hashtree-lmdb 0.2.83

//! LMDB-backed content-addressed blob storage.

mod configured;

pub use configured::{
    open_configured_lmdb_blob_store, open_shared_lmdb_blob_store, ConfiguredLmdbBlobStore,
    LOCAL_ADD_EXTERNAL_BLOB_DIR_NAME, SHARED_BLOB_MIN_MAP_SIZE_BYTES,
};

use async_trait::async_trait;
use hashtree_core::store::{PutManyReport, Store, StoreError, StoreStats};
use hashtree_core::{to_hex, types::Hash};
use heed::types::*;
use heed::{Database, EnvFlags, EnvOpenOptions, Error as HeedError, MdbError, PutFlags};
use std::collections::HashSet;
use std::fs::{self, File};
use std::io::{Read, Seek, SeekFrom, Write};
use std::path::{Path, PathBuf};
use std::sync::atomic::{AtomicU64, Ordering};
use std::time::{SystemTime, UNIX_EPOCH};

// Re-export sha256 for convenience
pub use hashtree_core::hash::sha256 as compute_sha256;

#[cfg(target_pointer_width = "64")]
const DEFAULT_MAP_SIZE: usize = 10 * 1024 * 1024 * 1024;
#[cfg(target_pointer_width = "32")]
const DEFAULT_MAP_SIZE: usize = 1024 * 1024 * 1024;
const DEFAULT_MAX_READERS: u32 = 1024;
const DATABASE_COUNT: u32 = 5;
const REOPEN_HEADROOM_BYTES: u64 = 64 * 1024 * 1024;
const EVICTION_BATCH_TARGET_BYTES: u64 = 256 * 1024 * 1024;
const EVICTION_BATCH_MAX_ITEMS: usize = 4096;
const LEGACY_BLOB_META_BYTES: usize = 16;
const BLOB_META_BYTES: usize = 24;
const ORDER_KEY_BYTES: usize = 40;
const PIN_COUNT_BYTES: usize = 4;
const STORE_TOTALS_BYTES: usize = 32;
const STORE_TOTALS_KEY: &[u8] = b"totals";
const NEXT_ORDER_BYTES: usize = 8;
const NEXT_ORDER_KEY: &[u8] = b"next_order";
const NEXT_ORDER_MIGRATION_FLOOR: u64 = 1 << 48;
const EXTERNAL_BLOB_MARKER_PREFIX: &[u8] = b"\0hashtree-lmdb-external-blob-v1\0";
const EXTERNAL_PACK_MARKER_PREFIX: &[u8] = b"\0hashtree-lmdb-external-pack-v1\0";
const EXTERNAL_PACK_RESERVED_MARKER_PREFIX: &[u8] = b"\0hashtree-lmdb-external-pack-reserved-v1\0";
const LMDB_NO_READ_AHEAD_ENV: &str = "HTREE_LMDB_NO_READ_AHEAD";
const LMDB_NO_SYNC_ENV: &str = "HTREE_LMDB_NO_SYNC";
const LMDB_NO_META_SYNC_ENV: &str = "HTREE_LMDB_NO_META_SYNC";
const LMDB_EXTERNAL_BLOB_MIN_BYTES_ENV: &str = "HTREE_LMDB_EXTERNAL_BLOB_MIN_BYTES";
const LMDB_EXTERNAL_BLOB_DIR_ENV: &str = "HTREE_LMDB_EXTERNAL_BLOB_DIR";
const LMDB_EXTERNAL_BLOB_SYNC_ENV: &str = "HTREE_LMDB_EXTERNAL_BLOB_SYNC";
const LMDB_EXTERNAL_BLOB_PACK_TARGET_BYTES_ENV: &str = "HTREE_LMDB_EXTERNAL_BLOB_PACK_TARGET_BYTES";
static EXTERNAL_PACK_COUNTER: AtomicU64 = AtomicU64::new(0);

#[derive(Debug, Clone, Copy)]
struct BlobMeta {
    order: u64,
    size: u64,
    last_accessed_at: u64,
}

#[derive(Debug, Default, Clone, Copy)]
struct StoreTotals {
    count: u64,
    total_bytes: u64,
    pinned_count: u64,
    pinned_bytes: u64,
}

#[derive(Debug, Clone)]
struct ExternalBlobConfig {
    base_path: PathBuf,
    min_bytes: usize,
    sync: bool,
    pack_target_bytes: Option<usize>,
}

/// Options for storing larger LMDB blobs in normal files.
#[derive(Debug, Clone)]
pub struct ExternalBlobOptions {
    pub base_path: PathBuf,
    pub min_bytes: usize,
    pub sync: bool,
    pub pack_target_bytes: Option<usize>,
}

impl ExternalBlobOptions {
    /// Build external-blob options from the standard hashtree LMDB environment.
    pub fn from_env(env_path: &Path) -> Option<Self> {
        ExternalBlobConfig::from_env(env_path).map(Into::into)
    }

    /// Override only the base directory, keeping the remaining env-derived knobs.
    pub fn with_base_path(mut self, base_path: PathBuf) -> Self {
        self.base_path = base_path;
        self
    }
}

impl ExternalBlobConfig {
    fn from_env(env_path: &Path) -> Option<Self> {
        let min_bytes = std::env::var(LMDB_EXTERNAL_BLOB_MIN_BYTES_ENV)
            .ok()
            .and_then(|value| value.parse::<usize>().ok())
            .filter(|value| *value > 0)?;
        let base_path = std::env::var(LMDB_EXTERNAL_BLOB_DIR_ENV)
            .ok()
            .filter(|value| !value.trim().is_empty())
            .map(PathBuf::from)
            .unwrap_or_else(|| env_path.join("external-blobs"));
        let sync = env_bool(LMDB_EXTERNAL_BLOB_SYNC_ENV).unwrap_or(true);
        let pack_target_bytes = std::env::var(LMDB_EXTERNAL_BLOB_PACK_TARGET_BYTES_ENV)
            .ok()
            .and_then(|value| value.parse::<usize>().ok())
            .filter(|value| *value > 0);
        Some(Self {
            base_path,
            min_bytes,
            sync,
            pack_target_bytes,
        })
    }
}

impl From<ExternalBlobConfig> for ExternalBlobOptions {
    fn from(config: ExternalBlobConfig) -> Self {
        Self {
            base_path: config.base_path,
            min_bytes: config.min_bytes,
            sync: config.sync,
            pack_target_bytes: config.pack_target_bytes,
        }
    }
}

impl From<ExternalBlobOptions> for ExternalBlobConfig {
    fn from(options: ExternalBlobOptions) -> Self {
        Self {
            base_path: options.base_path,
            min_bytes: options.min_bytes,
            sync: options.sync,
            pack_target_bytes: options.pack_target_bytes,
        }
    }
}

#[derive(Debug, Clone)]
struct ExternalPackRef {
    name: String,
    offset: u64,
    len: u64,
}

/// LMDB-backed blob store implementing hashtree's Store trait.
pub struct LmdbBlobStore {
    env: heed::Env,
    /// Maps SHA256 hash (32 bytes) → blob data
    blobs: Database<Bytes, Bytes>,
    /// Maps SHA256 hash (32 bytes) → [order: u64][size: u64]
    metadata: Database<Bytes, Bytes>,
    /// Maps [order: u64][hash: 32 bytes] → ()
    eviction_order: Database<Bytes, Unit>,
    /// Maps SHA256 hash (32 bytes) → pin count (u32)
    pins: Database<Bytes, Bytes>,
    /// Small aggregate counters used by quota checks and stats.
    stats: Database<Bytes, Bytes>,
    max_bytes: AtomicU64,
    next_order: AtomicU64,
    external_blobs: Option<ExternalBlobConfig>,
}

impl LmdbBlobStore {
    /// Open or create an LMDB blob store at the given path.
    pub fn new<P: AsRef<Path>>(path: P) -> Result<Self, StoreError> {
        Self::with_map_size(path, DEFAULT_MAP_SIZE)
    }

    /// Open or create with a maximum logical storage size.
    pub fn with_max_bytes<P: AsRef<Path>>(path: P, max_bytes: u64) -> Result<Self, StoreError> {
        Self::with_max_bytes_and_external_blob_options(
            path,
            max_bytes,
            ExternalBlobOptions::from_env,
        )
    }

    /// Open or create with a maximum logical storage size and custom external-blob options.
    pub fn with_max_bytes_and_external_blob_options<P, F>(
        path: P,
        max_bytes: u64,
        external_blobs: F,
    ) -> Result<Self, StoreError>
    where
        P: AsRef<Path>,
        F: FnOnce(&Path) -> Option<ExternalBlobOptions>,
    {
        let path_ref = path.as_ref();
        let existing_map_size = std::fs::metadata(path_ref.join("data.mdb"))
            .map(|metadata| metadata.len())
            .unwrap_or(0);
        let existing_headroom = if existing_map_size == 0 {
            0
        } else {
            existing_map_size
                .saturating_div(10)
                .max(REOPEN_HEADROOM_BYTES)
        };
        let requested_map_size = usize::try_from(Self::align_map_size_bytes(
            existing_map_size
                .saturating_add(existing_headroom)
                .max(max_bytes),
        ))
        .unwrap_or(usize::MAX)
        .max(DEFAULT_MAP_SIZE);
        let external_blobs = external_blobs(path_ref);
        let store = Self::with_map_size_and_external_blob_options(
            path_ref,
            requested_map_size,
            external_blobs,
        )?;
        store.max_bytes.store(max_bytes, Ordering::Relaxed);
        let current = store.total_bytes()?;
        if max_bytes > 0 && current > max_bytes {
            let target = max_bytes.saturating_mul(9) / 10;
            store.evict_to_target(current, target)?;
        }
        Ok(store)
    }

    /// Open or create with the default map size and explicit external-blob options.
    pub fn with_external_blob_options<P: AsRef<Path>>(
        path: P,
        external_blobs: Option<ExternalBlobOptions>,
    ) -> Result<Self, StoreError> {
        Self::with_map_size_and_external_blob_options(path, DEFAULT_MAP_SIZE, external_blobs)
    }

    fn align_map_size_bytes(bytes: u64) -> u64 {
        if bytes == 0 {
            return 0;
        }
        let page_size = (page_size::get() as u64).max(4096);
        let remainder = bytes % page_size;
        if remainder == 0 {
            bytes
        } else {
            bytes.saturating_add(page_size - remainder)
        }
    }

    /// Open or create with custom map size.
    pub fn with_map_size<P: AsRef<Path>>(path: P, map_size: usize) -> Result<Self, StoreError> {
        Self::with_map_size_and_settings(
            path,
            map_size,
            env_flags_from_env(),
            ExternalBlobConfig::from_env,
        )
    }

