                    if !strict_ref_resolves(&e.oid, pack, odb)? {
                        return Err(Error::CorruptObject(format!(
                            "strict: missing object {} referenced by tree",
                            e.oid.to_hex()
                        )));
                    }
                }
            }
            ObjectKind::Commit => {
                let c = parse_commit(data)?;
                if !strict_ref_resolves(&c.tree, pack, odb)? {
                    return Err(Error::CorruptObject(format!(
                        "strict: missing tree {} referenced by commit",
                        c.tree.to_hex()
                    )));
                }
                for p in &c.parents {
                    if !strict_ref_resolves(p, pack, odb)? {
                        return Err(Error::CorruptObject(format!(
                            "strict: missing parent {} referenced by commit",
                            p.to_hex()
                        )));
                    }
                }
            }
            ObjectKind::Tag => {
                let t = parse_tag(data)?;
                if !strict_ref_resolves(&t.object, pack, odb)? {
                    return Err(Error::CorruptObject(format!(
                        "strict: missing object {} referenced by tag",
                        t.object.to_hex()
                    )));
                }
            }
            ObjectKind::Blob => {}
        }
    }
    Ok(())
}

/// Whether `data` is a *thin* pack — i.e. it contains a `ref-delta` (type 7) whose base object is
/// not itself present in the pack. `git pack-objects --thin` produces such packs; a receiver that
/// rejects thin packs (`receive-pack --reject-thin-pack-for-testing`) uses this to refuse them.
///
/// Conservative: any parse error makes this return `false` (treat as non-thin) so a malformed pack
/// is handled by the normal ingestion path rather than mislabeled.
pub fn pack_is_thin(data: &[u8], algo: HashAlgo) -> bool {
    pack_is_thin_inner(data, algo).unwrap_or(false)
}

fn pack_is_thin_inner(data: &[u8], algo: HashAlgo) -> Result<bool> {
    use crate::pack_zlib::skip_zlib_at;

    let mut rd = PackReader::new(data);
    if rd.read_exact(4)? != b"PACK" {
        return Ok(false);
    }
    let _version = rd.read_u32_be()?;
    let nr_objects = rd.read_u32_be()? as usize;

    let mut ref_delta_bases: Vec<ObjectId> = Vec::new();
    let mut saw_ref_delta = false;

    for _ in 0..nr_objects {
        let obj_offset = rd.pos;
        let (type_code, size) = rd.read_type_size()?;
        match type_code {
            1..=4 => {
                let zlib_offset = rd.pos;
                let consumed = skip_zlib_at(data, zlib_offset, size)?;
                rd.pos = zlib_offset + consumed;
                let _ = obj_offset;
            }
            6 => {
                let neg = rd.read_ofs_neg_offset()?;
                let _base_offset = obj_offset.checked_sub(neg).ok_or_else(|| {
                    Error::CorruptObject("ofs-delta base offset underflow".to_owned())
                })?;
                let zlib_offset = rd.pos;
                let consumed = skip_zlib_at(data, zlib_offset, size)?;
                rd.pos = zlib_offset + consumed;
            }
            7 => {
                saw_ref_delta = true;
                let base_bytes = rd.read_exact(algo.len())?;
                let base_oid = ObjectId::from_bytes(base_bytes)?;
                ref_delta_bases.push(base_oid);
                let zlib_offset = rd.pos;
                let consumed = skip_zlib_at(data, zlib_offset, size)?;
                rd.pos = zlib_offset + consumed;
            }
            _ => return Ok(false),
        }
    }

    if !saw_ref_delta {
        return Ok(false);
    }

    pack_is_thin_with_ofs_deltas(data, algo, ref_delta_bases)
}

/// Resolve delta chains only far enough to test ref-delta bases (no full pack materialization).
fn pack_is_thin_with_ofs_deltas(
    data: &[u8],
    algo: HashAlgo,
    ref_delta_bases: Vec<ObjectId>,
) -> Result<bool> {
    let tmp = tempfile::tempdir().map_err(Error::Io)?;
    let git_dir = tmp.path().join(".git");
    std::fs::create_dir_all(git_dir.join("objects")).map_err(Error::Io)?;
    if algo == HashAlgo::Sha256 {
        std::fs::write(
            git_dir.join("config"),
            "[extensions]\n\tobjectformat = sha256\n",
        )
        .map_err(Error::Io)?;
    }
    let odb = Odb::new(&git_dir.join("objects")).with_config_git_dir(git_dir);
    match crate::pack_index_build::build_pack_index_records(
        data,
        &odb,
        crate::hash::Parallelism::resolve(Some(1)),
        false,
    ) {
        Ok(records) => {
            let in_pack: HashSet<ObjectId> = records.into_iter().map(|r| r.oid).collect();
            Ok(ref_delta_bases.iter().any(|b| !in_pack.contains(b)))
        }
        Err(Error::UnresolvedDeltas { .. }) => Ok(true),
        Err(e) => Err(e),
    }
}

/// Parse a pack byte stream and return every resolved object (after delta resolution) keyed by OID.
///
/// Does not write to any object database. Used for receive-pack connectivity checks before
/// applying a push to the permanent ODB.
///
/// Thin-pack bases may be resolved from `odb` when they are not present in the pack.
///
/// # Errors
///
/// Returns [`Error::UnresolvedDeltas`] when some delta bases are in neither the pack
/// nor `odb`, and [`Error::CorruptObject`] or [`Error::Zlib`] for a malformed pack.
pub fn pack_bytes_to_object_map(data: &[u8], odb: &Odb) -> Result<HashMap<ObjectId, Object>> {
    build_pack_object_map(PackReader::new(data), odb)
}

/// Resolved `(oid, pack_offset)` pairs for every object slot in a pack byte stream.
///
/// Offsets point at packed object headers (including delta slots). Thin-pack bases
/// may be resolved from `odb` when they are not present in the pack.
/// One row for a v2 pack index: resolved OID, header offset, CRC32 of raw pack bytes.
#[derive(Debug, Clone)]
pub struct PackIndexRecord {
    /// Object id (after delta resolution).
    pub oid: ObjectId,
    /// Byte offset of the object header in the pack.
    pub offset: u64,
    /// CRC32 of the packed object slice (header + zlib payload).
    pub crc32: u32,
    /// Object type after delta resolution.
    pub kind: ObjectKind,
}

/// Build index records using the repository's default parallelism (`pack.threads` when unset).
///
/// Does not copy the pack buffer or retain inflated objects beyond the delta chain each
/// worker is walking; see [`crate::objects::index_pack::build_pack_index_records`].
///
/// # Errors
///
/// Returns [`Error::CorruptObject`] for a malformed pack and [`Error::UnresolvedDeltas`]
/// for deltas whose bases are in neither the pack nor `odb`.
pub fn pack_index_records_from_bytes(data: &[u8], odb: &Odb) -> Result<Vec<PackIndexRecord>> {
    pack_index_records_with_threads(
        data,
        odb,
        crate::config::ConfigSet::pack_index_parallelism_for_git_dir(
            &crate::environment::Environment::empty(),
            odb.config_git_dir(),
        ),
    )
}

/// Build index records with an explicit worker thread count (`None`/`0` → available CPUs).
pub fn pack_index_records_with_threads(
    data: &[u8],
    odb: &Odb,
    parallelism: crate::hash::Parallelism,
) -> Result<Vec<PackIndexRecord>> {
    crate::pack_index_build::build_pack_index_records(data, odb, parallelism, true)
}

/// Like [`pack_index_records_with_threads`], reporting indexing and delta resolution
/// progress on `progress` when provided.
pub fn pack_index_records_with_threads_with_progress(
    data: &[u8],
    odb: &Odb,
    parallelism: crate::hash::Parallelism,
    progress: Option<&mut dyn crate::progress::ProgressSink>,
) -> Result<Vec<PackIndexRecord>> {
    crate::pack_index_build::build_pack_index_records_with_progress(
        data,
        odb,
        parallelism,
        true,
        progress,
        None,
    )
}

fn build_pack_object_map(mut rd: PackReader<'_>, odb: &Odb) -> Result<HashMap<ObjectId, Object>> {
    let algo = odb.hash_algo();
    let sig = rd.read_exact(4)?;
    if sig != b"PACK" {
        return Err(Error::CorruptObject(
            "not a pack stream: invalid signature".to_owned(),
        ));
    }
    let version = rd.read_u32_be()?;
    if version != 2 && version != 3 {
        return Err(Error::CorruptObject(format!(
            "unsupported pack version {version}"
        )));
    }
    let nr_objects = rd.read_u32_be()? as usize;

    let mut by_offset: HashMap<usize, (ObjectKind, Vec<u8>)> = HashMap::new();
    let mut by_oid: HashMap<ObjectId, (ObjectKind, Vec<u8>)> = HashMap::new();
    let mut pending: Vec<PendingDelta> = Vec::new();

    fn base_from_pack_or_odb(
        by_oid: &HashMap<ObjectId, (ObjectKind, Vec<u8>)>,
        odb: &Odb,
        id: &ObjectId,
    ) -> Option<(ObjectKind, Vec<u8>)> {
        if let Some(e) = by_oid.get(id) {
            return Some(e.clone());
        }
        odb.read(id).ok().map(|o| (o.kind, o.data))
    }

    for _ in 0..nr_objects {
        let obj_offset = rd.pos;
        let (type_code, size) = rd.read_type_size()?;