    /// Open or create with custom map size and explicit external-blob options.
    pub fn with_map_size_and_external_blob_options<P: AsRef<Path>>(
        path: P,
        map_size: usize,
        external_blobs: Option<ExternalBlobOptions>,
    ) -> Result<Self, StoreError> {
        Self::with_map_size_and_settings(path, map_size, env_flags_from_env(), |_| {
            external_blobs.map(Into::into)
        })
    }

    fn with_map_size_and_settings<P, F>(
        path: P,
        map_size: usize,
        flags: EnvFlags,
        external_blobs: F,
    ) -> Result<Self, StoreError>
    where
        P: AsRef<Path>,
        F: FnOnce(&Path) -> Option<ExternalBlobConfig>,
    {
        let path_ref = path.as_ref();
        std::fs::create_dir_all(path_ref).map_err(StoreError::Io)?;
        let existing_map_size = std::fs::metadata(path_ref.join("data.mdb"))
            .map(|metadata| metadata.len())
            .unwrap_or(0);
        let existing_headroom = if existing_map_size == 0 {
            0
        } else {
            existing_map_size
                .saturating_div(10)
                .max(REOPEN_HEADROOM_BYTES)
        };
        let requested_map_size = u64::try_from(map_size).unwrap_or(u64::MAX);
        let map_size = usize::try_from(Self::align_map_size_bytes(
            requested_map_size.max(existing_map_size.saturating_add(existing_headroom)),
        ))
        .unwrap_or(usize::MAX);

        let mut env_options = EnvOpenOptions::new();
        env_options
            .map_size(map_size)
            .max_dbs(DATABASE_COUNT)
            .max_readers(DEFAULT_MAX_READERS);
        unsafe {
            env_options.flags(flags);
        }
        let env = unsafe { env_options.open(path_ref).map_err(map_heed_error)? };
        let _ = env.clear_stale_readers();
        if env.info().map_size < map_size {
            unsafe { env.resize(map_size) }.map_err(map_heed_error)?;
        }

        let mut wtxn = env.write_txn().map_err(map_heed_error)?;
        let blobs = env
            .create_database(&mut wtxn, Some("blobs"))
            .map_err(map_heed_error)?;
        let metadata = env
            .create_database(&mut wtxn, Some("metadata"))
            .map_err(map_heed_error)?;
        let eviction_order = env
            .create_database(&mut wtxn, Some("eviction_order"))
            .map_err(map_heed_error)?;
        let pins = env
            .create_database(&mut wtxn, Some("pins"))
            .map_err(map_heed_error)?;
        let stats = env
            .create_database(&mut wtxn, Some("stats"))
            .map_err(map_heed_error)?;
        wtxn.commit().map_err(map_heed_error)?;

        let totals = Self::load_or_rebuild_totals(&env, metadata, pins, stats)?;
        let next_order = Self::load_or_initialize_next_order(&env, stats, totals.count > 0)?;

        Ok(Self {
            env,
            blobs,
            metadata,
            eviction_order,
            pins,
            stats,
            max_bytes: AtomicU64::new(0),
            next_order: AtomicU64::new(next_order),
            external_blobs: external_blobs(path_ref),
        })
    }

    fn load_or_rebuild_totals(
        env: &heed::Env,
        metadata: Database<Bytes, Bytes>,
        pins: Database<Bytes, Bytes>,
        stats: Database<Bytes, Bytes>,
    ) -> Result<StoreTotals, StoreError> {
        {
            let rtxn = env.read_txn().map_err(map_heed_error)?;
            if let Some(totals) = Self::read_store_totals(stats, &rtxn)? {
                return Ok(totals);
            }
        }

        let mut totals = StoreTotals::default();
        {
            let rtxn = env.read_txn().map_err(map_heed_error)?;
            for item in metadata.iter(&rtxn).map_err(map_heed_error)? {
                let (_, bytes) = item.map_err(map_heed_error)?;
                let meta = Self::decode_blob_meta(bytes)?;
                totals.count = totals.count.saturating_add(1);
                totals.total_bytes = totals.total_bytes.saturating_add(meta.size);
            }
            for item in pins.iter(&rtxn).map_err(map_heed_error)? {
                let (hash, pin_bytes) = item.map_err(map_heed_error)?;
                let count = Self::decode_pin_count(pin_bytes)?;
                if count == 0 {
                    continue;
                }
                let Some(meta_bytes) = metadata.get(&rtxn, hash).map_err(map_heed_error)? else {
                    continue;
                };
                let meta = Self::decode_blob_meta(meta_bytes)?;
                totals.pinned_count = totals.pinned_count.saturating_add(1);
                totals.pinned_bytes = totals.pinned_bytes.saturating_add(meta.size);
            }
        }

        let mut wtxn = env.write_txn().map_err(map_heed_error)?;
        Self::write_store_totals(stats, &mut wtxn, totals).map_err(map_heed_error)?;
        wtxn.commit().map_err(map_heed_error)?;
        Ok(totals)
    }

    fn load_or_initialize_next_order(
        env: &heed::Env,
        stats: Database<Bytes, Bytes>,
        has_existing_blobs: bool,
    ) -> Result<u64, StoreError> {
        {
            let rtxn = env.read_txn().map_err(map_heed_error)?;
            if let Some(next_order) = Self::read_next_order(stats, &rtxn)? {
                return Ok(next_order);
            }
        }

        let next_order = if has_existing_blobs {
            Self::legacy_next_order_floor()
        } else {
            0
        };
        let mut wtxn = env.write_txn().map_err(map_heed_error)?;
        let next_order = Self::read_next_order_lossy(stats, &wtxn)
            .map_err(map_heed_error)?
            .unwrap_or(next_order);
        Self::write_next_order(stats, &mut wtxn, next_order).map_err(map_heed_error)?;
        wtxn.commit().map_err(map_heed_error)?;
        Ok(next_order)
    }

    fn legacy_next_order_floor() -> u64 {
        let micros = SystemTime::now()
            .duration_since(UNIX_EPOCH)
            .ok()
            .and_then(|duration| u64::try_from(duration.as_micros()).ok())
            .unwrap_or(NEXT_ORDER_MIGRATION_FLOOR);
        micros.max(NEXT_ORDER_MIGRATION_FLOOR)
    }

    fn read_store_totals(
        stats: Database<Bytes, Bytes>,
        txn: &heed::RoTxn,
    ) -> Result<Option<StoreTotals>, StoreError> {
        stats
            .get(txn, STORE_TOTALS_KEY)
            .map_err(map_heed_error)?
            .map(Self::decode_store_totals)
            .transpose()
    }

    fn read_store_totals_lossy(
        stats: Database<Bytes, Bytes>,
        txn: &heed::RoTxn,
    ) -> std::result::Result<StoreTotals, HeedError> {
        Ok(stats
            .get(txn, STORE_TOTALS_KEY)?
            .and_then(Self::decode_store_totals_lossy)
            .unwrap_or_default())
    }

    fn write_store_totals(
        stats: Database<Bytes, Bytes>,
        txn: &mut heed::RwTxn,
        totals: StoreTotals,
    ) -> std::result::Result<(), HeedError> {
        let encoded = Self::encode_store_totals(totals);
        stats.put(txn, STORE_TOTALS_KEY, &encoded)
    }

    fn read_next_order(
        stats: Database<Bytes, Bytes>,
        txn: &heed::RoTxn,
    ) -> Result<Option<u64>, StoreError> {
        stats
            .get(txn, NEXT_ORDER_KEY)
            .map_err(map_heed_error)?
            .map(Self::decode_next_order)
            .transpose()
    }

    fn read_next_order_lossy(
        stats: Database<Bytes, Bytes>,
        txn: &heed::RoTxn,
    ) -> std::result::Result<Option<u64>, HeedError> {
        Ok(stats
            .get(txn, NEXT_ORDER_KEY)?
            .and_then(Self::decode_next_order_lossy))
    }

    fn write_next_order(
        stats: Database<Bytes, Bytes>,
        txn: &mut heed::RwTxn,
        next_order: u64,
    ) -> std::result::Result<(), HeedError> {
        let encoded = Self::encode_next_order(next_order);
        stats.put(txn, NEXT_ORDER_KEY, &encoded)
    }

    fn allocate_order_range_in_txn(
        &self,
        txn: &mut heed::RwTxn,
        count: usize,
    ) -> std::result::Result<u64, HeedError> {
        let start = Self::read_next_order_lossy(self.stats, txn)?
            .unwrap_or_else(|| self.next_order.load(Ordering::Relaxed));
        let end = start.saturating_add(count as u64);
        Self::write_next_order(self.stats, txn, end)?;
        self.next_order.store(end, Ordering::Relaxed);
        Ok(start)
    }

    fn increment_totals_in_txn(
        &self,
        txn: &mut heed::RwTxn,
        count: u64,
        total_bytes: u64,
        pinned_count: u64,
        pinned_bytes: u64,
    ) -> std::result::Result<(), HeedError> {
        let mut totals = Self::read_store_totals_lossy(self.stats, txn)?;
        totals.count = totals.count.saturating_add(count);
        totals.total_bytes = totals.total_bytes.saturating_add(total_bytes);
        totals.pinned_count = totals.pinned_count.saturating_add(pinned_count);
        totals.pinned_bytes = totals.pinned_bytes.saturating_add(pinned_bytes);
        Self::write_store_totals(self.stats, txn, totals)
    }

    fn decrement_totals_in_txn(
        &self,
        txn: &mut heed::RwTxn,
        count: u64,
        total_bytes: u64,
        pinned_count: u64,
        pinned_bytes: u64,
    ) -> std::result::Result<(), HeedError> {
        let mut totals = Self::read_store_totals_lossy(self.stats, txn)?;
        totals.count = totals.count.saturating_sub(count);
        totals.total_bytes = totals.total_bytes.saturating_sub(total_bytes);
        totals.pinned_count = totals.pinned_count.saturating_sub(pinned_count);
        totals.pinned_bytes = totals.pinned_bytes.saturating_sub(pinned_bytes);
        Self::write_store_totals(self.stats, txn, totals)
    }

    /// Check if a hash exists (sync version for internal use).
    pub fn exists(&self, hash: &Hash) -> Result<bool, StoreError> {
        let rtxn = self
            .env
            .read_txn()
            .map_err(|e| StoreError::Other(e.to_string()))?;

        if self
            .metadata
            .get(&rtxn, hash)
            .map_err(|e| StoreError::Other(e.to_string()))?
            .is_some()
        {
            return Ok(true);
        }

        Ok(self
            .blobs
            .get(&rtxn, hash)
            .map_err(|e| StoreError::Other(e.to_string()))?
            .is_some())
    }

    fn mark_existing_hashes_in_db(
        db: Database<Bytes, Bytes>,
        rtxn: &heed::RoTxn,
        sorted_hashes: &[Hash],
        existing: &mut [bool],
    ) -> Result<(), StoreError> {
        debug_assert_eq!(sorted_hashes.len(), existing.len());
        debug_assert!(sorted_hashes.windows(2).all(|pair| pair[0] <= pair[1]));

        if sorted_hashes.is_empty() {
            return Ok(());
        }

        for (index, hash) in sorted_hashes.iter().enumerate() {
            if existing[index] {
                continue;
            }
            if db.get(rtxn, hash).map_err(map_heed_error)?.is_some() {
                existing[index] = true;
            }
        }