        match type_code {
            1..=4 => {
                let kind = type_code_to_kind(type_code)?;
                let data = rd.decompress(size)?;
                let oid = odb.hash(kind, &data)?;
                by_offset.insert(obj_offset, (kind, data.clone()));
                by_oid.insert(oid, (kind, data));
            }
            6 => {
                let neg = rd.read_ofs_neg_offset()?;
                let base_offset = obj_offset.checked_sub(neg).ok_or_else(|| {
                    Error::CorruptObject("ofs-delta base offset underflow".to_owned())
                })?;
                let delta_data = rd.decompress(size)?;
                pending.push(PendingDelta {
                    offset: obj_offset,
                    base_oid: None,
                    base_offset: Some(base_offset),
                    delta_data,
                });
            }
            7 => {
                let base_bytes = rd.read_exact(algo.len())?;
                let base_oid = ObjectId::from_bytes(base_bytes)?;
                let delta_data = rd.decompress(size)?;
                pending.push(PendingDelta {
                    offset: obj_offset,
                    base_oid: Some(base_oid),
                    base_offset: None,
                    delta_data,
                });
            }
            other => {
                return Err(Error::CorruptObject(format!(
                    "unknown packed-object type {other}"
                )))
            }
        }
    }

    let consumed = rd.pos;
    {
        let mut hasher = ChecksumHasher::new(algo);
        hasher.update(&rd.data[..consumed]);
        let digest = hasher.finalize();
        let trailing = rd.read_exact(algo.len())?;
        if digest.as_slice() != trailing {
            return Err(Error::CorruptObject(
                "pack trailing checksum mismatch".to_owned(),
            ));
        }
    }

    let mut remaining = pending;
    loop {
        if remaining.is_empty() {
            break;
        }
        let before = remaining.len();
        let mut still_pending: Vec<PendingDelta> = Vec::new();

        for delta in remaining {
            let base = if let Some(base_off) = delta.base_offset {
                by_offset.get(&base_off).cloned()
            } else if let Some(ref base_id) = delta.base_oid {
                base_from_pack_or_odb(&by_oid, odb, base_id)
            } else {
                None
            };

            if let Some((base_kind, base_data)) = base {
                let result = apply_delta(&base_data, &delta.delta_data)?;
                let oid = odb.hash(base_kind, &result)?;
                by_offset.insert(delta.offset, (base_kind, result.clone()));
                by_oid.insert(oid, (base_kind, result));
            } else {
                still_pending.push(delta);
            }
        }

        remaining = still_pending;
        if remaining.len() == before {
            return Err(Error::UnresolvedDeltas {
                count: u64::try_from(remaining.len()).unwrap_or(u64::MAX),
            });
        }
    }

    Ok(by_oid
        .into_iter()
        .map(|(oid, (kind, data))| (oid, Object::new(kind, data)))
        .collect())
}

/// Convert a pack object type code to an [`ObjectKind`].
pub(crate) fn type_code_to_kind(code: u8) -> Result<ObjectKind> {
    match code {
        1 => Ok(ObjectKind::Commit),
        2 => Ok(ObjectKind::Tree),
        3 => Ok(ObjectKind::Blob),
        4 => Ok(ObjectKind::Tag),
        _ => Err(Error::CorruptObject(format!(
            "type code {code} is not a regular object type"
        ))),
    }
}

/// Low-level cursor over a buffered pack byte stream (in-memory pack parsing).
pub(crate) struct PackReader<'a> {
    data: &'a [u8],
    pub(crate) pos: usize,
}

impl<'a> PackReader<'a> {
    pub(crate) fn new(data: &'a [u8]) -> Self {
        Self { data, pos: 0 }
    }

    /// Read exactly `n` bytes and advance the cursor, returning a slice into
    /// the internal buffer.
    pub(crate) fn read_exact(&mut self, n: usize) -> Result<&[u8]> {
        if self.pos + n > self.data.len() {
            return Err(Error::CorruptObject(format!(
                "pack stream truncated: need {n} bytes at offset {}",
                self.pos
            )));
        }
        let slice = &self.data[self.pos..self.pos + n];
        self.pos += n;
        Ok(slice)
    }

    /// Read a single byte and advance the cursor.
    pub(crate) fn read_byte(&mut self) -> Result<u8> {
        if self.pos >= self.data.len() {
            return Err(Error::CorruptObject(
                "unexpected end of pack stream".to_owned(),
            ));
        }
        let b = self.data[self.pos];
        self.pos += 1;
        Ok(b)
    }

    /// Read a big-endian `u32`.
    pub(crate) fn read_u32_be(&mut self) -> Result<u32> {
        let bytes = self.read_exact(4)?;
        Ok(u32::from_be_bytes(bytes.try_into().map_err(|_| {
            Error::CorruptObject("u32 read failed".to_owned())
        })?))
    }

    /// Read the packed-object type + size header (variable-length big-endian
    /// encoding with the type in bits 4-6 of the first byte).
    ///
    /// Returns `(type_code, uncompressed_size)`.
    pub(crate) fn read_type_size(&mut self) -> Result<(u8, usize)> {
        let c = self.read_byte()?;
        let type_code = (c >> 4) & 0x7;
        let mut size = (c & 0x0f) as usize;
        let mut shift = 4u32;
        let mut cur = c;
        while cur & 0x80 != 0 {
            cur = self.read_byte()?;
            size |= ((cur & 0x7f) as usize) << shift;
            shift += 7;
        }
        Ok((type_code, size))
    }

    /// Read an `OFS_DELTA` negative-offset value.
    ///
    /// The encoding uses a big-endian variable-length integer with a +1 bias
    /// on each continuation byte, yielding values ≥ 1.
    pub(crate) fn read_ofs_neg_offset(&mut self) -> Result<usize> {
        let mut c = self.read_byte()?;
        let mut value = (c & 0x7f) as usize;
        while c & 0x80 != 0 {
            c = self.read_byte()?;
            value = (value + 1) << 7 | (c & 0x7f) as usize;
        }
        Ok(value)
    }

    /// Decompress zlib-compressed data starting at the current cursor position.
    ///
    /// Advances the cursor by exactly the number of compressed bytes consumed.
    /// Returns an error if the decompressed length differs from `expected_size`.
    pub(crate) fn decompress(&mut self, expected_size: usize) -> Result<Vec<u8>> {
        let (out, consumed) = decompress_zlib_at(self.data, self.pos, expected_size)?;
        self.pos += consumed;
        Ok(out)
    }
}

/// Decompress a zlib stream at `offset` in `pack` into `out` (cleared first; capacity reused).
pub(crate) fn decompress_zlib_at_into(
    pack: &[u8],
    offset: usize,
    expected_size: usize,
    out: &mut Vec<u8>,
) -> Result<usize> {
    out.clear();
    out.reserve(expected_size);
    let slice = pack.get(offset..).ok_or_else(|| {
        Error::CorruptObject(format!(
            "pack stream truncated: need zlib at offset {offset}"
        ))
    })?;
    let mut decoder = ZlibDecoder::new(slice);
    decoder
        .read_to_end(out)
        .map_err(|e| Error::Zlib(e.to_string()))?;
    if out.len() != expected_size {
        return Err(Error::CorruptObject(format!(
            "decompressed {} bytes but expected {}",
            out.len(),
            expected_size
        )));
    }
    Ok(decoder.total_in() as usize)
}

/// Decompress a zlib stream at `offset` in `pack` without advancing a [`PackReader`].
pub(crate) fn decompress_zlib_at(
    pack: &[u8],
    offset: usize,
    expected_size: usize,
) -> Result<(Vec<u8>, usize)> {
    let mut out = Vec::new();
    let consumed = decompress_zlib_at_into(pack, offset, expected_size, &mut out)?;
    Ok((out, consumed))
}

fn io_to_corrupt_eof(e: io::Error, stream_pos: usize, context: &str) -> Error {
    if e.kind() == io::ErrorKind::UnexpectedEof {
        Error::CorruptObject(format!(
            "pack stream truncated ({context}) at offset {stream_pos}"
        ))
    } else {
        Error::Io(e)
    }
}

/// Streaming cursor over a pack file: hashes body bytes incrementally (no full-buffer read).
///
/// Raw pack bytes are either consumed as object headers (via [`Self::read_byte`]) or as zlib
/// payloads.  Zlib decoders may read ahead; overflow bytes stay in [`Self::pending`] so the next
/// object header or zlib stream starts at the correct offset.
struct StreamingPackReader<'a> {
    inner: &'a mut dyn Read,
    pack_hasher: ChecksumHasher,
    stream_pos: usize,
    max_input_bytes: Option<u64>,
    /// Compressed (or other) bytes already read from `inner` and hashed but not yet consumed by
    /// the current parsing step.
    pending: Vec<u8>,
}

impl<'a> StreamingPackReader<'a> {
    fn new(inner: &'a mut dyn Read, max_input_bytes: Option<u64>, algo: HashAlgo) -> Self {
        Self {
            inner,
            pack_hasher: ChecksumHasher::new(algo),
            stream_pos: 0,
            max_input_bytes,
            pending: Vec::new(),
        }
    }

    fn stream_pos(&self) -> usize {
        self.stream_pos
    }

    fn enforce_max_input(&self) -> Result<()> {
        if let Some(limit) = self.max_input_bytes {
            let pos = u64::try_from(self.stream_pos)
                .map_err(|_| Error::CorruptObject("pack stream position overflow".to_owned()))?;
            if pos > limit {
                return Err(Error::CorruptObject(
                    "pack exceeds maximum allowed size".to_owned(),
                ));
            }
        }
        Ok(())
    }

    /// Read pack-body bytes (hashed). Used for headers and non-zlib payload reads only.
    fn read_from_source(&mut self, buf: &mut [u8]) -> Result<usize> {
        let n = if !self.pending.is_empty() {
            let take = buf.len().min(self.pending.len());
            buf[..take].copy_from_slice(&self.pending[..take]);
            self.pending.drain(..take);
            take
        } else {
            self.inner.read(buf).map_err(Error::Io)?
        };
        if n > 0 {
            self.pack_hasher.update(&buf[..n]);
            self.stream_pos += n;
            self.enforce_max_input()?;
        }
        Ok(n)
    }

    fn read_byte(&mut self) -> Result<u8> {
        let mut b = [0u8; 1];
        let n = self.read_from_source(&mut b)?;
        if n == 0 {
            return Err(Error::CorruptObject(format!(
                "pack stream truncated (read byte) at offset {}",
                self.stream_pos
            )));
        }
        Ok(b[0])
    }

    fn read_exact_n(&mut self, n: usize) -> Result<Vec<u8>> {
        let mut v = vec![0u8; n];
        let mut got = 0usize;
        while got < n {
            let m = self.read_from_source(&mut v[got..n])?;
            if m == 0 {
                return Err(Error::CorruptObject(format!(
                    "pack stream truncated (read exact) at offset {}",
                    self.stream_pos
                )));
            }
            got += m;
        }
        Ok(v)
    }

    fn read_u32_be(&mut self) -> Result<u32> {
        let mut b = [0u8; 4];
        let mut got = 0usize;
        while got < 4 {
            let m = self.read_from_source(&mut b[got..4])?;
            if m == 0 {
                return Err(Error::CorruptObject(format!(
                    "pack stream truncated (read u32) at offset {}",
                    self.stream_pos
                )));
            }
            got += m;
        }
        Ok(u32::from_be_bytes(b))
    }

    fn read_type_size(&mut self) -> Result<(u8, usize)> {
        let c = self.read_byte()?;
        let type_code = (c >> 4) & 0x7;
        let mut size = (c & 0x0f) as usize;
        let mut shift = 4u32;
        let mut cur = c;
        while cur & 0x80 != 0 {
            cur = self.read_byte()?;
            size |= ((cur & 0x7f) as usize) << shift;
            shift += 7;
        }
        Ok((type_code, size))
    }

    fn read_ofs_neg_offset(&mut self) -> Result<usize> {
        let mut c = self.read_byte()?;
        let mut value = (c & 0x7f) as usize;
        while c & 0x80 != 0 {
            c = self.read_byte()?;
            value = (value + 1) << 7 | (c & 0x7f) as usize;
        }
        Ok(value)
    }

    /// Pull zlib-compressed bytes until one object inflates to `expected_size` bytes.
    ///
    /// Bytes read from `inner` into `pending` are not hashed until we know how many belong to the
    /// zlib stream (`total_in()`). Lookahead past the zlib end (including the 20-byte pack
    /// trailer) must never be fed to the pack checksum.
    ///
    /// When the pack arrives in small chunks (e.g. side-band-64k from `upload-pack`), `flate2` may
    /// return an error before the full deflate stream is in `pending`. Retry after reading more
    /// from `inner` (same idea as [`PackReader::decompress`], which sees the whole zlib at once).
    /// Inflate one pack object to `expected_size` bytes, invoking `chunk_sink` for each step.
    pub(crate) fn decompress_into(
        &mut self,
        expected_size: usize,
        mut chunk_sink: impl FnMut(&[u8]) -> Result<()>,
    ) -> Result<()> {
        if expected_size == 0 {
            const CHUNK: usize = 64 * 1024;
            let mut scratch = [0u8; CHUNK];
            loop {
                let mut cursor = std::io::Cursor::new(self.pending.as_slice());
                let mut z = ZlibDecoder::new(&mut cursor);
                let mut sink = [0u8; 1];
                match z.read(&mut sink) {
                    Ok(0) => {
                        let consumed = z.total_in() as usize;
                        if consumed > self.pending.len() {
                            return Err(Error::CorruptObject(
                                "zlib total_in exceeds pending buffer".to_owned(),
                            ));
                        }
                        if consumed == 0 {
                            let n = self.inner.read(&mut scratch).map_err(Error::Io)?;
                            if n == 0 {
                                return Err(Error::CorruptObject(format!(
                                    "pack stream truncated (zlib) at offset {}",
                                    self.stream_pos
                                )));
                            }
                            self.pending.extend_from_slice(&scratch[..n]);
                            continue;
                        }
                        self.pack_hasher.update(&self.pending[..consumed]);
                        self.stream_pos += consumed;
                        self.pending.drain(..consumed);
                        self.enforce_max_input()?;
                        return Ok(());
                    }
                    Ok(_) => {
                        return Err(Error::CorruptObject(
                            "0-byte packed object inflated to non-empty output".to_owned(),
                        ));
                    }
                    Err(e) if e.kind() == io::ErrorKind::UnexpectedEof => {
                        let n = self.inner.read(&mut scratch).map_err(Error::Io)?;
                        if n == 0 {
                            return Err(Error::CorruptObject(format!(
                                "pack stream truncated (zlib) at offset {}",
                                self.stream_pos
                            )));
                        }
                        self.pending.extend_from_slice(&scratch[..n]);
                    }
                    Err(e) => return Err(Error::Zlib(e.to_string())),
                }
            }
        }

        const CHUNK: usize = 64 * 1024;
        let mut scratch = [0u8; CHUNK];
        let mut scratch_out = [0u8; CHUNK];

        let mut z = Decompress::new(true);
        let mut produced = 0usize;
        let mut eof = false;
        loop {
            if self.pending.is_empty() && !eof {
                let n = self.inner.read(&mut scratch).map_err(Error::Io)?;
                if n == 0 {
                    eof = true;
                } else {
                    self.pending.extend_from_slice(&scratch[..n]);
                }
            }

            let flush = if eof && self.pending.is_empty() {
                FlushDecompress::Finish
            } else {
                FlushDecompress::None
            };

            let before_in = z.total_in();
            let before_out = z.total_out();
            let status = z
                .decompress(self.pending.as_slice(), &mut scratch_out, flush)
                .map_err(|e| Error::Zlib(e.to_string()))?;
            let consumed = (z.total_in() - before_in) as usize;
            if consumed > self.pending.len() {
                return Err(Error::CorruptObject(
                    "zlib consumed more than pending buffer".to_owned(),
                ));
            }
            self.pack_hasher.update(&self.pending[..consumed]);
            self.stream_pos += consumed;
            self.pending.drain(..consumed);
            self.enforce_max_input()?;
            let chunk_len = (z.total_out() - before_out) as usize;
            if chunk_len > 0 {
                let chunk = &scratch_out[..chunk_len];
                let take = chunk.len().min(expected_size - produced);
                chunk_sink(&chunk[..take])?;
                produced += take;
                if take < chunk.len() {
                    return Err(Error::CorruptObject(format!(
                        "decompressed size mismatch: got {}, want {expected_size}",
                        produced + (chunk.len() - take)
                    )));
                }
            }

            match status {
                Status::StreamEnd => {
                    if produced != expected_size {
                        return Err(Error::CorruptObject(format!(
                            "decompressed size mismatch: got {produced}, want {expected_size}"
                        )));
                    }
                    return Ok(());
                }
                Status::Ok | Status::BufError => {
                    if consumed == 0 && !eof {
                        let n = self.inner.read(&mut scratch).map_err(Error::Io)?;
                        if n == 0 {
                            eof = true;
                        } else {
                            self.pending.extend_from_slice(&scratch[..n]);
                        }
                    } else if eof && self.pending.is_empty() && consumed == 0 {
                        return Err(Error::CorruptObject(format!(
                            "pack stream truncated (zlib) at offset {}",
                            self.stream_pos
                        )));
                    }
                }
            }
        }
    }

    fn decompress(&mut self, expected_size: usize) -> Result<Vec<u8>> {
        let mut out = vec![0u8; expected_size];
        let mut pos = 0usize;
        self.decompress_into(expected_size, |chunk| {
            out[pos..pos + chunk.len()].copy_from_slice(chunk);
            pos += chunk.len();
            Ok(())
        })?;
        Ok(out)
    }

    /// Hash over all pack bytes read so far (objects only; trailer not yet read).
    fn finalize_hasher(&self) -> Vec<u8> {
        self.pack_hasher.clone().finalize()
    }

    /// Trailing pack checksum (hash-width bytes); not included in [`Self::finalize_hasher`].
    fn read_trailer(&mut self) -> Result<Vec<u8>> {
        let hash_len = self.pack_hasher.algo().len();
        let mut b = vec![0u8; hash_len];
        if self.pending.len() >= hash_len {
            b.copy_from_slice(&self.pending[..hash_len]);
            self.pending.drain(..hash_len);
            self.stream_pos += hash_len;
            self.enforce_max_input()?;
            return Ok(b);
        }
        let tail = self.pending.len();
        if tail > 0 {
            b[..tail].copy_from_slice(&self.pending[..]);
            self.pending.clear();
        }
        self.inner
            .read_exact(&mut b[tail..])
            .map_err(|e| io_to_corrupt_eof(e, self.stream_pos, "trailer"))?;
        self.stream_pos += hash_len;
        self.enforce_max_input()?;
        Ok(b)
    }
}

/// Apply a git "patch delta" to `base`, producing the patched result.
///
/// The delta binary format is:
/// 1. Source size: variable-length little-endian integer (must equal
///    `base.len()`).
/// 2. Destination size: variable-length little-endian integer.
/// 3. A sequence of COPY (MSB set) and INSERT (MSB clear) instructions.
///
/// # Errors
///
/// Returns [`Error::CorruptObject`] if the delta is malformed, the source-size
/// field does not match `base.len()`, or the result length does not match the
/// declared destination size.
pub fn apply_delta(base: &[u8], delta: &[u8]) -> Result<Vec<u8>> {
    let mut result = Vec::new();
    apply_delta_into(&mut result, base, delta)?;
    Ok(result)
}