        Ok(())
    }

    /// Mark which sorted hashes already exist using bounded LMDB range scans.
    pub fn existing_hashes_in_sorted_candidates(
        &self,
        sorted_hashes: &[Hash],
    ) -> Result<Vec<bool>, StoreError> {
        let mut existing = vec![false; sorted_hashes.len()];
        if sorted_hashes.is_empty() {
            return Ok(existing);
        }

        let rtxn = self.env.read_txn().map_err(map_heed_error)?;
        Self::mark_existing_hashes_in_db(self.metadata, &rtxn, sorted_hashes, &mut existing)?;
        if existing.iter().all(|exists| *exists) {
            return Ok(existing);
        }
        Self::mark_existing_hashes_in_db(self.blobs, &rtxn, sorted_hashes, &mut existing)?;
        Ok(existing)
    }

    pub fn blob_size_sync(&self, hash: &Hash) -> Result<Option<u64>, StoreError> {
        let rtxn = self
            .env
            .read_txn()
            .map_err(|e| StoreError::Other(e.to_string()))?;
        self.blob_size_in_txn(&rtxn, hash)
    }

    pub fn map_size_bytes(&self) -> usize {
        self.env.info().map_size
    }

    pub fn force_sync(&self) -> Result<(), StoreError> {
        self.env.force_sync().map_err(map_heed_error)
    }

    fn total_bytes(&self) -> Result<u64, StoreError> {
        let rtxn = self.env.read_txn().map_err(map_heed_error)?;
        Ok(Self::read_store_totals(self.stats, &rtxn)?
            .unwrap_or_default()
            .total_bytes)
    }

    fn evict_for_write_pressure(&self, incoming_bytes: u64) -> Result<u64, StoreError> {
        let current = self.total_bytes()?;
        if current == 0 {
            return Ok(0);
        }

        let headroom = incoming_bytes.max(current / 10).max(1);
        let target = current.saturating_sub(headroom);
        self.evict_to_target(current, target)
    }

    fn enforce_max_bytes_after_insert(&self, inserted_bytes: u64) -> Result<u64, StoreError> {
        let max = self.max_bytes.load(Ordering::Relaxed);
        if max == 0 || inserted_bytes == 0 {
            return Ok(0);
        }

        let current = self.total_bytes()?;
        if current <= max {
            return Ok(0);
        }

        let target = if inserted_bytes >= max {
            inserted_bytes
        } else {
            max.saturating_mul(9)
                .saturating_div(10)
                .saturating_add(inserted_bytes)
                .min(max)
        };
        self.evict_to_target(current, target)
    }

    fn write_metadata_for_inserted_blobs(
        &self,
        wtxn: &mut heed::RwTxn,
        inserted_entries: &[(Hash, u64)],
    ) -> Result<(), StoreError> {
        if inserted_entries.is_empty() {
            return Ok(());
        }

        let now = unix_timestamp_now();
        let mut inserted_bytes = 0u64;
        let mut inserted_pinned = 0u64;
        let mut inserted_pinned_bytes = 0u64;
        let mut order = self
            .allocate_order_range_in_txn(wtxn, inserted_entries.len())
            .map_err(map_heed_error)?;

        for (hash, data_len) in inserted_entries {
            let meta = Self::encode_blob_meta(BlobMeta {
                order,
                size: *data_len,
                last_accessed_at: now,
            });
            let order_key = Self::encode_order_key(order, hash);
            order = order.saturating_add(1);
            self.metadata
                .put(wtxn, hash, &meta)
                .map_err(map_heed_error)?;
            self.eviction_order
                .put(wtxn, &order_key, &())
                .map_err(map_heed_error)?;
            inserted_bytes = inserted_bytes.saturating_add(*data_len);
            if self
                .read_pin_count_lossy(wtxn, hash)
                .map_err(map_heed_error)?
                > 0
            {
                inserted_pinned = inserted_pinned.saturating_add(1);
                inserted_pinned_bytes = inserted_pinned_bytes.saturating_add(*data_len);
            }
        }

        self.increment_totals_in_txn(
            wtxn,
            inserted_entries.len() as u64,
            inserted_bytes,
            inserted_pinned,
            inserted_pinned_bytes,
        )
        .map_err(map_heed_error)?;
        Ok(())
    }

    fn put_sync_attempt(&self, hash: Hash, data: &[u8]) -> Result<bool, StoreError> {
        let mut wtxn = self.env.write_txn().map_err(map_heed_error)?;
        let external_config = self
            .external_blobs
            .as_ref()
            .filter(|config| data.len() >= config.min_bytes);
        let external_marker = external_config.map(|_| Self::external_blob_ref(&hash));
        let value = external_marker.as_deref().unwrap_or(data);

        match self
            .blobs
            .put_with_flags(&mut wtxn, PutFlags::NO_OVERWRITE, &hash, value)
        {
            Ok(()) => {}
            Err(HeedError::Mdb(MdbError::KeyExist)) => return Ok(false),
            Err(err) => return Err(map_heed_error(err)),
        }

        if let Some(config) = external_config {
            self.write_external_blob(&hash, data, config)?;
        }
        self.write_metadata_for_inserted_blobs(&mut wtxn, &[(hash, data.len() as u64)])?;
        wtxn.commit().map_err(map_heed_error)?;
        Ok(true)
    }

    fn put_many_sync_attempt(
        &self,
        total: usize,
        items: &[(Hash, &[u8])],
    ) -> Result<PutManyReport, StoreError> {
        let mut wtxn = self.env.write_txn().map_err(map_heed_error)?;
        let mut report = PutManyReport {
            total,
            ..PutManyReport::default()
        };
        let mut inserted_entries: Vec<(Hash, u64)> = Vec::new();
        let mut external_blobs: Vec<(Hash, &[u8])> = Vec::new();
        let mut external_pack_entries: Vec<(usize, Hash, &[u8])> = Vec::new();
        let external_config = self.external_blobs.as_ref();

        for (hash, data) in items {
            let external = external_config.filter(|config| data.len() >= config.min_bytes);
            let pack_external = external.is_some_and(|config| config.pack_target_bytes.is_some());
            let reserved_marker;
            let external_marker;
            let value = if pack_external {
                reserved_marker = Self::external_pack_reserved_ref(hash);
                reserved_marker.as_slice()
            } else if external.is_some() {
                external_marker = Self::external_blob_ref(hash);
                external_marker.as_slice()
            } else {
                *data
            };

            match self
                .blobs
                .put_with_flags(&mut wtxn, PutFlags::NO_OVERWRITE, hash, value)
            {
                Ok(()) => {}
                Err(HeedError::Mdb(MdbError::KeyExist)) => continue,
                Err(err) => return Err(map_heed_error(err)),
            }

            let inserted_index = inserted_entries.len();
            let data_len = data.len() as u64;
            inserted_entries.push((*hash, data_len));
            report.inserted = report.inserted.saturating_add(1);
            report.inserted_bytes = report.inserted_bytes.saturating_add(data_len);
            report.inserted_hashes.push(*hash);

            if pack_external {
                external_pack_entries.push((inserted_index, *hash, *data));
            } else if external.is_some() {
                external_blobs.push((*hash, *data));
            }
        }

        if inserted_entries.is_empty() {
            return Ok(report);
        }

        if let Some(config) = external_config {
            for (hash, data) in external_blobs {
                self.write_external_blob(&hash, data, config)?;
            }
            if let Some(pack_target_bytes) = config.pack_target_bytes {
                for (inserted_index, marker) in self.write_external_blob_packs(
                    &external_pack_entries,
                    config,
                    pack_target_bytes,
                )? {
                    let hash = inserted_entries[inserted_index].0;
                    self.blobs
                        .put(&mut wtxn, &hash, &marker)
                        .map_err(map_heed_error)?;
                }
            }
        }

        self.write_metadata_for_inserted_blobs(&mut wtxn, &inserted_entries)?;

        wtxn.commit().map_err(map_heed_error)?;
        Ok(report)
    }

    /// Get storage statistics.
    pub fn stats(&self) -> Result<LmdbStats, StoreError> {
        let rtxn = self
            .env
            .read_txn()
            .map_err(|e| StoreError::Other(e.to_string()))?;
        let totals = Self::read_store_totals(self.stats, &rtxn)?.unwrap_or_default();

        Ok(LmdbStats {
            count: totals.count as usize,
            total_bytes: totals.total_bytes,
            pinned_count: totals.pinned_count as usize,
            pinned_bytes: totals.pinned_bytes,
        })
    }

    /// List all hashes in the store.
    pub fn list(&self) -> Result<Vec<Hash>, StoreError> {
        let rtxn = self
            .env
            .read_txn()
            .map_err(|e| StoreError::Other(e.to_string()))?;

        let mut hashes = Vec::new();
        for item in self.metadata.iter(&rtxn).map_err(map_heed_error)? {
            let (hash, _) = item.map_err(|e| StoreError::Other(e.to_string()))?;
            let hash_arr: Hash = hash
                .try_into()
                .map_err(|_| StoreError::Other("invalid hash length".into()))?;
            hashes.push(hash_arr);
        }

        if hashes.is_empty() {
            for item in self
                .blobs
                .iter(&rtxn)
                .map_err(|e| StoreError::Other(e.to_string()))?
            {
                let (hash, _) = item.map_err(|e| StoreError::Other(e.to_string()))?;
                let hash_arr: Hash = hash
                    .try_into()
                    .map_err(|_| StoreError::Other("invalid hash length".into()))?;
                hashes.push(hash_arr);
            }
        }

        Ok(hashes)
    }

    /// Sync put operation (for use in sync contexts).
    pub fn put_sync(&self, hash: Hash, data: &[u8]) -> Result<bool, StoreError> {
        let incoming_bytes = data.len() as u64;

        let mut retried_after_eviction = false;
        loop {
            match self.put_sync_attempt(hash, data) {
                Ok(inserted) => {
                    if inserted {
                        self.enforce_max_bytes_after_insert(incoming_bytes)?;
                    }
                    return Ok(inserted);
                }
                Err(err) if is_map_full_store_error(&err) && !retried_after_eviction => {
                    let freed = self.evict_for_write_pressure(incoming_bytes)?;
                    if freed == 0 {
                        return Err(err);
                    }
                    retried_after_eviction = true;
                }
                Err(err) => return Err(err),
            }
        }
    }

    /// Sync batch put operation with exact insert accounting.
    pub fn put_many_report_sync(
        &self,
        items: &[(Hash, Vec<u8>)],
    ) -> Result<PutManyReport, StoreError> {
        let total = items.len();
        if items.is_empty() {
            return Ok(PutManyReport::default());
        }

        let mut seen_missing = HashSet::new();
        let write_items = items
            .iter()
            .filter_map(|(hash, data)| {
                if !seen_missing.insert(*hash) {
                    None
                } else {
                    Some((*hash, data.as_slice()))
                }
            })
            .collect::<Vec<_>>();
        let incoming_bytes = write_items
            .iter()
            .map(|(_, data)| data.len() as u64)
            .fold(0u64, |total, size| total.saturating_add(size));

        let mut retried_after_eviction = false;
        loop {
            match self.put_many_sync_attempt(total, &write_items) {
                Ok(report) => {
                    if report.inserted_bytes > 0 {
                        self.enforce_max_bytes_after_insert(report.inserted_bytes)?;
                    }
                    return Ok(report);
                }
                Err(err) if is_map_full_store_error(&err) && !retried_after_eviction => {
                    let freed = self.evict_for_write_pressure(incoming_bytes)?;
                    if freed == 0 {
                        return Err(err);
                    }
                    retried_after_eviction = true;
                }
                Err(err) => return Err(err),
            }
        }
    }

    /// Sync batch put operation (for use in sync contexts).
    pub fn put_many_sync(&self, items: &[(Hash, Vec<u8>)]) -> Result<usize, StoreError> {
        self.put_many_report_sync(items)
            .map(|report| report.inserted)
    }