/// Apply `delta` onto `base`, reusing `out` storage (cleared first, capacity grown as needed).
///
/// Same semantics as [`apply_delta`]; prefer this when resolving multi-hop chains.
pub fn apply_delta_into(out: &mut Vec<u8>, base: &[u8], delta: &[u8]) -> Result<()> {
    let mut pos = 0usize;

    let src_size = read_delta_varint(delta, &mut pos)?;
    if src_size != base.len() {
        return Err(Error::CorruptObject(format!(
            "delta source size {src_size} != base size {}",
            base.len()
        )));
    }
    let dest_size = read_delta_varint(delta, &mut pos)?;
    out.clear();
    if out.capacity() < dest_size {
        out.reserve(dest_size - out.capacity());
    }

    while pos < delta.len() {
        let cmd = delta[pos];
        pos += 1;
        if cmd == 0 {
            return Err(Error::CorruptObject(
                "reserved opcode 0 in delta stream".to_owned(),
            ));
        }
        if cmd & 0x80 != 0 {
            // COPY instruction: up to 4 offset bytes (bits 0-3) and up to 3
            // size bytes (bits 4-6) are present, each controlled by a flag bit.
            let mut offset = 0usize;
            let mut size = 0usize;

            macro_rules! maybe_read_byte {
                ($flag:expr, $shift:expr, $target:expr) => {
                    if cmd & $flag != 0 {
                        let b = *delta.get(pos).ok_or_else(|| {
                            Error::CorruptObject("truncated delta COPY operand".to_owned())
                        })?;
                        pos += 1;
                        $target |= (b as usize) << $shift;
                    }
                };
            }

            maybe_read_byte!(0x01, 0, offset);
            maybe_read_byte!(0x02, 8, offset);
            maybe_read_byte!(0x04, 16, offset);
            maybe_read_byte!(0x08, 24, offset);
            maybe_read_byte!(0x10, 0, size);
            maybe_read_byte!(0x20, 8, size);
            maybe_read_byte!(0x40, 16, size);

            if size == 0 {
                size = 0x10000;
            }

            let end = offset.checked_add(size).ok_or_else(|| {
                Error::CorruptObject("delta COPY range overflows usize".to_owned())
            })?;
            let chunk = base.get(offset..end).ok_or_else(|| {
                Error::CorruptObject(format!(
                    "delta COPY [{offset},{end}) out of range (base is {} bytes)",
                    base.len()
                ))
            })?;
            out.extend_from_slice(chunk);
        } else {
            // INSERT instruction: copy the next `cmd` literal bytes verbatim.
            let n = cmd as usize;
            let chunk = delta
                .get(pos..pos + n)
                .ok_or_else(|| Error::CorruptObject("truncated delta INSERT data".to_owned()))?;
            out.extend_from_slice(chunk);
            pos += n;
        }
    }

    if out.len() != dest_size {
        return Err(Error::CorruptObject(format!(
            "delta produced {} bytes but expected {dest_size}",
            out.len()
        )));
    }

    Ok(())
}

/// Apply `delta` onto `base`, materializing at most `max_out` bytes in `out`.
///
/// The full delta stream is validated against `dest_size`; COPY/INSERT bytes beyond what is
/// needed for `out` are not copied once `out` already holds `max_out` bytes and
/// `prefix_sufficient` returns true.
///
/// # Errors
///
/// Same as [`apply_delta_into`].
pub(crate) fn apply_delta_prefix_into(
    out: &mut Vec<u8>,
    base: &[u8],
    delta: &[u8],
    max_out: usize,
    prefix_sufficient: impl Fn(&[u8]) -> bool,
) -> Result<()> {
    let mut pos = 0usize;

    let src_size = read_delta_varint(delta, &mut pos)?;
    if src_size != base.len() {
        return Err(Error::CorruptObject(format!(
            "delta source size {src_size} != base size {}",
            base.len()
        )));
    }
    let dest_size = read_delta_varint(delta, &mut pos)?;
    out.clear();
    if out.capacity() < max_out.min(dest_size) {
        out.reserve(max_out.min(dest_size) - out.capacity());
    }

    let mut result_len = 0usize;
    let mut done = false;

    while pos < delta.len() {
        let cmd = delta[pos];
        pos += 1;
        if cmd == 0 {
            return Err(Error::CorruptObject(
                "reserved opcode 0 in delta stream".to_owned(),
            ));
        }
        if cmd & 0x80 != 0 {
            let mut offset = 0usize;
            let mut size = 0usize;

            macro_rules! maybe_read_byte {
                ($flag:expr, $shift:expr, $target:expr) => {
                    if cmd & $flag != 0 {
                        let b = *delta.get(pos).ok_or_else(|| {
                            Error::CorruptObject("truncated delta COPY operand".to_owned())
                        })?;
                        pos += 1;
                        $target |= (b as usize) << $shift;
                    }
                };
            }

            maybe_read_byte!(0x01, 0, offset);
            maybe_read_byte!(0x02, 8, offset);
            maybe_read_byte!(0x04, 16, offset);
            maybe_read_byte!(0x08, 24, offset);
            maybe_read_byte!(0x10, 0, size);
            maybe_read_byte!(0x20, 8, size);
            maybe_read_byte!(0x40, 16, size);

            if size == 0 {
                size = 0x10000;
            }

            let end = offset.checked_add(size).ok_or_else(|| {
                Error::CorruptObject("delta COPY range overflows usize".to_owned())
            })?;
            let chunk = base.get(offset..end).ok_or_else(|| {
                Error::CorruptObject(format!(
                    "delta COPY [{offset},{end}) out of range (base is {} bytes)",
                    base.len()
                ))
            })?;
            result_len = result_len
                .checked_add(size)
                .ok_or_else(|| Error::CorruptObject("delta result length overflow".to_owned()))?;
            if !done {
                let room = max_out.saturating_sub(out.len());
                if room == 0 {
                    if prefix_sufficient(out) {
                        done = true;
                    }
                } else if chunk.len() <= room {
                    out.extend_from_slice(chunk);
                    if out.len() >= max_out && prefix_sufficient(out) {
                        done = true;
                        out.truncate(max_out);
                    }
                } else {
                    out.extend_from_slice(&chunk[..room]);
                    if prefix_sufficient(out) {
                        done = true;
                    }
                }
            }
        } else {
            let n = cmd as usize;
            let chunk = delta
                .get(pos..pos + n)
                .ok_or_else(|| Error::CorruptObject("truncated delta INSERT data".to_owned()))?;
            pos += n;
            result_len = result_len
                .checked_add(n)
                .ok_or_else(|| Error::CorruptObject("delta result length overflow".to_owned()))?;
            if !done {
                let room = max_out.saturating_sub(out.len());
                if room == 0 {
                    if prefix_sufficient(out) {
                        done = true;
                    }
                } else if chunk.len() <= room {
                    out.extend_from_slice(chunk);
                    if out.len() >= max_out && prefix_sufficient(out) {
                        done = true;
                        out.truncate(max_out);
                    }
                } else {
                    out.extend_from_slice(&chunk[..room]);
                    if prefix_sufficient(out) {
                        done = true;
                    }
                }
            }
        }
    }

    if result_len != dest_size {
        return Err(Error::CorruptObject(format!(
            "delta produced {result_len} bytes but expected {dest_size}"
        )));
    }

    Ok(())
}

/// Uncompressed size of the object produced when `delta` is applied to its base.
///
/// Reads only the second varint in the Git binary delta stream (after `src_size`).
/// Returns [`None`] when `data` ends before both varints are complete (incremental zlib prefix
/// decoding); callers with a full delta buffer should treat [`None`] as a truncated stream.
pub(crate) fn delta_uncompressed_result_size_if_complete(data: &[u8]) -> Result<Option<usize>> {
    let mut pos = 0;
    let Some(_src_size) = read_delta_varint_if_complete(data, &mut pos)? else {
        return Ok(None);
    };
    read_delta_varint_if_complete(data, &mut pos)
}

fn read_delta_varint_if_complete(data: &[u8], pos: &mut usize) -> Result<Option<usize>> {
    let start = *pos;
    let mut value = 0usize;
    let mut shift = 0u32;
    while *pos < data.len() {
        let b = data[*pos];
        *pos += 1;
        value |= ((b & 0x7f) as usize) << shift;
        shift += 7;
        if b & 0x80 == 0 {
            return Ok(Some(value));
        }
        if shift > 35 {
            return Err(Error::CorruptObject(
                "delta varint exceeds supported width".to_owned(),
            ));
        }
    }
    *pos = start;
    Ok(None)
}

/// Read a variable-length little-endian integer from `data` starting at `*pos`.
///
/// Advances `*pos` past the consumed bytes.
fn read_delta_varint(data: &[u8], pos: &mut usize) -> Result<usize> {
    let mut value = 0usize;
    let mut shift = 0u32;
    loop {
        let b = *data
            .get(*pos)
            .ok_or_else(|| Error::CorruptObject("truncated delta varint".to_owned()))?;
        *pos += 1;
        value |= ((b & 0x7f) as usize) << shift;
        shift += 7;
        if b & 0x80 == 0 {
            break;
        }
    }
    Ok(value)
}

#[cfg(test)]
mod tests {
    #![allow(clippy::vec_init_then_push)]
    use super::*;

    // Helper: build a minimal pack from a list of (kind, data) pairs.
    // Returns the raw pack bytes.
    fn make_pack(objects: &[(ObjectKind, &[u8])]) -> Vec<u8> {
        use flate2::write::ZlibEncoder;
        use std::io::Write;

        let mut entries: Vec<Vec<u8>> = Vec::new();
        for (kind, data) in objects {
            let type_code: u8 = match kind {
                ObjectKind::Commit => 1,
                ObjectKind::Tree => 2,
                ObjectKind::Blob => 3,
                ObjectKind::Tag => 4,
            };
            // Encode type+size header.
            let mut header = Vec::new();
            let mut size = data.len();
            let first = ((type_code & 0x7) << 4) | (size & 0x0f) as u8;
            size >>= 4;
            if size > 0 {
                header.push(first | 0x80);
                while size > 0 {
                    let b = (size & 0x7f) as u8;
                    size >>= 7;
                    header.push(if size > 0 { b | 0x80 } else { b });
                }
            } else {
                header.push(first);
            }
            // zlib-compress data.
            let mut enc = ZlibEncoder::new(Vec::new(), flate2::Compression::default());
            enc.write_all(data).unwrap();
            let compressed = enc.finish().unwrap();
            let mut entry = header;
            entry.extend_from_slice(&compressed);
            entries.push(entry);
        }

        // Assemble: PACK + version(2) + count + entries + SHA-1.
        let mut pack = Vec::new();
        pack.extend_from_slice(b"PACK");
        pack.extend_from_slice(&2u32.to_be_bytes());
        pack.extend_from_slice(&(objects.len() as u32).to_be_bytes());
        for entry in &entries {
            pack.extend_from_slice(entry);
        }
        pack.extend_from_slice(&HashAlgo::Sha1.checksum_digest(&pack));
        pack
    }

    #[test]
    fn test_apply_delta_simple() {
        // Build a trivial delta: insert "hello world".
        let base = b"hello";
        let mut delta = Vec::new();
        // src_size = 5
        delta.push(5u8);
        // dest_size = 11
        delta.push(11u8);
        // COPY instruction: copy base[0..5]
        // cmd = 0x80 | 0x01 (offset present, byte 0) | 0x10 (size byte 0)
        delta.push(0x80 | 0x01 | 0x10); // 0x91
        delta.push(0u8); // offset = 0
        delta.push(5u8); // size = 5
                         // INSERT " world" (6 bytes)
        delta.push(6u8);
        delta.extend_from_slice(b" world");

        let result = apply_delta(base, &delta).unwrap();
        assert_eq!(result, b"hello world");
    }

    #[test]
    fn test_apply_delta_prefix_into_stops_after_prefix() {
        let base = b"author A <a@b.com> 1 +0000\n\nbody";
        let mut delta = Vec::new();
        delta.push(base.len() as u8);
        delta.push(base.len() as u8);
        delta.push(0x80 | 0x01 | 0x10);
        delta.push(0u8);
        delta.push(base.len() as u8);

        let mut out = Vec::new();
        apply_delta_prefix_into(&mut out, base, &delta, 32, |buf| {
            buf.windows(7).any(|w| w == b"author ")
        })
        .unwrap();
        assert!(out.len() <= 32);
        assert!(out.starts_with(b"author A"));
        assert_eq!(apply_delta(base, &delta).unwrap(), base);
    }

    #[test]
    fn test_apply_delta_insert_only() {
        let base = b"";
        let mut delta = Vec::new();
        delta.push(0u8); // src_size = 0
        delta.push(5u8); // dest_size = 5
        delta.push(5u8); // INSERT 5 bytes
        delta.extend_from_slice(b"hello");

        let result = apply_delta(base, &delta).unwrap();
        assert_eq!(result, b"hello");
    }

    #[test]
    fn test_apply_delta_copy_only() {
        let base = b"abcdef";
        let mut delta = Vec::new();
        delta.push(6u8); // src_size = 6
        delta.push(3u8); // dest_size = 3
                         // COPY base[2..5]: offset=2, size=3
                         // cmd = 0x80 | 0x01 | 0x10
        delta.push(0x91u8);
        delta.push(2u8); // offset = 2
        delta.push(3u8); // size = 3

        let result = apply_delta(base, &delta).unwrap();
        assert_eq!(result, b"cde");
    }

    #[test]
    fn test_apply_delta_size_zero_means_65536() {
        // A COPY with size bytes all zero means 0x10000 = 65536.
        let base = vec![0xABu8; 65536];
        let mut delta = Vec::new();
        // src_size = 65536, encoded as 3 bytes little-endian varint
        delta.push(0x80 | (65536 & 0x7f) as u8); // 0
        delta.push(0x80 | ((65536 >> 7) & 0x7f) as u8); // 0x80
        delta.push(((65536 >> 14) & 0x7f) as u8); // 4
                                                  // dest_size = 65536, same
        delta.push(0x80 | (65536 & 0x7f) as u8);
        delta.push(0x80 | ((65536 >> 7) & 0x7f) as u8);
        delta.push(((65536 >> 14) & 0x7f) as u8);
        // COPY: offset=0 (no offset bytes), size=0 (no size bytes) → means 0x10000
        // cmd = 0x80 (no offset/size bytes present at all → offset=0, size=0→65536)
        delta.push(0x80u8);

        let result = apply_delta(&base, &delta).unwrap();
        assert_eq!(result.len(), 65536);
        assert!(result.iter().all(|&b| b == 0xAB));
    }

    #[cfg_attr(
        not(feature = "full-tests"),
        ignore = "starts system git or is heavy; run with --features full-tests"
    )]
    #[test]
    fn test_unpack_objects_blobs() {
        use tempfile::TempDir;
        let tmp = TempDir::new().unwrap();
        let objects_dir = tmp.path().join("objects");
        std::fs::create_dir_all(&objects_dir).unwrap();
        let odb = Odb::new(&objects_dir);

        let pack = make_pack(&[
            (ObjectKind::Blob, b"hello\n"),
            (ObjectKind::Blob, b"world\n"),
        ]);

        let opts = UnpackOptions::default();
        let count = unpack_objects(&mut pack.as_slice(), &odb, &opts).unwrap();
        assert_eq!(count, 2);

        // Verify both blobs can be read back.
        let oid1 = HashAlgo::Sha1
            .hash_object(ObjectKind::Blob, b"hello\n")
            .expect("hash");
        let oid2 = HashAlgo::Sha1
            .hash_object(ObjectKind::Blob, b"world\n")
            .expect("hash");
        let obj1 = odb.read(&oid1).unwrap();
        let obj2 = odb.read(&oid2).unwrap();
        assert_eq!(obj1.data, b"hello\n");
        assert_eq!(obj2.data, b"world\n");
    }

    #[cfg_attr(
        not(feature = "full-tests"),
        ignore = "starts system git or is heavy; run with --features full-tests"
    )]
    #[test]
    fn test_unpack_objects_empty_tree() {
        use tempfile::TempDir;
        let tmp = TempDir::new().unwrap();
        let objects_dir = tmp.path().join("objects");
        std::fs::create_dir_all(&objects_dir).unwrap();
        let odb = Odb::new(&objects_dir);

        let pack = make_pack(&[(ObjectKind::Tree, b"")]);
        let opts = UnpackOptions::default();
        assert_eq!(
            unpack_objects(&mut pack.as_slice(), &odb, &opts).unwrap(),
            1
        );
        let oid = HashAlgo::Sha1
            .hash_object(ObjectKind::Tree, b"")
            .expect("hash");
        assert!(odb.exists(&oid));
        let loose = objects_dir
            .join(oid.loose_prefix())
            .join(oid.loose_suffix());
        assert!(
            loose.is_file(),
            "empty tree must be materialized as a loose object during unpack"
        );
    }

    #[cfg_attr(
        not(feature = "full-tests"),
        ignore = "starts system git or is heavy; run with --features full-tests"
    )]
    #[test]
    fn test_strict_skips_gitlink_tree_entries() {
        use crate::index::{MODE_GITLINK, MODE_REGULAR};
        use crate::objects::{serialize_tree, TreeEntry};

        // A submodule commit oid that is NOT in the pack/ODB (lives in the
        // submodule repository, like a 160000 gitlink target on push).
        let submodule_oid = ObjectId::from_hex(&"7f".repeat(20)).unwrap();

        // Superproject tree referencing the submodule via a gitlink entry.
        let tree_data = serialize_tree(&[TreeEntry {
            mode: MODE_GITLINK,
            name: b"sub".to_vec(),
            oid: submodule_oid,
        }]);
        let tree_oid = HashAlgo::Sha1
            .hash_object(ObjectKind::Tree, &tree_data)
            .expect("hash");

        // Strict connectivity must NOT flag the gitlink target as missing,
        // matching upstream git (git/fsck.c skips S_ISGITLINK entries).
        let mut pack = HashMap::new();
        pack.insert(tree_oid, (ObjectKind::Tree, tree_data.clone()));
        assert!(strict_verify_packed_references(None, &pack).is_ok());

        // Regression guard: a non-gitlink (regular file) entry pointing at an
        // absent blob must still be reported as a strict connectivity error.
        let bad_tree = serialize_tree(&[TreeEntry {
            mode: MODE_REGULAR,
            name: b"file".to_vec(),
            oid: ObjectId::from_hex(&"ab".repeat(20)).unwrap(),
        }]);
        let bad_oid = HashAlgo::Sha1
            .hash_object(ObjectKind::Tree, &bad_tree)
            .expect("hash");
        let mut bad_pack = HashMap::new();
        bad_pack.insert(bad_oid, (ObjectKind::Tree, bad_tree));
        assert!(matches!(
            strict_verify_packed_references(None, &bad_pack),
            Err(Error::CorruptObject(_))
        ));
    }

    /// `Read` that returns at most `max_len` bytes per call (simulates side-band chunking).
    struct ChunkedReader<'a> {
        data: &'a [u8],
        pos: usize,
        max_len: usize,
    }

    impl io::Read for ChunkedReader<'_> {
        fn read(&mut self, buf: &mut [u8]) -> io::Result<usize> {
            if self.pos >= self.data.len() {
                return Ok(0);
            }
            let take = (self.data.len() - self.pos)
                .min(self.max_len)
                .min(buf.len());
            buf[..take].copy_from_slice(&self.data[self.pos..self.pos + take]);
            self.pos += take;
            Ok(take)
        }
    }