    /// Sync get operation (for use in sync contexts).
    pub fn get_sync(&self, hash: &Hash) -> Result<Option<Vec<u8>>, StoreError> {
        let rtxn = self
            .env
            .read_txn()
            .map_err(|e| StoreError::Other(e.to_string()))?;

        let Some(blob) = self
            .blobs
            .get(&rtxn, hash)
            .map_err(|e| StoreError::Other(e.to_string()))?
        else {
            return Ok(None);
        };

        self.decode_blob_value(hash, blob).map(Some)
    }

    pub fn get_range_sync(
        &self,
        hash: &Hash,
        start: u64,
        end_inclusive: u64,
    ) -> Result<Option<Vec<u8>>, StoreError> {
        let rtxn = self
            .env
            .read_txn()
            .map_err(|e| StoreError::Other(e.to_string()))?;
        let Some(blob) = self
            .blobs
            .get(&rtxn, hash)
            .map_err(|e| StoreError::Other(e.to_string()))?
        else {
            return Ok(None);
        };

        if self.is_external_blob_ref(hash, blob) {
            return self.read_external_blob_range(hash, start, end_inclusive);
        }
        if let Some(pack_ref) = Self::decode_external_pack_ref(blob)? {
            return self.read_external_pack_range(&pack_ref, start, end_inclusive);
        }

        if blob.is_empty() || end_inclusive < start {
            return Ok(Some(Vec::new()));
        }
        let len = blob.len() as u64;
        if start >= len {
            return Ok(Some(Vec::new()));
        }

        let actual_end = end_inclusive.min(len - 1);
        let start = usize::try_from(start)
            .map_err(|_| StoreError::Other("blob range start is too large".to_string()))?;
        let end_exclusive = usize::try_from(actual_end.saturating_add(1))
            .map_err(|_| StoreError::Other("blob range end is too large".to_string()))?;
        Ok(Some(blob[start..end_exclusive].to_vec()))
    }

    pub fn touch_accessed_sync(&self, hash: &Hash, now: u64) -> Result<bool, StoreError> {
        self.touch_many_accessed_sync(std::slice::from_ref(hash), now)
            .map(|updated| updated > 0)
    }

    pub fn touch_many_accessed_sync(&self, hashes: &[Hash], now: u64) -> Result<usize, StoreError> {
        if hashes.is_empty() {
            return Ok(0);
        }

        let mut wtxn = self
            .env
            .write_txn()
            .map_err(|e| StoreError::Other(e.to_string()))?;
        let mut to_touch = Vec::new();

        for hash in hashes {
            let meta = self
                .metadata
                .get(&wtxn, hash)
                .map_err(|e| StoreError::Other(e.to_string()))?
                .map(Self::decode_blob_meta)
                .transpose()?;
            let Some(meta) = meta else {
                continue;
            };

            if meta.last_accessed_at >= now {
                continue;
            }
            to_touch.push((*hash, meta));
        }

        let updated = to_touch.len();
        if updated == 0 {
            wtxn.commit()
                .map_err(|e| StoreError::Other(e.to_string()))?;
            return Ok(0);
        }

        let mut order = self
            .allocate_order_range_in_txn(&mut wtxn, updated)
            .map_err(|e| StoreError::Other(e.to_string()))?;

        for (hash, mut meta) in to_touch {
            let old_order_key = Self::encode_order_key(meta.order, &hash);
            let _ = self
                .eviction_order
                .delete(&mut wtxn, &old_order_key)
                .map_err(|e| StoreError::Other(e.to_string()))?;

            meta.order = order;
            order = order.saturating_add(1);
            meta.last_accessed_at = now;
            let meta_bytes = Self::encode_blob_meta(meta);
            let new_order_key = Self::encode_order_key(meta.order, &hash);
            self.metadata
                .put(&mut wtxn, &hash, &meta_bytes)
                .map_err(|e| StoreError::Other(e.to_string()))?;
            self.eviction_order
                .put(&mut wtxn, &new_order_key, &())
                .map_err(|e| StoreError::Other(e.to_string()))?;
        }

        wtxn.commit()
            .map_err(|e| StoreError::Other(e.to_string()))?;
        Ok(updated)
    }

    pub fn last_accessed_at_sync(&self, hash: &Hash) -> Result<Option<u64>, StoreError> {
        let rtxn = self
            .env
            .read_txn()
            .map_err(|e| StoreError::Other(e.to_string()))?;
        self.metadata
            .get(&rtxn, hash)
            .map_err(|e| StoreError::Other(e.to_string()))?
            .map(Self::decode_blob_meta)
            .transpose()
            .map(|meta| meta.map(|meta| meta.last_accessed_at))
    }

    pub fn many_last_accessed_at_sync(
        &self,
        hashes: &[Hash],
    ) -> Result<Vec<(Hash, u64)>, StoreError> {
        let rtxn = self
            .env
            .read_txn()
            .map_err(|e| StoreError::Other(e.to_string()))?;
        let mut results = Vec::new();
        for hash in hashes {
            let Some(meta) = self
                .metadata
                .get(&rtxn, hash)
                .map_err(|e| StoreError::Other(e.to_string()))?
                .map(Self::decode_blob_meta)
                .transpose()?
            else {
                continue;
            };
            results.push((*hash, meta.last_accessed_at));
        }
        Ok(results)
    }

    /// Sync delete operation (for use in sync contexts).
    pub fn delete_sync(&self, hash: &Hash) -> Result<bool, StoreError> {
        let mut wtxn = self
            .env
            .write_txn()
            .map_err(|e| StoreError::Other(e.to_string()))?;
        let external_path = self.external_blob_path_in_txn(&wtxn, hash)?;
        let (existed, _) = self.delete_blob_in_txn(&mut wtxn, hash)?;

        wtxn.commit()
            .map_err(|e| StoreError::Other(e.to_string()))?;
        if existed {
            self.remove_external_blob_file(external_path);
        }

        Ok(existed)
    }

    fn pin_sync(&self, hash: &Hash) -> Result<(), StoreError> {
        let mut wtxn = self
            .env
            .write_txn()
            .map_err(|e| StoreError::Other(e.to_string()))?;
        let previous = self.read_pin_count(&wtxn, hash)?;
        let count = previous.saturating_add(1);
        let encoded = count.to_be_bytes();
        self.pins
            .put(&mut wtxn, hash, &encoded)
            .map_err(|e| StoreError::Other(e.to_string()))?;
        if previous == 0 {
            if let Some(size) = self.blob_size_in_txn(&wtxn, hash)? {
                self.increment_totals_in_txn(&mut wtxn, 0, 0, 1, size)
                    .map_err(|e| StoreError::Other(e.to_string()))?;
            }
        }
        wtxn.commit()
            .map_err(|e| StoreError::Other(e.to_string()))?;
        Ok(())
    }

    fn unpin_sync(&self, hash: &Hash) -> Result<(), StoreError> {
        let mut wtxn = self
            .env
            .write_txn()
            .map_err(|e| StoreError::Other(e.to_string()))?;
        let count = self.read_pin_count(&wtxn, hash)?;
        if count == 1 {
            if let Some(size) = self.blob_size_in_txn(&wtxn, hash)? {
                self.decrement_totals_in_txn(&mut wtxn, 0, 0, 1, size)
                    .map_err(|e| StoreError::Other(e.to_string()))?;
            }
        }
        if count <= 1 {
            let _ = self
                .pins
                .delete(&mut wtxn, hash)
                .map_err(|e| StoreError::Other(e.to_string()))?;
        } else {
            let encoded = (count - 1).to_be_bytes();
            self.pins
                .put(&mut wtxn, hash, &encoded)
                .map_err(|e| StoreError::Other(e.to_string()))?;
        }
        wtxn.commit()
            .map_err(|e| StoreError::Other(e.to_string()))?;
        Ok(())
    }

    fn evict_to_target(&self, current_bytes: u64, target_bytes: u64) -> Result<u64, StoreError> {
        if current_bytes <= target_bytes {
            return Ok(0);
        }

        let rtxn = self
            .env
            .read_txn()
            .map_err(|e| StoreError::Other(e.to_string()))?;
        let order_keys: Vec<Vec<u8>> = self
            .eviction_order
            .iter(&rtxn)
            .map_err(|e| StoreError::Other(e.to_string()))?
            .map(|item| {
                item.map(|(key, _)| key.to_vec())
                    .map_err(|e| StoreError::Other(e.to_string()))
            })
            .collect::<Result<_, _>>()?;
        drop(rtxn);

        let mut freed_total = 0u64;
        let to_free = current_bytes - target_bytes;
        let mut index = 0usize;

        while freed_total < to_free && index < order_keys.len() {
            let mut wtxn = self
                .env
                .write_txn()
                .map_err(|e| StoreError::Other(e.to_string()))?;
            let mut batch_freed = 0u64;
            let mut batch_items = 0usize;
            let mut batch_external_paths = Vec::new();

            while freed_total + batch_freed < to_free && index < order_keys.len() {
                let order_key = &order_keys[index];
                index += 1;

                let hash = Self::decode_hash_from_order_key(order_key)?;
                if self.read_pin_count(&wtxn, &hash)? > 0 {
                    continue;
                }

                let external_path = self.external_blob_path_in_txn(&wtxn, &hash)?;
                let (_, bytes_freed) = self.delete_blob_in_txn(&mut wtxn, &hash)?;
                if bytes_freed == 0 {
                    let _ = self
                        .eviction_order
                        .delete(&mut wtxn, order_key)
                        .map_err(|e| StoreError::Other(e.to_string()))?;
                    continue;
                }

                batch_freed = batch_freed.saturating_add(bytes_freed);
                batch_items += 1;
                if let Some(path) = external_path {
                    batch_external_paths.push(path);
                }

                if batch_freed >= EVICTION_BATCH_TARGET_BYTES
                    || batch_items >= EVICTION_BATCH_MAX_ITEMS
                {
                    break;
                }
            }

            wtxn.commit()
                .map_err(|e| StoreError::Other(e.to_string()))?;
            for path in batch_external_paths {
                let _ = fs::remove_file(path);
            }
            if batch_freed > 0 {
                freed_total = freed_total.saturating_add(batch_freed);
            }
        }

        Ok(freed_total)
    }

    fn delete_blob_in_txn(
        &self,
        wtxn: &mut heed::RwTxn,
        hash: &Hash,
    ) -> Result<(bool, u64), StoreError> {
        let pin_count = self.read_pin_count(wtxn, hash)?;
        let data_len = self
            .blobs
            .get(wtxn, hash)
            .map_err(|e| StoreError::Other(e.to_string()))?
            .map(|data| data.len() as u64);
        let meta = self
            .metadata
            .get(wtxn, hash)
            .map_err(|e| StoreError::Other(e.to_string()))?
            .map(Self::decode_blob_meta)
            .transpose()?;

        let existed = self
            .blobs
            .delete(wtxn, hash)
            .map_err(|e| StoreError::Other(e.to_string()))?;
        let _ = self
            .metadata
            .delete(wtxn, hash)
            .map_err(|e| StoreError::Other(e.to_string()))?;
        let _ = self
            .pins
            .delete(wtxn, hash)
            .map_err(|e| StoreError::Other(e.to_string()))?;
        if let Some(meta) = meta {
            let order_key = Self::encode_order_key(meta.order, hash);
            let _ = self
                .eviction_order
                .delete(wtxn, &order_key)
                .map_err(|e| StoreError::Other(e.to_string()))?;
        }
        let bytes_freed = meta.map(|m| m.size).or(data_len).unwrap_or(0);
        if existed || meta.is_some() {
            self.decrement_totals_in_txn(
                wtxn,
                1,
                bytes_freed,
                u64::from(pin_count > 0),
                if pin_count > 0 { bytes_freed } else { 0 },
            )
            .map_err(|e| StoreError::Other(e.to_string()))?;
        }