    #[cfg_attr(
        not(feature = "full-tests"),
        ignore = "starts system git or is heavy; run with --features full-tests"
    )]
    #[test]
    fn test_unpack_objects_chunked_read_matches_full_buffer() {
        use tempfile::TempDir;
        let pack = make_pack(&[(ObjectKind::Blob, b"chunked-stream")]);
        let opts = UnpackOptions::default();
        let oid = HashAlgo::Sha1
            .hash_object(ObjectKind::Blob, b"chunked-stream")
            .expect("hash");

        let tmp = TempDir::new().unwrap();
        let objects_dir = tmp.path().join("objects");
        std::fs::create_dir_all(&objects_dir).unwrap();
        let odb = Odb::new(&objects_dir);
        assert_eq!(
            unpack_objects(&mut pack.as_slice(), &odb, &opts).unwrap(),
            1
        );
        assert!(odb.exists(&oid));

        let tmp2 = TempDir::new().unwrap();
        let objects_dir2 = tmp2.path().join("objects");
        std::fs::create_dir_all(&objects_dir2).unwrap();
        let odb2 = Odb::new(&objects_dir2);
        let mut chunked = ChunkedReader {
            data: pack.as_slice(),
            pos: 0,
            max_len: 8,
        };
        assert_eq!(unpack_objects(&mut chunked, &odb2, &opts).unwrap(), 1);
        assert!(odb2.exists(&oid));
    }

    #[cfg_attr(
        not(feature = "full-tests"),
        ignore = "starts system git or is heavy; run with --features full-tests"
    )]
    #[test]
    fn test_unpack_objects_dry_run_writes_nothing() {
        use tempfile::TempDir;
        let tmp = TempDir::new().unwrap();
        let objects_dir = tmp.path().join("objects");
        std::fs::create_dir_all(&objects_dir).unwrap();
        let odb = Odb::new(&objects_dir);

        let pack = make_pack(&[(ObjectKind::Blob, b"test content")]);

        let opts = UnpackOptions {
            dry_run: true,
            quiet: true,
            strict: false,
            allowed_missing: Default::default(),
            allow_promisor_missing_references: false,
            max_input_bytes: None,
            ..Default::default()
        };
        let count = unpack_objects(&mut pack.as_slice(), &odb, &opts).unwrap();
        assert_eq!(count, 1);

        // Nothing should be written.
        let oid = HashAlgo::Sha1
            .hash_object(ObjectKind::Blob, b"test content")
            .expect("hash");
        assert!(!odb.exists(&oid));
    }

    #[cfg_attr(
        not(feature = "full-tests"),
        ignore = "starts system git or is heavy; run with --features full-tests"
    )]
    #[test]
    fn test_unpack_objects_bad_signature() {
        use tempfile::TempDir;
        let tmp = TempDir::new().unwrap();
        let objects_dir = tmp.path().join("objects");
        std::fs::create_dir_all(&objects_dir).unwrap();
        let odb = Odb::new(&objects_dir);

        let mut bad = b"NOPE\x00\x00\x00\x02\x00\x00\x00\x00".to_vec();
        bad.extend_from_slice(&[0u8; 20]);
        let opts = UnpackOptions::default();
        let err = unpack_objects(&mut bad.as_slice(), &odb, &opts).unwrap_err();
        assert!(err.to_string().contains("invalid signature"));
    }

    #[cfg_attr(
        not(feature = "full-tests"),
        ignore = "starts system git or is heavy; run with --features full-tests"
    )]
    #[test]
    fn test_unpack_objects_checksum_mismatch() {
        use tempfile::TempDir;
        let tmp = TempDir::new().unwrap();
        let objects_dir = tmp.path().join("objects");
        std::fs::create_dir_all(&objects_dir).unwrap();
        let odb = Odb::new(&objects_dir);

        let mut pack = make_pack(&[(ObjectKind::Blob, b"data")]);
        // Corrupt the trailing checksum.
        let n = pack.len();
        pack[n - 1] ^= 0xFF;

        let opts = UnpackOptions::default();
        let err = unpack_objects(&mut pack.as_slice(), &odb, &opts).unwrap_err();
        assert!(err.to_string().contains("checksum"));
    }

    #[test]
    fn test_apply_delta_source_size_mismatch() {
        let base = b"hi";
        let delta = [3u8, 2u8, 2u8, b'h', b'i']; // src_size=3 != base.len()=2
        let err = apply_delta(base, &delta).unwrap_err();
        assert!(err.to_string().contains("source size"));
    }

    #[test]
    fn unpack_many_objects_does_not_retain_all_payloads() {
        let _lock = TEST_UNPACK_RETAIN_LOCK.lock().expect("retain lock");
        const N: usize = 2500;
        let blobs: Vec<(ObjectKind, Vec<u8>)> = (0..N)
            .map(|i| (ObjectKind::Blob, format!("blob-{i}\n").into_bytes()))
            .collect();
        let blob_refs: Vec<(ObjectKind, &[u8])> =
            blobs.iter().map(|(k, d)| (*k, d.as_slice())).collect();
        let pack = make_pack(&blob_refs);
        let tmp = tempfile::tempdir().expect("tempdir");
        let odb = Odb::new(tmp.path());
        let cache_budget = 256 * 1024;
        test_set_unpack_base_cache_byte_limit(cache_budget);
        let count =
            unpack_objects(&mut pack.as_slice(), &odb, &UnpackOptions::default()).expect("unpack");
        assert_eq!(count, N);
        assert!(
            test_peak_unpack_retained_bytes() <= cache_budget + 128 * 1024,
            "peak retained {} exceeds cache budget {}",
            test_peak_unpack_retained_bytes(),
            cache_budget
        );
        test_set_unpack_base_cache_byte_limit(0);
    }

    #[test]
    fn unpack_many_deltified_blobs_respects_base_cache_budget() {
        let _lock = TEST_UNPACK_RETAIN_LOCK.lock().expect("retain lock");
        use crate::objects::{serialize_commit, CommitData};
        use grit_test_support::objects::{HashAlgo as FixtureAlgo, ObjectKind as PK, PackBuilder};

        const N_DELTAS: usize = 350;
        const BLOB_SIZE: usize = 48 * 1024;
        let base: Vec<u8> = (0..BLOB_SIZE).map(|i| (i % 251) as u8).collect();
        let tree_oid = HashAlgo::Sha1
            .hash_object(ObjectKind::Tree, b"")
            .expect("hash");
        let commit = CommitData {
            tree: tree_oid,
            parents: vec![],
            author: "A <a@b.com>".into(),
            committer: "A <a@b.com>".into(),
            author_raw: Vec::new(),
            committer_raw: Vec::new(),
            encoding: None,
            message: "m".into(),
            raw_message: None,
            extra_headers: Vec::new(),
        };
        let commit_data = serialize_commit(&commit);

        let mut pb = PackBuilder::new(FixtureAlgo::Sha1);
        pb.add_full(PK::Tree, b"");
        pb.add_full(PK::Commit, &commit_data);
        let base_idx = pb.add_full(PK::Blob, &base);
        for i in 0..N_DELTAS {
            let mut variant = base.clone();
            variant[BLOB_SIZE - 1] = (i % 256) as u8;
            let delta = crate::delta_encode::encode_lcp_delta(&base, &variant).expect("delta");
            pb.add_ofs_delta(base_idx, &delta, delta.len());
        }
        let pack = pb.build().bytes;

        let tmp = tempfile::tempdir().expect("tempdir");
        let odb = Odb::new(tmp.path());
        let cache_budget = 512 * 1024;
        test_set_unpack_base_cache_byte_limit(cache_budget);
        let opts = UnpackOptions {
            strict: true,
            ..Default::default()
        };
        let count = unpack_objects(&mut pack.as_slice(), &odb, &opts).expect("unpack");
        assert_eq!(count, N_DELTAS + 3);
        assert!(
            test_peak_unpack_retained_bytes() <= cache_budget + 256 * 1024,
            "peak retained {} exceeds cache budget {}",
            test_peak_unpack_retained_bytes(),
            cache_budget
        );
        test_set_unpack_base_cache_byte_limit(0);
    }

    #[test]
    fn pack_index_many_blobs_without_retaining_all_payloads() {
        const N: usize = 1500;
        let blobs: Vec<(ObjectKind, Vec<u8>)> = (0..N)
            .map(|i| (ObjectKind::Blob, format!("blob-{i}\n").into_bytes()))
            .collect();
        let blob_refs: Vec<(ObjectKind, &[u8])> =
            blobs.iter().map(|(k, d)| (*k, d.as_slice())).collect();
        let pack = make_pack(&blob_refs);
        let tmp = tempfile::tempdir().expect("tempdir");
        let odb = Odb::new(tmp.path());
        let started = std::time::Instant::now();
        let records = pack_index_records_from_bytes(&pack, &odb).expect("index records");
        let elapsed = started.elapsed();
        assert_eq!(records.len(), N, "one index row per object");
        assert!(
            elapsed.as_secs() < 30,
            "indexing {N} blobs took {:?}; expected linear scan without full retention blow-up",
            elapsed
        );
    }

    fn pack_with_ref_delta(base: &[u8], target: &[u8]) -> (Vec<u8>, ObjectId, ObjectId) {
        use grit_test_support::objects::{HashAlgo as FixtureAlgo, ObjectKind as PK, PackBuilder};

        let delta = crate::delta_encode::encode_lcp_delta(base, target).expect("delta");
        let base_oid = HashAlgo::Sha1
            .hash_object(ObjectKind::Blob, base)
            .expect("hash");
        let target_oid = HashAlgo::Sha1
            .hash_object(ObjectKind::Blob, target)
            .expect("hash");
        let mut pb = PackBuilder::new(FixtureAlgo::Sha1);
        pb.add_full(PK::Blob, base);
        pb.add_ref_delta(base_oid.as_bytes(), &delta, delta.len());
        (pb.build().bytes, base_oid, target_oid)
    }

    #[cfg_attr(
        not(feature = "full-tests"),
        ignore = "starts system git or is heavy; run with --features full-tests"
    )]
    #[test]
    fn test_unpack_objects_resolves_ref_and_ofs_deltas() {
        use grit_test_support::objects::{HashAlgo as FixtureAlgo, ObjectKind as PK, PackBuilder};

        let base = b"base payload";
        let target = b"base payload-extended";
        let delta = crate::delta_encode::encode_lcp_delta(base, target).unwrap();
        let base_oid = HashAlgo::Sha1
            .hash_object(ObjectKind::Blob, base)
            .expect("hash");

        let mut pb = PackBuilder::new(FixtureAlgo::Sha1);
        let base_idx = pb.add_full(PK::Blob, base);
        pb.add_ofs_delta(base_idx, &delta, delta.len());
        let ofs_pack = pb.build().bytes;

        let tmp = tempfile::tempdir().unwrap();
        let odb = Odb::new(tmp.path());
        let opts = UnpackOptions::default();
        assert_eq!(
            unpack_objects(&mut ofs_pack.as_slice(), &odb, &opts).unwrap(),
            2
        );
        let target_oid = HashAlgo::Sha1
            .hash_object(ObjectKind::Blob, target)
            .expect("hash");
        assert_eq!(odb.read(&target_oid).unwrap().data, target);

        let (ref_pack, _, ref_target_oid) = pack_with_ref_delta(base, target);
        let tmp2 = tempfile::tempdir().unwrap();
        let odb2 = Odb::new(tmp2.path());
        assert_eq!(
            unpack_objects(&mut ref_pack.as_slice(), &odb2, &opts).unwrap(),
            2
        );
        assert_eq!(odb2.read(&ref_target_oid).unwrap().data, target);
        assert!(odb2.exists(&base_oid));
    }

    #[cfg_attr(
        not(feature = "full-tests"),
        ignore = "starts system git or is heavy; run with --features full-tests"
    )]
    #[test]
    fn test_unpack_objects_ref_delta_base_from_loose() {
        let base = b"loose-base";
        let target = b"loose-base-v2";
        let (_, base_oid, target_oid) = pack_with_ref_delta(base, target);

        let tmp = tempfile::tempdir().unwrap();
        let odb = Odb::new(tmp.path());
        odb.write_local(ObjectKind::Blob, base).unwrap();
        // Pack only contains the delta slot (thin pack).
        let delta_only = {
            use grit_test_support::objects::{HashAlgo as FixtureAlgo, PackBuilder};
            let delta = crate::delta_encode::encode_lcp_delta(base, target).unwrap();
            let mut pb = PackBuilder::new(FixtureAlgo::Sha1);
            pb.add_ref_delta(base_oid.as_bytes(), &delta, delta.len());
            pb.build().bytes
        };
        let pack = delta_only;

        let opts = UnpackOptions::default();
        assert_eq!(
            unpack_objects(&mut pack.as_slice(), &odb, &opts).unwrap(),
            1
        );
        assert_eq!(odb.read(&target_oid).unwrap().data, target);
    }

    #[cfg_attr(
        not(feature = "full-tests"),
        ignore = "starts system git or is heavy; run with --features full-tests"
    )]
    #[test]
    fn test_unpack_objects_strict_commit_tree_tag() {
        use crate::objects::{serialize_commit, serialize_tag, CommitData, TagData};

        let tree_data = b"";
        let tree_oid = HashAlgo::Sha1
            .hash_object(ObjectKind::Tree, tree_data)
            .expect("hash");
        let commit = CommitData {
            tree: tree_oid,
            parents: vec![],
            author: "A <a@b.com>".into(),
            committer: "A <a@b.com>".into(),
            author_raw: Vec::new(),
            committer_raw: Vec::new(),
            encoding: None,
            message: "msg".into(),
            raw_message: None,
            extra_headers: Vec::new(),
        };
        let commit_data = serialize_commit(&commit);
        let commit_oid = HashAlgo::Sha1
            .hash_object(ObjectKind::Commit, &commit_data)
            .expect("hash");
        let tag = TagData {
            object: commit_oid,
            object_type: "commit".into(),
            tag: "v1".into(),
            tagger: Some("T <t@e.com>".into()),
            extra_headers: Vec::new(),
            message: "annotated".into(),
        };
        let tag_data = serialize_tag(&tag);
        let pack = make_pack(&[
            (ObjectKind::Tree, tree_data),
            (ObjectKind::Commit, &commit_data),
            (ObjectKind::Tag, &tag_data),
        ]);

        let tmp = tempfile::tempdir().unwrap();
        let odb = Odb::new(tmp.path());
        let opts = UnpackOptions {
            strict: true,
            ..Default::default()
        };
        assert_eq!(
            unpack_objects(&mut pack.as_slice(), &odb, &opts).unwrap(),
            3
        );
        assert!(odb.exists(&commit_oid));
    }

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

        let tree_oid = HashAlgo::Sha1
            .hash_object(ObjectKind::Tree, b"")
            .expect("hash");
        let missing_parent = ObjectId::from_hex(&"cc".repeat(20)).unwrap();
        let commit = CommitData {
            tree: tree_oid,
            parents: vec![missing_parent],
            author: "A <a@b.com>".into(),
            committer: "A <a@b.com>".into(),
            author_raw: Vec::new(),
            committer_raw: Vec::new(),
            encoding: None,
            message: "msg".into(),
            raw_message: None,
            extra_headers: Vec::new(),
        };
        let commit_data = serialize_commit(&commit);
        let commit_oid = HashAlgo::Sha1
            .hash_object(ObjectKind::Commit, &commit_data)
            .expect("hash");
        let pack = make_pack(&[(ObjectKind::Tree, b""), (ObjectKind::Commit, &commit_data)]);

        let tmp = tempfile::tempdir().unwrap();
        let odb = Odb::new(tmp.path());
        let mut shallow = HashSet::new();
        shallow.insert(commit_oid);
        let opts = UnpackOptions {
            strict: true,
            shallow_boundaries: shallow,
            ..Default::default()
        };
        assert_eq!(
            unpack_objects(&mut pack.as_slice(), &odb, &opts).unwrap(),
            2
        );
    }

    #[cfg_attr(
        not(feature = "full-tests"),
        ignore = "starts system git or is heavy; run with --features full-tests"
    )]
    #[test]
    fn test_unpack_objects_max_input_bytes() {
        let pack = make_pack(&[(ObjectKind::Blob, b"small")]);
        let tmp = tempfile::tempdir().unwrap();
        let odb = Odb::new(tmp.path());
        let opts = UnpackOptions {
            max_input_bytes: Some(4),
            ..Default::default()
        };
        let err = unpack_objects(&mut pack.as_slice(), &odb, &opts).unwrap_err();
        assert!(err.to_string().contains("maximum allowed size"));
    }

    #[cfg_attr(
        not(feature = "full-tests"),
        ignore = "starts system git or is heavy; run with --features full-tests"
    )]
    #[test]
    fn test_unpack_objects_unresolved_ref_delta() {
        let missing = ObjectId::from_hex(&"dd".repeat(20)).unwrap();
        use grit_test_support::objects::{HashAlgo as FixtureAlgo, PackBuilder};
        let delta = [0u8, 1u8, 1u8, b'x'];
        let mut pb = PackBuilder::new(FixtureAlgo::Sha1);
        pb.add_ref_delta(missing.as_bytes(), &delta, delta.len());
        let pack = pb.build().bytes;

        let tmp = tempfile::tempdir().unwrap();
        let odb = Odb::new(tmp.path());
        let err =
            unpack_objects(&mut pack.as_slice(), &odb, &UnpackOptions::default()).unwrap_err();
        assert!(err.to_string().contains("could not be resolved"));
    }

    #[cfg_attr(
        not(feature = "full-tests"),
        ignore = "starts system git or is heavy; run with --features full-tests"
    )]
    #[test]
    fn missing_ref_delta_base_is_typed_on_every_ingest_path() {
        use grit_test_support::objects::{HashAlgo as FixtureAlgo, PackBuilder};
        let delta = [0u8, 1u8, 1u8, b'x'];
        let mut pb = PackBuilder::new(FixtureAlgo::Sha1);
        pb.add_ref_delta(&[0xdd; 20], &delta, delta.len());
        pb.add_ref_delta(&[0xee; 20], &delta, delta.len());
        let pack = pb.build().bytes;
        let tmp = tempfile::tempdir().unwrap();
        let odb = Odb::new(tmp.path());

        let unpacked = unpack_objects(&mut pack.as_slice(), &odb, &UnpackOptions::default());
        assert!(
            matches!(unpacked, Err(Error::UnresolvedDeltas { count: 2 })),
            "{unpacked:?}"
        );
        let mapped = pack_bytes_to_object_map(&pack, &odb);
        assert!(
            matches!(mapped, Err(Error::UnresolvedDeltas { count: 2 })),
            "{mapped:?}"
        );
        let indexed = pack_index_records_from_bytes(&pack, &odb);
        assert!(
            matches!(indexed, Err(Error::UnresolvedDeltas { count: 2 })),
            "{indexed:?}"
        );
        assert!(pack_is_thin(&pack, HashAlgo::Sha1));
    }

    #[test]
    fn test_apply_delta_into_matches_apply_delta() {
        let base = b"abcdef";
        let mut delta = Vec::new();
        delta.push(6u8);
        delta.push(3u8);
        delta.push(0x91u8);
        delta.push(2u8);
        delta.push(3u8);

        let expected = apply_delta(base, &delta).unwrap();
        let mut out = Vec::new();
        apply_delta_into(&mut out, base, &delta).unwrap();
        assert_eq!(out, expected);
    }

    #[test]
    fn test_pack_bytes_to_object_map_ref_delta() {
        let base = b"map-base";
        let target = b"map-base-2";
        let (pack, _, target_oid) = pack_with_ref_delta(base, target);
        let tmp = tempfile::tempdir().unwrap();
        let odb = Odb::new(tmp.path());
        let map = pack_bytes_to_object_map(&pack, &odb).unwrap();
        assert_eq!(map.get(&target_oid).unwrap().data, target);
    }

    #[test]
    fn test_pack_is_thin_and_non_thin() {
        let base = b"thin";
        let target = b"thin-pack";
        let (full, base_oid, _) = pack_with_ref_delta(base, target);
        assert!(!pack_is_thin(&full, HashAlgo::Sha1));

        let delta = crate::delta_encode::encode_lcp_delta(base, target).unwrap();
        use grit_test_support::objects::{HashAlgo as FixtureAlgo, PackBuilder};
        let mut pb = PackBuilder::new(FixtureAlgo::Sha1);
        pb.add_ref_delta(base_oid.as_bytes(), &delta, delta.len());
        let thin = pb.build().bytes;
        assert!(pack_is_thin(&thin, HashAlgo::Sha1));
    }