        Ok((existed || meta.is_some(), bytes_freed))
    }

    fn blob_size_in_txn(&self, txn: &heed::RoTxn, hash: &Hash) -> Result<Option<u64>, StoreError> {
        if let Some(meta) = self
            .metadata
            .get(txn, hash)
            .map_err(|e| StoreError::Other(e.to_string()))?
            .map(Self::decode_blob_meta)
            .transpose()?
        {
            return Ok(Some(meta.size));
        }

        self.blobs
            .get(txn, hash)
            .map_err(|e| StoreError::Other(e.to_string()))
            .map(|data| data.map(|bytes| bytes.len() as u64))
    }

    fn read_pin_count(&self, txn: &heed::RoTxn, hash: &[u8]) -> Result<u32, StoreError> {
        self.pins
            .get(txn, hash)
            .map_err(|e| StoreError::Other(e.to_string()))?
            .map(Self::decode_pin_count)
            .transpose()?
            .map_or(Ok(0), Ok)
    }

    fn read_pin_count_lossy(
        &self,
        txn: &heed::RoTxn,
        hash: &[u8],
    ) -> std::result::Result<u32, HeedError> {
        Ok(self
            .pins
            .get(txn, hash)?
            .and_then(Self::decode_pin_count_lossy)
            .unwrap_or(0))
    }

    fn encode_blob_meta(meta: BlobMeta) -> [u8; BLOB_META_BYTES] {
        let mut encoded = [0u8; BLOB_META_BYTES];
        encoded[..8].copy_from_slice(&meta.order.to_be_bytes());
        encoded[8..16].copy_from_slice(&meta.size.to_be_bytes());
        encoded[16..].copy_from_slice(&meta.last_accessed_at.to_be_bytes());
        encoded
    }

    fn decode_blob_meta(bytes: &[u8]) -> Result<BlobMeta, StoreError> {
        if bytes.len() != LEGACY_BLOB_META_BYTES && bytes.len() != BLOB_META_BYTES {
            return Err(StoreError::Other(format!(
                "invalid blob metadata length: {}",
                bytes.len()
            )));
        }
        Ok(BlobMeta {
            order: Self::decode_order(&bytes[..8])?,
            size: u64::from_be_bytes(
                bytes[8..16]
                    .try_into()
                    .map_err(|_| StoreError::Other("invalid blob size bytes".into()))?,
            ),
            last_accessed_at: if bytes.len() >= BLOB_META_BYTES {
                u64::from_be_bytes(
                    bytes[16..24]
                        .try_into()
                        .map_err(|_| StoreError::Other("invalid blob access time bytes".into()))?,
                )
            } else {
                0
            },
        })
    }

    fn encode_store_totals(totals: StoreTotals) -> [u8; STORE_TOTALS_BYTES] {
        let mut encoded = [0u8; STORE_TOTALS_BYTES];
        encoded[0..8].copy_from_slice(&totals.count.to_be_bytes());
        encoded[8..16].copy_from_slice(&totals.total_bytes.to_be_bytes());
        encoded[16..24].copy_from_slice(&totals.pinned_count.to_be_bytes());
        encoded[24..32].copy_from_slice(&totals.pinned_bytes.to_be_bytes());
        encoded
    }

    fn decode_store_totals(bytes: &[u8]) -> Result<StoreTotals, StoreError> {
        Self::decode_store_totals_lossy(bytes).ok_or_else(|| {
            StoreError::Other(format!("invalid store totals length: {}", bytes.len()))
        })
    }

    fn decode_store_totals_lossy(bytes: &[u8]) -> Option<StoreTotals> {
        if bytes.len() != STORE_TOTALS_BYTES {
            return None;
        }
        Some(StoreTotals {
            count: u64::from_be_bytes(bytes[0..8].try_into().ok()?),
            total_bytes: u64::from_be_bytes(bytes[8..16].try_into().ok()?),
            pinned_count: u64::from_be_bytes(bytes[16..24].try_into().ok()?),
            pinned_bytes: u64::from_be_bytes(bytes[24..32].try_into().ok()?),
        })
    }

    fn encode_next_order(next_order: u64) -> [u8; NEXT_ORDER_BYTES] {
        next_order.to_be_bytes()
    }

    fn decode_next_order(bytes: &[u8]) -> Result<u64, StoreError> {
        Self::decode_next_order_lossy(bytes)
            .ok_or_else(|| StoreError::Other(format!("invalid next order length: {}", bytes.len())))
    }

    fn decode_next_order_lossy(bytes: &[u8]) -> Option<u64> {
        if bytes.len() != NEXT_ORDER_BYTES {
            return None;
        }
        Some(u64::from_be_bytes(bytes.try_into().ok()?))
    }

    fn encode_order_key(order: u64, hash: &Hash) -> [u8; ORDER_KEY_BYTES] {
        let mut key = [0u8; ORDER_KEY_BYTES];
        key[..8].copy_from_slice(&order.to_be_bytes());
        key[8..].copy_from_slice(hash);
        key
    }

    fn decode_order(bytes: &[u8]) -> Result<u64, StoreError> {
        if bytes.len() != 8 {
            return Err(StoreError::Other(format!(
                "invalid order length: {}",
                bytes.len()
            )));
        }
        Ok(u64::from_be_bytes(bytes.try_into().map_err(|_| {
            StoreError::Other("invalid order bytes".into())
        })?))
    }

    fn decode_hash_from_order_key(bytes: &[u8]) -> Result<Hash, StoreError> {
        if bytes.len() != ORDER_KEY_BYTES {
            return Err(StoreError::Other(format!(
                "invalid order key length: {}",
                bytes.len()
            )));
        }
        let mut hash = [0u8; 32];
        hash.copy_from_slice(&bytes[8..]);
        Ok(hash)
    }

    fn decode_pin_count(bytes: &[u8]) -> Result<u32, StoreError> {
        Self::decode_pin_count_lossy(bytes)
            .ok_or_else(|| StoreError::Other(format!("invalid pin count length: {}", bytes.len())))
    }

    fn decode_pin_count_lossy(bytes: &[u8]) -> Option<u32> {
        if bytes.len() != PIN_COUNT_BYTES {
            return None;
        }
        Some(u32::from_be_bytes(bytes.try_into().ok()?))
    }

    fn external_blob_ref(hash: &Hash) -> Vec<u8> {
        let mut marker = Vec::with_capacity(EXTERNAL_BLOB_MARKER_PREFIX.len() + hash.len());
        marker.extend_from_slice(EXTERNAL_BLOB_MARKER_PREFIX);
        marker.extend_from_slice(hash);
        marker
    }

    fn external_pack_reserved_ref(hash: &Hash) -> Vec<u8> {
        let mut marker =
            Vec::with_capacity(EXTERNAL_PACK_RESERVED_MARKER_PREFIX.len() + hash.len());
        marker.extend_from_slice(EXTERNAL_PACK_RESERVED_MARKER_PREFIX);
        marker.extend_from_slice(hash);
        marker
    }

    fn external_pack_blob_ref(
        pack_name: &str,
        offset: u64,
        len: u64,
    ) -> Result<Vec<u8>, StoreError> {
        let name_len = u16::try_from(pack_name.len())
            .map_err(|_| StoreError::Other("external pack file name is too long".to_string()))?;
        let mut marker =
            Vec::with_capacity(EXTERNAL_PACK_MARKER_PREFIX.len() + 2 + pack_name.len() + 16);
        marker.extend_from_slice(EXTERNAL_PACK_MARKER_PREFIX);
        marker.extend_from_slice(&name_len.to_be_bytes());
        marker.extend_from_slice(pack_name.as_bytes());
        marker.extend_from_slice(&offset.to_be_bytes());
        marker.extend_from_slice(&len.to_be_bytes());
        Ok(marker)
    }

    fn is_external_blob_ref(&self, hash: &Hash, value: &[u8]) -> bool {
        value.len() == EXTERNAL_BLOB_MARKER_PREFIX.len() + hash.len()
            && value.starts_with(EXTERNAL_BLOB_MARKER_PREFIX)
            && &value[EXTERNAL_BLOB_MARKER_PREFIX.len()..] == hash
    }

    fn decode_external_pack_ref(value: &[u8]) -> Result<Option<ExternalPackRef>, StoreError> {
        if !value.starts_with(EXTERNAL_PACK_MARKER_PREFIX) {
            return Ok(None);
        }

        let rest = &value[EXTERNAL_PACK_MARKER_PREFIX.len()..];
        if rest.len() < 2 + 8 + 8 {
            return Err(StoreError::Other(
                "invalid external LMDB pack marker length".to_string(),
            ));
        }
        let name_len = u16::from_be_bytes(
            rest[..2]
                .try_into()
                .map_err(|_| StoreError::Other("invalid external pack name length".into()))?,
        ) as usize;
        let expected_len = 2usize.saturating_add(name_len).saturating_add(16);
        if rest.len() != expected_len {
            return Err(StoreError::Other(format!(
                "invalid external LMDB pack marker length: {}",
                value.len()
            )));
        }
        let name_bytes = &rest[2..2 + name_len];
        let name = std::str::from_utf8(name_bytes)
            .map_err(|_| StoreError::Other("external pack name is not UTF-8".into()))?;
        if name.len() < 2
            || name.contains('/')
            || name.contains('\\')
            || name.starts_with('.')
            || name.contains("..")
        {
            return Err(StoreError::Other(
                "external pack name is not a safe relative file name".into(),
            ));
        }
        let offset_start = 2 + name_len;
        let offset = u64::from_be_bytes(
            rest[offset_start..offset_start + 8]
                .try_into()
                .map_err(|_| StoreError::Other("invalid external pack offset".into()))?,
        );
        let len = u64::from_be_bytes(
            rest[offset_start + 8..offset_start + 16]
                .try_into()
                .map_err(|_| StoreError::Other("invalid external pack length".into()))?,
        );
        Ok(Some(ExternalPackRef {
            name: name.to_string(),
            offset,
            len,
        }))
    }

    fn external_blob_path_for_config(config: &ExternalBlobConfig, hash: &Hash) -> PathBuf {
        let hex = to_hex(hash);
        config
            .base_path
            .join(&hex[..2])
            .join(&hex[2..4])
            .join(&hex[4..])
    }

    fn external_blob_path(&self, hash: &Hash) -> Option<PathBuf> {
        self.external_blobs
            .as_ref()
            .map(|config| Self::external_blob_path_for_config(config, hash))
    }

    fn external_pack_path_for_config(config: &ExternalBlobConfig, pack_name: &str) -> PathBuf {
        config
            .base_path
            .join("packs")
            .join(&pack_name[..2])
            .join(pack_name)
    }

    fn external_pack_path(&self, pack_ref: &ExternalPackRef) -> Option<PathBuf> {
        self.external_blobs
            .as_ref()
            .map(|config| Self::external_pack_path_for_config(config, &pack_ref.name))
    }

    fn external_pack_name(first_hash: &Hash) -> String {
        let hash_hex = to_hex(first_hash);
        let nanos = SystemTime::now()
            .duration_since(UNIX_EPOCH)
            .unwrap_or_default()
            .as_nanos();
        let counter = EXTERNAL_PACK_COUNTER.fetch_add(1, Ordering::Relaxed);
        format!(
            "{}-{:032x}-{}-{:016x}.pack",
            &hash_hex[..12],
            nanos,
            std::process::id(),
            counter
        )
    }

    fn write_external_blob(
        &self,
        hash: &Hash,
        data: &[u8],
        config: &ExternalBlobConfig,
    ) -> Result<(), StoreError> {
        let path = Self::external_blob_path_for_config(config, hash);
        if path.exists() {
            return Ok(());
        }

        let parent = path
            .parent()
            .ok_or_else(|| StoreError::Other("external blob path has no parent".to_string()))?;
        fs::create_dir_all(parent)?;
        let temp_path = unique_temp_path(&path);
        {
            let mut file = File::options()
                .write(true)
                .create_new(true)
                .open(&temp_path)?;
            file.write_all(data)?;
            if config.sync {
                file.sync_all()?;
            }
        }

        if let Err(error) = fs::rename(&temp_path, &path) {
            let _ = fs::remove_file(&temp_path);
            return Err(error.into());
        }
        if config.sync {
            File::open(parent)?.sync_all()?;
        }
        Ok(())
    }

    fn write_external_blob_packs(
        &self,
        entries: &[(usize, Hash, &[u8])],
        config: &ExternalBlobConfig,
        pack_target_bytes: usize,
    ) -> Result<Vec<(usize, Vec<u8>)>, StoreError> {
        let mut markers = Vec::with_capacity(entries.len());
        let mut pack_entries = Vec::new();
        let mut pack_bytes = 0usize;

        for entry in entries {
            let data_len = entry.2.len();
            let would_exceed_target =
                !pack_entries.is_empty() && pack_bytes.saturating_add(data_len) > pack_target_bytes;
            if would_exceed_target {
                markers.extend(self.write_external_blob_pack(&pack_entries, config)?);
                pack_entries.clear();
                pack_bytes = 0;
            }

            pack_entries.push(*entry);
            pack_bytes = pack_bytes.saturating_add(data_len);
        }

        if !pack_entries.is_empty() {
            markers.extend(self.write_external_blob_pack(&pack_entries, config)?);
        }

        Ok(markers)
    }

    fn write_external_blob_pack(
        &self,
        entries: &[(usize, Hash, &[u8])],
        config: &ExternalBlobConfig,
    ) -> Result<Vec<(usize, Vec<u8>)>, StoreError> {
        if entries.is_empty() {
            return Ok(Vec::new());
        }

        let pack_name = Self::external_pack_name(&entries[0].1);
        let path = Self::external_pack_path_for_config(config, &pack_name);
        let parent = path
            .parent()
            .ok_or_else(|| StoreError::Other("external pack path has no parent".to_string()))?;
        fs::create_dir_all(parent)?;
        let temp_path = unique_temp_path(&path);
        let mut markers = Vec::with_capacity(entries.len());
        let write_result = (|| -> Result<(), StoreError> {
            let mut file = File::options()
                .write(true)
                .create_new(true)
                .open(&temp_path)?;
            let mut offset = 0u64;
            for (index, _, data) in entries {
                let len = data.len() as u64;
                file.write_all(data)?;
                markers.push((
                    *index,
                    Self::external_pack_blob_ref(&pack_name, offset, len)?,
                ));
                offset = offset.saturating_add(len);
            }
            if config.sync {
                file.sync_all()?;
            }
            Ok(())
        })();

        if let Err(error) = write_result {
            let _ = fs::remove_file(&temp_path);
            return Err(error);
        }
        if let Err(error) = fs::rename(&temp_path, &path) {
            let _ = fs::remove_file(&temp_path);
            return Err(error.into());
        }
        if config.sync {
            File::open(parent)?.sync_all()?;
        }
        Ok(markers)
    }

    fn decode_blob_value(&self, hash: &Hash, value: &[u8]) -> Result<Vec<u8>, StoreError> {
        if self.is_external_blob_ref(hash, value) {
            let path = self.external_blob_path(hash).ok_or_else(|| {
                StoreError::Other(
                    "external LMDB blob marker found but external blobs are disabled".into(),
                )
            })?;
            return fs::read(path).map_err(StoreError::Io);
        }
        if let Some(pack_ref) = Self::decode_external_pack_ref(value)? {
            return self.read_external_pack_blob(&pack_ref);
        }
        Ok(value.to_vec())
    }

    fn read_external_blob_range(
        &self,
        hash: &Hash,
        start: u64,
        end_inclusive: u64,
    ) -> Result<Option<Vec<u8>>, StoreError> {
        let path = self.external_blob_path(hash).ok_or_else(|| {
            StoreError::Other(
                "external LMDB blob marker found but external blobs are disabled".into(),
            )
        })?;
        let mut file = File::open(path)?;
        let len = file.metadata()?.len();
        if len == 0 || start >= len || end_inclusive < start {
            return Ok(Some(Vec::new()));
        }

        let actual_end = end_inclusive.min(len - 1);
        let read_len = actual_end.saturating_sub(start).saturating_add(1);
        let read_len = usize::try_from(read_len)
            .map_err(|_| StoreError::Other("blob range is too large to read".to_string()))?;
        let mut data = vec![0; read_len];
        file.seek(SeekFrom::Start(start))?;
        file.read_exact(&mut data)?;
        Ok(Some(data))
    }

    fn read_external_pack_blob(&self, pack_ref: &ExternalPackRef) -> Result<Vec<u8>, StoreError> {
        let read_len = usize::try_from(pack_ref.len).map_err(|_| {
            StoreError::Other("external pack blob is too large to read".to_string())
        })?;
        let path = self.external_pack_path(pack_ref).ok_or_else(|| {
            StoreError::Other(
                "external LMDB pack marker found but external blobs are disabled".into(),
            )
        })?;
        let mut file = File::open(path)?;
        file.seek(SeekFrom::Start(pack_ref.offset))?;
        let mut data = vec![0; read_len];
        file.read_exact(&mut data)?;
        Ok(data)
    }

    fn read_external_pack_range(
        &self,
        pack_ref: &ExternalPackRef,
        start: u64,
        end_inclusive: u64,
    ) -> Result<Option<Vec<u8>>, StoreError> {
        if pack_ref.len == 0 || start >= pack_ref.len || end_inclusive < start {
            return Ok(Some(Vec::new()));
        }

        let actual_end = end_inclusive.min(pack_ref.len - 1);
        let read_len = actual_end.saturating_sub(start).saturating_add(1);
        let read_len = usize::try_from(read_len).map_err(|_| {
            StoreError::Other("external pack blob range is too large to read".to_string())
        })?;
        let path = self.external_pack_path(pack_ref).ok_or_else(|| {
            StoreError::Other(
                "external LMDB pack marker found but external blobs are disabled".into(),
            )
        })?;
        let mut file = File::open(path)?;
        file.seek(SeekFrom::Start(pack_ref.offset.saturating_add(start)))?;
        let mut data = vec![0; read_len];
        file.read_exact(&mut data)?;
        Ok(Some(data))
    }

    fn external_blob_path_for_value(
        &self,
        hash: &Hash,
        value: &[u8],
    ) -> Result<Option<PathBuf>, StoreError> {
        if self.is_external_blob_ref(hash, value) {
            return Ok(self.external_blob_path(hash));
        }
        if Self::decode_external_pack_ref(value)?.is_some() {
            return Ok(None);
        }
        Ok(None)
    }

    fn external_blob_path_in_txn(
        &self,
        txn: &heed::RoTxn,
        hash: &Hash,
    ) -> Result<Option<PathBuf>, StoreError> {
        let Some(value) = self
            .blobs
            .get(txn, hash)
            .map_err(|e| StoreError::Other(e.to_string()))?
        else {
            return Ok(None);
        };
        self.external_blob_path_for_value(hash, value)
    }

    fn remove_external_blob_file(&self, path: Option<PathBuf>) {
        if let Some(path) = path {
            let _ = fs::remove_file(path);
        }
    }
}

fn map_heed_error(error: HeedError) -> StoreError {
    match error {
        HeedError::Io(io_error) => StoreError::Io(io_error),
        other => StoreError::Other(other.to_string()),
    }
}

fn is_map_full_store_error(err: &StoreError) -> bool {
    let message = err.to_string();
    message.contains("MDB_MAP_FULL") || message.contains("MapFull")
}

fn env_bool(name: &str) -> Option<bool> {
    std::env::var(name).ok().and_then(|value| {
        let value = value.trim();
        if value == "1" || value.eq_ignore_ascii_case("true") || value.eq_ignore_ascii_case("yes") {
            Some(true)
        } else if value == "0"
            || value.eq_ignore_ascii_case("false")
            || value.eq_ignore_ascii_case("no")
        {
            Some(false)
        } else {
            None
        }
    })
}

fn env_flags_from_env() -> EnvFlags {
    env_flags_from_bools(
        env_bool(LMDB_NO_READ_AHEAD_ENV).unwrap_or(false),
        env_bool(LMDB_NO_SYNC_ENV).unwrap_or(false),
        env_bool(LMDB_NO_META_SYNC_ENV).unwrap_or(false),
    )
}

fn env_flags_from_bools(no_read_ahead: bool, no_sync: bool, no_meta_sync: bool) -> EnvFlags {
    let mut flags = EnvFlags::empty();
    if no_read_ahead {
        flags |= EnvFlags::NO_READ_AHEAD;
    }
    if no_sync {
        flags |= EnvFlags::NO_SYNC;
    }
    if no_meta_sync {
        flags |= EnvFlags::NO_META_SYNC;
    }
    flags
}

fn unique_temp_path(path: &Path) -> PathBuf {
    let file_name = path
        .file_name()
        .and_then(|name| name.to_str())
        .unwrap_or("blob");
    let nanos = SystemTime::now()
        .duration_since(UNIX_EPOCH)
        .unwrap_or_default()
        .as_nanos();
    path.with_file_name(format!(".{file_name}.tmp.{}.{}", std::process::id(), nanos))
}

fn unix_timestamp_now() -> u64 {
    SystemTime::now()
        .duration_since(UNIX_EPOCH)
        .unwrap_or_default()
        .as_secs()
}

#[derive(Debug, Clone)]
pub struct LmdbStats {
    pub count: usize,
    pub total_bytes: u64,
    pub pinned_count: usize,
    pub pinned_bytes: u64,
}

#[async_trait]
impl Store for LmdbBlobStore {
    async fn put(&self, hash: Hash, data: Vec<u8>) -> Result<bool, StoreError> {
        self.put_sync(hash, &data)
    }

    async fn put_many(&self, items: Vec<(Hash, Vec<u8>)>) -> Result<usize, StoreError> {
        self.put_many_sync(&items)
    }

    async fn get(&self, hash: &Hash) -> Result<Option<Vec<u8>>, StoreError> {
        self.get_sync(hash)
    }

    async fn get_range(
        &self,
        hash: &Hash,
        start: u64,
        end_inclusive: u64,
    ) -> Result<Option<Vec<u8>>, StoreError> {
        self.get_range_sync(hash, start, end_inclusive)
    }

    async fn blob_size(&self, hash: &Hash) -> Result<Option<u64>, StoreError> {
        self.blob_size_sync(hash)
    }

    async fn has(&self, hash: &Hash) -> Result<bool, StoreError> {
        self.exists(hash)
    }

    async fn delete(&self, hash: &Hash) -> Result<bool, StoreError> {
        self.delete_sync(hash)
    }

    fn set_max_bytes(&self, max: u64) {
        self.max_bytes.store(max, Ordering::Relaxed);
    }

    fn max_bytes(&self) -> Option<u64> {
        let max = self.max_bytes.load(Ordering::Relaxed);
        if max > 0 {
            Some(max)
        } else {
            None
        }
    }

    async fn stats(&self) -> StoreStats {
        match self.stats() {
            Ok(stats) => StoreStats {
                count: stats.count as u64,
                bytes: stats.total_bytes,
                pinned_count: stats.pinned_count as u64,
                pinned_bytes: stats.pinned_bytes,
            },
            Err(_) => StoreStats::default(),
        }
    }

    async fn evict_if_needed(&self) -> Result<u64, StoreError> {
        let max = self.max_bytes.load(Ordering::Relaxed);
        if max == 0 {
            return Ok(0);
        }

        let current = self.total_bytes()?;
        if current <= max {
            return Ok(0);
        }

        let target = max * 9 / 10;
        self.evict_to_target(current, target)
    }

    async fn pin(&self, hash: &Hash) -> Result<(), StoreError> {
        self.pin_sync(hash)
    }

    async fn unpin(&self, hash: &Hash) -> Result<(), StoreError> {
        self.unpin_sync(hash)
    }

    fn pin_count(&self, hash: &Hash) -> u32 {
        let Ok(rtxn) = self.env.read_txn() else {
            return 0;
        };
        self.read_pin_count(&rtxn, hash).unwrap_or(0)
    }
}

#[cfg(test)]
mod tests {
    use super::*;
    use hashtree_core::sha256;
    use heed::EnvOpenOptions;
    #[cfg(unix)]
    use std::path::{Path, PathBuf};
    #[cfg(unix)]
    use std::process::Command;
    use std::sync::{
        atomic::{AtomicBool, Ordering},
        Arc, Barrier,
    };
    use std::time::Duration;
    use tempfile::TempDir;