    #[cfg_attr(
        not(feature = "full-tests"),
        ignore = "starts system git or is heavy; run with --features full-tests"
    )]
    #[test]
    fn test_unpack_zero_byte_blob_via_pack_builder() {
        use grit_test_support::objects::{HashAlgo as FixtureAlgo, ObjectKind as PK, PackBuilder};
        let mut pb = PackBuilder::new(FixtureAlgo::Sha1);
        pb.add_full(PK::Blob, b"");
        let pack = pb.build().bytes;
        let tmp = tempfile::tempdir().unwrap();
        let odb = Odb::new(tmp.path());
        assert_eq!(
            unpack_objects(&mut pack.as_slice(), &odb, &UnpackOptions::default()).unwrap(),
            1
        );
        let oid = HashAlgo::Sha1
            .hash_object(ObjectKind::Blob, b"")
            .expect("hash");
        assert!(odb.exists(&oid));
    }

    #[test]
    fn test_strict_verify_rejects_missing_tree_reference() {
        let tree_oid = HashAlgo::Sha1
            .hash_object(ObjectKind::Tree, b"")
            .expect("hash");
        let missing = ObjectId::from_hex(&"ee".repeat(20)).unwrap();
        let bad_tree = crate::objects::serialize_tree(&[crate::objects::TreeEntry {
            mode: crate::index::MODE_REGULAR,
            name: b"f".to_vec(),
            oid: missing,
        }]);
        let bad_oid = HashAlgo::Sha1
            .hash_object(ObjectKind::Tree, &bad_tree)
            .expect("hash");
        let mut pack = HashMap::new();
        pack.insert(tree_oid, (ObjectKind::Tree, b"".to_vec()));
        pack.insert(bad_oid, (ObjectKind::Tree, bad_tree));
        assert!(strict_verify_packed_references(None, &pack).is_err());
    }

    #[cfg_attr(
        not(feature = "full-tests"),
        ignore = "starts system git or is heavy; run with --features full-tests"
    )]
    #[test]
    fn test_unpack_large_blob_materializes_on_disk() {
        let big: Vec<u8> = vec![b'x'; 2 * 1024 * 1024 + 1];
        let pack = make_pack(&[(ObjectKind::Blob, &big)]);
        let tmp = tempfile::tempdir().unwrap();
        let odb = Odb::new(tmp.path());
        assert_eq!(
            unpack_objects(&mut pack.as_slice(), &odb, &UnpackOptions::default()).unwrap(),
            1
        );
        let oid = HashAlgo::Sha1
            .hash_object(ObjectKind::Blob, &big)
            .expect("hash");
        assert_eq!(odb.read(&oid).unwrap().data, big);
    }

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

        let payload = vec![0u8; 4096];
        let mut pack = make_pack(&[(ObjectKind::Blob, &payload)]);
        pack.truncate(pack.len() - 21);
        let digest = HashAlgo::Sha1.checksum_digest(&pack);
        pack.extend_from_slice(&digest);

        let tmp = tempfile::tempdir().unwrap();
        let odb = Odb::new(tmp.path());
        let started = Instant::now();
        let err =
            unpack_objects(&mut pack.as_slice(), &odb, &UnpackOptions::default()).unwrap_err();
        assert!(
            started.elapsed() < Duration::from_secs(2),
            "unpack hung on truncated zlib"
        );
        let msg = err.to_string();
        assert!(
            msg.contains("truncated") || msg.contains("zlib") || msg.contains("mismatch"),
            "unexpected error: {msg}"
        );
    }

    #[test]
    fn strict_dot_special_does_not_reload_streamed_blobs() {
        use std::cell::Cell;

        let tmp = tempfile::tempdir().unwrap();
        let odb = Odb::new(tmp.path());
        let big: Vec<u8> = vec![b'x'; 2 * 1024 * 1024 + 1];
        let big_oid = HashAlgo::Sha1
            .hash_object(ObjectKind::Blob, &big)
            .expect("hash");
        let tree_oid = HashAlgo::Sha1
            .hash_object(ObjectKind::Tree, b"")
            .expect("hash");

        struct CountingSink<'a> {
            inner: &'a Odb,
            reads: Cell<usize>,
        }

        impl UnpackSink for CountingSink<'_> {
            fn hash_algo(&self) -> HashAlgo {
                Odb::hash_algo(self.inner)
            }

            fn write(&self, kind: ObjectKind, data: &[u8], dry_run: bool) -> Result<ObjectId> {
                if dry_run {
                    return self.inner.hash(kind, data);
                }
                self.inner.write(kind, data)
            }

            fn read_stored(&self, oid: &ObjectId) -> Result<Option<Object>> {
                self.reads.set(self.reads.get() + 1);
                self.inner.read_stored(oid)
            }

            fn contains(&self, oid: &ObjectId) -> Result<bool> {
                Odb::contains(self.inner, oid)
            }
        }

        let mut by_oid = HashMap::new();
        by_oid.insert(
            big_oid,
            PackedObjectEntry::Stored {
                kind: ObjectKind::Blob,
                oid: big_oid,
            },
        );
        by_oid.insert(
            tree_oid,
            PackedObjectEntry::DryRun {
                kind: ObjectKind::Tree,
                data: Vec::new(),
            },
        );

        let sink = CountingSink {
            inner: &odb,
            reads: Cell::new(0),
        };
        verify_packed_dot_special_entries(&by_oid, &sink).expect("dot-special walk");
        assert_eq!(
            sink.reads.get(),
            0,
            "must not read streamed blobs back for strict fsck"
        );
    }

    #[test]
    fn strict_verify_does_not_reload_streamed_blobs() {
        use std::cell::Cell;

        use crate::index::MODE_REGULAR;
        use crate::objects::{serialize_commit, serialize_tree, CommitData, TreeEntry};

        let tmp = tempfile::tempdir().unwrap();
        let odb = Odb::new(tmp.path());
        let big: Vec<u8> = vec![b'x'; 2 * 1024 * 1024 + 1];
        let big_oid = odb.write(ObjectKind::Blob, &big).expect("write blob");

        let tree_data = serialize_tree(&[TreeEntry {
            mode: MODE_REGULAR,
            name: b"big.bin".to_vec(),
            oid: big_oid,
        }]);
        let tree_oid = HashAlgo::Sha1
            .hash_object(ObjectKind::Tree, &tree_data)
            .expect("hash");
        let commit = CommitData {
            tree: tree_oid,
            parents: vec![],
            author: "A <a@b.com>".into(),
            committer: "A <a@b.com>".into(),
            author_raw: Vec::new(),
            committer_raw: Vec::new(),
            encoding: None,
            message: "m".into(),
            raw_message: None,
            extra_headers: Vec::new(),
        };
        let commit_data = serialize_commit(&commit);
        let commit_oid = HashAlgo::Sha1
            .hash_object(ObjectKind::Commit, &commit_data)
            .expect("hash");

        struct CountingSink<'a> {
            inner: &'a Odb,
            reads: Cell<usize>,
        }

        impl UnpackSink for CountingSink<'_> {
            fn hash_algo(&self) -> HashAlgo {
                Odb::hash_algo(self.inner)
            }

            fn write(&self, kind: ObjectKind, data: &[u8], dry_run: bool) -> Result<ObjectId> {
                if dry_run {
                    return self.inner.hash(kind, data);
                }
                self.inner.write(kind, data)
            }

            fn read_stored(&self, oid: &ObjectId) -> Result<Option<Object>> {
                self.reads.set(self.reads.get() + 1);
                self.inner.read_stored(oid)
            }

            fn contains(&self, oid: &ObjectId) -> Result<bool> {
                Odb::contains(self.inner, oid)
            }
        }

        let mut by_oid = HashMap::new();
        by_oid.insert(
            big_oid,
            PackedObjectEntry::Stored {
                kind: ObjectKind::Blob,
                oid: big_oid,
            },
        );
        by_oid.insert(
            tree_oid,
            PackedObjectEntry::DryRun {
                kind: ObjectKind::Tree,
                data: tree_data,
            },
        );
        by_oid.insert(
            commit_oid,
            PackedObjectEntry::DryRun {
                kind: ObjectKind::Commit,
                data: commit_data,
            },
        );

        let sink = CountingSink {
            inner: &odb,
            reads: Cell::new(0),
        };
        strict_verify_packed_references_map(
            &sink,
            &by_oid,
            &HashSet::new(),
            false,
            &HashSet::new(),
            None,
        )
        .expect("strict connectivity");
        assert_eq!(
            sink.reads.get(),
            0,
            "strict verify must not read streamed blob payloads"
        );

        let missing = ObjectId::from_hex(&"ab".repeat(20)).unwrap();
        let bad_tree = serialize_tree(&[TreeEntry {
            mode: MODE_REGULAR,
            name: b"missing".to_vec(),
            oid: missing,
        }]);
        let bad_tree_oid = HashAlgo::Sha1
            .hash_object(ObjectKind::Tree, &bad_tree)
            .expect("hash");
        let mut bad = HashMap::new();
        bad.insert(
            bad_tree_oid,
            PackedObjectEntry::DryRun {
                kind: ObjectKind::Tree,
                data: bad_tree,
            },
        );
        let sink2 = CountingSink {
            inner: &odb,
            reads: Cell::new(0),
        };
        assert!(matches!(
            strict_verify_packed_references_map(
                &sink2,
                &bad,
                &HashSet::new(),
                false,
                &HashSet::new(),
                None,
            ),
            Err(Error::CorruptObject(_))
        ));
        assert_eq!(sink2.reads.get(), 0);
    }
}