    #[cfg(unix)]
    const STALE_READER_HELPER_ENV: &str = "HASHTREE_LMDB_STALE_READER_HELPER";
    #[cfg(unix)]
    const STALE_READER_HELPER_MODE_ENV: &str = "HASHTREE_LMDB_STALE_READER_HELPER_MODE";
    #[cfg(unix)]
    const STALE_READER_DB_PATH_ENV: &str = "HASHTREE_LMDB_STALE_READER_DB_PATH";
    #[cfg(unix)]
    const STALE_READER_MARKER_PATH_ENV: &str = "HASHTREE_LMDB_STALE_READER_MARKER_PATH";
    #[cfg(unix)]
    const TEST_MAX_READERS: u32 = 4;

    fn count_files_under(path: &std::path::Path) -> std::io::Result<usize> {
        if !path.exists() {
            return Ok(0);
        }
        let mut count = 0usize;
        for entry in std::fs::read_dir(path)? {
            let entry = entry?;
            let metadata = entry.metadata()?;
            if metadata.is_dir() {
                count += count_files_under(&entry.path())?;
            } else if metadata.is_file() {
                count += 1;
            }
        }
        Ok(count)
    }

    fn persisted_next_order(store: &LmdbBlobStore) -> Result<u64, StoreError> {
        let rtxn = store.env.read_txn().map_err(map_heed_error)?;
        LmdbBlobStore::read_next_order(store.stats, &rtxn)?
            .ok_or_else(|| StoreError::Other("missing persisted next_order".to_string()))
    }

    fn metadata_order(store: &LmdbBlobStore, hash: &Hash) -> Result<u64, StoreError> {
        let rtxn = store.env.read_txn().map_err(map_heed_error)?;
        let meta = store
            .metadata
            .get(&rtxn, hash)
            .map_err(map_heed_error)?
            .ok_or_else(|| StoreError::Other("missing blob metadata".to_string()))?;
        LmdbBlobStore::decode_blob_meta(meta).map(|meta| meta.order)
    }

    fn delete_persisted_next_order(store: &LmdbBlobStore) -> Result<(), StoreError> {
        let mut wtxn = store.env.write_txn().map_err(map_heed_error)?;
        store
            .stats
            .delete(&mut wtxn, NEXT_ORDER_KEY)
            .map_err(map_heed_error)?;
        wtxn.commit().map_err(map_heed_error)
    }

    #[cfg(unix)]
    fn run_helper(mode: &str, path: &Path, marker: &Path) {
        let output = Command::new(std::env::current_exe().expect("test binary path"))
            .arg("--ignored")
            .arg("--exact")
            .arg("tests::lmdb_stale_reader_helper")
            .env(STALE_READER_HELPER_ENV, "1")
            .env(STALE_READER_HELPER_MODE_ENV, mode)
            .env(STALE_READER_DB_PATH_ENV, path)
            .env(STALE_READER_MARKER_PATH_ENV, marker)
            .env("RUST_TEST_THREADS", "1")
            .output()
            .expect("spawn stale-reader helper");

        assert!(
            output.status.success(),
            "stale-reader helper failed: stdout={} stderr={}",
            String::from_utf8_lossy(&output.stdout),
            String::from_utf8_lossy(&output.stderr)
        );
        assert!(
            marker.exists(),
            "stale-reader helper did not run: stdout={} stderr={}",
            String::from_utf8_lossy(&output.stdout),
            String::from_utf8_lossy(&output.stderr)
        );
    }

    #[test]
    fn env_flags_from_bools_enables_bulk_ingest_flags() {
        let flags = env_flags_from_bools(true, true, true);

        assert!(flags.contains(EnvFlags::NO_READ_AHEAD));
        assert!(flags.contains(EnvFlags::NO_SYNC));
        assert!(flags.contains(EnvFlags::NO_META_SYNC));
    }

    #[tokio::test]
    async fn test_put_get() -> Result<(), StoreError> {
        let temp = TempDir::new().unwrap();
        let store = LmdbBlobStore::new(temp.path().join("blobs"))?;

        let data = b"hello lmdb";
        let hash = sha256(data);
        store.put(hash, data.to_vec()).await?;

        assert!(store.has(&hash).await?);
        assert_eq!(store.get(&hash).await?, Some(data.to_vec()));

        Ok(())
    }

    #[test]
    fn external_blob_spill_keeps_large_values_out_of_lmdb() -> Result<(), StoreError> {
        let temp = TempDir::new().unwrap();
        let external_dir = temp.path().join("external");
        let store = LmdbBlobStore::with_map_size_and_settings(
            temp.path().join("blobs"),
            16 * 1024 * 1024,
            EnvFlags::empty(),
            |_| {
                Some(ExternalBlobConfig {
                    base_path: external_dir.clone(),
                    min_bytes: 8,
                    sync: false,
                    pack_target_bytes: None,
                })
            },
        )?;

        let small = b"tiny";
        let small_hash = sha256(small);
        assert!(store.put_sync(small_hash, small)?);

        let large = b"large external blob payload";
        let large_hash = sha256(large);
        assert!(store.put_sync(large_hash, large)?);

        assert_eq!(store.get_sync(&small_hash)?, Some(small.to_vec()));
        assert_eq!(store.get_sync(&large_hash)?, Some(large.to_vec()));
        assert_eq!(
            store.get_range_sync(&large_hash, 6, 13)?,
            Some(b"external".to_vec())
        );

        let rtxn = store.env.read_txn().map_err(map_heed_error)?;
        let inline_value = store
            .blobs
            .get(&rtxn, &small_hash)
            .map_err(map_heed_error)?
            .expect("small inline value");
        assert_eq!(inline_value, small);
        let external_value = store
            .blobs
            .get(&rtxn, &large_hash)
            .map_err(map_heed_error)?
            .expect("large marker value");
        assert!(store.is_external_blob_ref(&large_hash, external_value));
        drop(rtxn);

        let external_path = store.external_blob_path(&large_hash).unwrap();
        assert_eq!(std::fs::read(&external_path)?, large);
        let stats = store.stats()?;
        assert_eq!(stats.count, 2);
        assert_eq!(stats.total_bytes, (small.len() + large.len()) as u64);

        assert!(store.delete_sync(&large_hash)?);
        assert!(!external_path.exists());
        Ok(())
    }

    #[test]
    fn external_blob_pack_batches_large_values() -> Result<(), StoreError> {
        let temp = TempDir::new().unwrap();
        let external_dir = temp.path().join("external");
        let store = LmdbBlobStore::with_map_size_and_settings(
            temp.path().join("blobs"),
            16 * 1024 * 1024,
            EnvFlags::empty(),
            |_| {
                Some(ExternalBlobConfig {
                    base_path: external_dir.clone(),
                    min_bytes: 8,
                    sync: true,
                    pack_target_bytes: Some(1024 * 1024),
                })
            },
        )?;

        let first = b"first packed blob payload".to_vec();
        let second = b"second packed blob payload".to_vec();
        let tiny = b"tiny".to_vec();
        let first_hash = sha256(&first);
        let second_hash = sha256(&second);
        let tiny_hash = sha256(&tiny);
        let items = vec![
            (first_hash, first.clone()),
            (tiny_hash, tiny.clone()),
            (second_hash, second.clone()),
        ];

        assert_eq!(store.put_many_sync(&items)?, 3);
        assert_eq!(store.get_sync(&first_hash)?, Some(first.clone()));
        assert_eq!(store.get_sync(&second_hash)?, Some(second.clone()));
        assert_eq!(store.get_sync(&tiny_hash)?, Some(tiny.clone()));
        assert_eq!(
            store.get_range_sync(&second_hash, 7, 12)?,
            Some(b"packed".to_vec())
        );

        let rtxn = store.env.read_txn().map_err(map_heed_error)?;
        let first_value = store
            .blobs
            .get(&rtxn, &first_hash)
            .map_err(map_heed_error)?
            .expect("first pack marker");
        let second_value = store
            .blobs
            .get(&rtxn, &second_hash)
            .map_err(map_heed_error)?
            .expect("second pack marker");
        let tiny_value = store
            .blobs
            .get(&rtxn, &tiny_hash)
            .map_err(map_heed_error)?
            .expect("tiny inline value");
        let first_pack =
            LmdbBlobStore::decode_external_pack_ref(first_value)?.expect("first external pack ref");
        let second_pack = LmdbBlobStore::decode_external_pack_ref(second_value)?
            .expect("second external pack ref");
        assert_eq!(first_pack.name, second_pack.name);
        assert_eq!(tiny_value, tiny.as_slice());
        drop(rtxn);

        let pack_path = store.external_pack_path(&first_pack).unwrap();
        assert!(pack_path.exists());
        assert_eq!(
            std::fs::read(&pack_path)?,
            [first.as_slice(), second.as_slice()].concat()
        );
        let pack_count_after_first_write = count_files_under(&external_dir.join("packs"))?;

        assert_eq!(store.put_many_sync(&items)?, 0);
        assert_eq!(
            count_files_under(&external_dir.join("packs"))?,
            pack_count_after_first_write,
            "rewriting an already-present batch must not create orphan external pack files"
        );

        assert!(store.delete_sync(&first_hash)?);
        assert!(pack_path.exists());
        assert_eq!(store.get_sync(&second_hash)?, Some(second));
        Ok(())
    }

    #[tokio::test]
    async fn test_delete() -> Result<(), StoreError> {
        let temp = TempDir::new().unwrap();
        let store = LmdbBlobStore::new(temp.path().join("blobs"))?;

        let data = b"delete me";
        let hash = sha256(data);
        store.put(hash, data.to_vec()).await?;
        assert!(store.has(&hash).await?);

        assert!(store.delete(&hash).await?);
        assert!(!store.has(&hash).await?);
        assert!(!store.delete(&hash).await?);

        Ok(())
    }

    #[tokio::test]
    async fn test_list() -> Result<(), StoreError> {
        let temp = TempDir::new().unwrap();
        let store = LmdbBlobStore::new(temp.path().join("blobs"))?;

        let d1 = b"one";
        let d2 = b"two";
        let d3 = b"three";
        let h1 = sha256(d1);
        let h2 = sha256(d2);
        let h3 = sha256(d3);

        store.put(h1, d1.to_vec()).await?;
        store.put(h2, d2.to_vec()).await?;
        store.put(h3, d3.to_vec()).await?;

        let hashes = store.list()?;
        assert_eq!(hashes.len(), 3);
        assert!(hashes.contains(&h1));
        assert!(hashes.contains(&h2));
        assert!(hashes.contains(&h3));

        Ok(())
    }

    #[test]
    fn existing_hashes_in_sorted_candidates_marks_present_hashes() -> Result<(), StoreError> {
        let temp = TempDir::new().unwrap();
        let store = LmdbBlobStore::new(temp.path().join("blobs"))?;

        let h1 = sha256(b"range present one");
        let h2 = sha256(b"range present two");
        let missing = sha256(b"range missing");
        store.put_sync(h1, b"range present one")?;
        store.put_sync(h2, b"range present two")?;

        let mut candidates = vec![missing, h2, h1, h1];
        candidates.sort_unstable();
        let existing = store.existing_hashes_in_sorted_candidates(&candidates)?;

        assert_eq!(existing.len(), candidates.len());
        for (hash, exists) in candidates.iter().zip(existing) {
            assert_eq!(exists, *hash == h1 || *hash == h2);
        }

        Ok(())
    }

    #[tokio::test]
    async fn test_blob_last_accessed_persists_and_updates_eviction_order() -> Result<(), StoreError>
    {
        let temp = TempDir::new().unwrap();
        let store = LmdbBlobStore::new(temp.path().join("blobs"))?;

        let first = sha256(b"first");
        let second = sha256(b"second");
        store.put(first, b"first".to_vec()).await?;
        store.put(second, b"second".to_vec()).await?;

        assert!(store.last_accessed_at_sync(&first)?.unwrap_or(0) > 0);
        let access_time = unix_timestamp_now().saturating_add(1000);
        store.touch_accessed_sync(&first, access_time)?;

        assert_eq!(store.last_accessed_at_sync(&first)?, Some(access_time));
        assert!(store.delete_sync(&first)?);
        assert!(store.exists(&second)?);

        Ok(())
    }

    #[test]
    fn duplicate_put_is_noop_and_preserves_blob_last_accessed() -> Result<(), StoreError> {
        let temp = TempDir::new().unwrap();
        let store = LmdbBlobStore::new(temp.path().join("blobs"))?;

        let data = b"already here";
        let hash = sha256(data);
        assert!(store.put_sync(hash, data)?);
        let accessed = store.last_accessed_at_sync(&hash)?;
        let next_order = persisted_next_order(&store)?;

        assert!(!store.put_sync(hash, data)?);
        assert_eq!(store.last_accessed_at_sync(&hash)?, accessed);
        assert_eq!(persisted_next_order(&store)?, next_order);

        let stats = store.stats()?;
        assert_eq!(stats.count, 1);
        assert_eq!(stats.total_bytes, data.len() as u64);

        Ok(())
    }

    #[test]
    fn next_order_persists_across_reopen_and_counts_only_new_blobs() -> Result<(), StoreError> {
        let temp = TempDir::new().unwrap();
        let path = temp.path().join("blobs");
        let first = sha256(b"order first");
        let second = sha256(b"order second");
        let third = sha256(b"order third");

        {
            let store = LmdbBlobStore::new(&path)?;
            assert!(store.put_sync(first, b"order first")?);
            assert!(store.put_sync(second, b"order second")?);
            assert_eq!(metadata_order(&store, &first)?, 0);
            assert_eq!(metadata_order(&store, &second)?, 1);
            assert_eq!(persisted_next_order(&store)?, 2);

            assert!(!store.put_sync(first, b"order first")?);
            assert_eq!(
                store.put_many_sync(&[(second, b"order second".to_vec())])?,
                0
            );
            assert_eq!(persisted_next_order(&store)?, 2);
        }

        let reopened = LmdbBlobStore::new(&path)?;
        assert_eq!(persisted_next_order(&reopened)?, 2);
        assert!(reopened.put_sync(third, b"order third")?);
        assert_eq!(metadata_order(&reopened, &third)?, 2);
        assert_eq!(persisted_next_order(&reopened)?, 3);

        Ok(())
    }

    #[test]
    fn legacy_store_without_next_order_seeds_high_counter_without_tail_scan(
    ) -> Result<(), StoreError> {
        let temp = TempDir::new().unwrap();
        let path = temp.path().join("blobs");
        let existing = sha256(b"legacy existing");
        let new = sha256(b"legacy new");

        {
            let store = LmdbBlobStore::new(&path)?;
            assert!(store.put_sync(existing, b"legacy existing")?);
            delete_persisted_next_order(&store)?;
        }

        let reopened = LmdbBlobStore::new(&path)?;
        let migrated_next_order = persisted_next_order(&reopened)?;
        assert!(migrated_next_order >= NEXT_ORDER_MIGRATION_FLOOR);
        assert!(reopened.put_sync(new, b"legacy new")?);
        assert_eq!(metadata_order(&reopened, &new)?, migrated_next_order);
        assert_eq!(
            persisted_next_order(&reopened)?,
            migrated_next_order.saturating_add(1)
        );

        Ok(())
    }

    #[test]
    fn put_many_report_counts_only_new_hashes_and_bytes() -> Result<(), StoreError> {
        let temp = TempDir::new().unwrap();
        let store = LmdbBlobStore::new(temp.path().join("blobs"))?;

        let existing = b"existing";
        let existing_hash = sha256(existing);
        assert!(store.put_sync(existing_hash, existing)?);

        let new_one = b"new one";
        let new_two = b"new two";
        let new_one_hash = sha256(new_one);
        let new_two_hash = sha256(new_two);
        let items = vec![
            (existing_hash, existing.to_vec()),
            (new_one_hash, new_one.to_vec()),
            (new_one_hash, new_one.to_vec()),
            (new_two_hash, new_two.to_vec()),
        ];

        let report = store.put_many_report_sync(&items)?;

        assert_eq!(report.total, 4);
        assert_eq!(report.inserted, 2);
        assert_eq!(
            report.inserted_bytes,
            (new_one.len() + new_two.len()) as u64
        );
        assert_eq!(report.inserted_hashes, vec![new_one_hash, new_two_hash]);
        assert_eq!(store.put_many_sync(&items)?, 0);
        assert_eq!(
            store.stats()?.total_bytes,
            (existing.len() + new_one.len() + new_two.len()) as u64
        );

        Ok(())
    }

    #[test]
    fn duplicate_heavy_batch_does_not_evict_by_candidate_bytes() -> Result<(), StoreError> {
        let temp = TempDir::new().unwrap();
        let store = LmdbBlobStore::new(temp.path().join("blobs"))?;
        store.set_max_bytes(35);

        let first = [1u8; 10];
        let second = [2u8; 10];
        let third = [3u8; 10];
        let new = [4u8; 5];
        let first_hash = sha256(&first);
        let second_hash = sha256(&second);
        let third_hash = sha256(&third);
        let new_hash = sha256(&new);
        assert!(store.put_sync(first_hash, &first)?);
        assert!(store.put_sync(second_hash, &second)?);
        assert!(store.put_sync(third_hash, &third)?);
        assert_eq!(store.stats()?.total_bytes, 30);

        let report = store.put_many_report_sync(&[
            (first_hash, first.to_vec()),
            (second_hash, second.to_vec()),
            (new_hash, new.to_vec()),
        ])?;

        assert_eq!(report.inserted, 1);
        assert_eq!(report.inserted_bytes, 5);
        assert_eq!(store.stats()?.total_bytes, 35);
        assert!(store.exists(&first_hash)?);
        assert!(store.exists(&second_hash)?);
        assert!(store.exists(&third_hash)?);
        assert!(store.exists(&new_hash)?);

        Ok(())
    }

    #[test]
    fn test_decodes_legacy_blob_metadata_without_access_time() -> Result<(), StoreError> {
        let mut encoded = [0u8; LEGACY_BLOB_META_BYTES];
        encoded[..8].copy_from_slice(&7u64.to_be_bytes());
        encoded[8..].copy_from_slice(&42u64.to_be_bytes());

        let meta = LmdbBlobStore::decode_blob_meta(&encoded)?;

        assert_eq!(meta.order, 7);
        assert_eq!(meta.size, 42);
        assert_eq!(meta.last_accessed_at, 0);
        Ok(())
    }

    #[tokio::test]
    async fn test_stats() -> Result<(), StoreError> {
        let temp = TempDir::new().unwrap();
        let store = LmdbBlobStore::new(temp.path().join("blobs"))?;

        let d1 = b"hello";
        let d2 = b"world";
        store.put(sha256(d1), d1.to_vec()).await?;
        store.put(sha256(d2), d2.to_vec()).await?;

        let stats = store.stats()?;
        assert_eq!(stats.count, 2);
        assert_eq!(stats.total_bytes, 10);

        Ok(())
    }

    #[tokio::test]
    async fn test_stats_persist_across_reopen_and_mutations() -> Result<(), StoreError> {
        let temp = TempDir::new().unwrap();
        let path = temp.path().join("blobs");
        let h1 = sha256(b"hello");
        let h2 = sha256(b"world!");
        let h3 = sha256(b"prepin");

        {
            let store = LmdbBlobStore::new(&path)?;
            store.put(h1, b"hello".to_vec()).await?;
            store.put(h2, b"world!".to_vec()).await?;
            store.pin(&h1).await?;
            store.pin(&h3).await?;
            store.put(h3, b"prepin".to_vec()).await?;

            let stats = store.stats()?;
            assert_eq!(stats.count, 3);
            assert_eq!(stats.total_bytes, 17);
            assert_eq!(stats.pinned_count, 2);
            assert_eq!(stats.pinned_bytes, 11);
        }

        {
            let reopened = LmdbBlobStore::new(&path)?;
            let stats = reopened.stats()?;
            assert_eq!(stats.count, 3);
            assert_eq!(stats.total_bytes, 17);
            assert_eq!(stats.pinned_count, 2);
            assert_eq!(stats.pinned_bytes, 11);

            assert!(reopened.delete(&h1).await?);
            let stats = reopened.stats()?;
            assert_eq!(stats.count, 2);
            assert_eq!(stats.total_bytes, 12);
            assert_eq!(stats.pinned_count, 1);
            assert_eq!(stats.pinned_bytes, 6);
        }

        let reopened = LmdbBlobStore::new(&path)?;
        let stats = reopened.stats()?;
        assert_eq!(stats.count, 2);
        assert_eq!(stats.total_bytes, 12);
        assert_eq!(stats.pinned_count, 1);
        assert_eq!(stats.pinned_bytes, 6);

        Ok(())
    }

    #[tokio::test]
    async fn test_deduplication() -> Result<(), StoreError> {
        let temp = TempDir::new().unwrap();
        let store = LmdbBlobStore::new(temp.path().join("blobs"))?;

        let data = b"same";
        let hash = sha256(data);
        assert!(store.put(hash, data.to_vec()).await?); // Returns true (newly stored)
        assert!(!store.put(hash, data.to_vec()).await?); // Returns false (already existed)

        assert_eq!(store.list()?.len(), 1);

        Ok(())
    }

    #[tokio::test]
    async fn test_max_bytes() -> Result<(), StoreError> {
        let temp = TempDir::new().unwrap();
        let store = LmdbBlobStore::new(temp.path().join("blobs"))?;

        assert!(store.max_bytes().is_none());

        store.set_max_bytes(1000);
        assert_eq!(store.max_bytes(), Some(1000));

        store.set_max_bytes(0);
        assert!(store.max_bytes().is_none());

        Ok(())
    }

    #[test]
    fn test_with_max_bytes_expands_lmdb_map_size() -> Result<(), StoreError> {
        let temp = TempDir::new().unwrap();
        let requested = (DEFAULT_MAP_SIZE as u64) + 64 * 1024 * 1024;
        let store = LmdbBlobStore::with_max_bytes(temp.path().join("blobs"), requested)?;

        assert!(
            store.map_size_bytes() as u64 >= requested,
            "expected LMDB map to grow to at least {requested} bytes, got {}",
            store.map_size_bytes()
        );
        assert_eq!(store.max_bytes(), Some(requested));

        Ok(())
    }

    #[tokio::test]
    async fn test_eviction_over_limit() -> Result<(), StoreError> {
        let temp = TempDir::new().unwrap();
        let store = LmdbBlobStore::new(temp.path().join("blobs"))?;
        store.set_max_bytes(25);

        let h1 = sha256(b"aaaaaaaaaa");
        let h2 = sha256(b"bbbbbbbbbb");
        let h3 = sha256(b"cccccccccc");

        store.put(h1, b"aaaaaaaaaa".to_vec()).await?;
        store.put(h2, b"bbbbbbbbbb".to_vec()).await?;
        store.put(h3, b"cccccccccc".to_vec()).await?;

        let freed = store.evict_if_needed().await?;
        assert_eq!(
            freed, 0,
            "write path should have already evicted stale blobs"
        );

        assert!(
            !store.has(&h1).await?,
            "oldest blob should be evicted before the third write"
        );
        assert!(store.has(&h2).await?);
        assert!(store.has(&h3).await?);

        let stats = store.stats()?;
        assert!(
            stats.total_bytes <= 22,
            "store should be reduced to 90% target"
        );

        Ok(())
    }

    #[tokio::test]
    async fn test_eviction_respects_pins() -> Result<(), StoreError> {
        let temp = TempDir::new().unwrap();
        let store = LmdbBlobStore::new(temp.path().join("blobs"))?;
        store.set_max_bytes(25);

        let h1 = sha256(b"aaaaaaaaaa");
        let h2 = sha256(b"bbbbbbbbbb");
        let h3 = sha256(b"cccccccccc");

        store.put(h1, b"aaaaaaaaaa".to_vec()).await?;
        store.put(h2, b"bbbbbbbbbb".to_vec()).await?;
        store.pin(&h1).await?;
        store.put(h3, b"cccccccccc".to_vec()).await?;

        let freed = store.evict_if_needed().await?;
        assert_eq!(
            freed, 0,
            "write path should have already evicted stale blobs"
        );

        assert!(store.has(&h1).await?, "pinned blob must not be evicted");
        assert!(
            !store.has(&h2).await?,
            "oldest unpinned blob should be evicted before the third write"
        );
        assert!(store.has(&h3).await?);

        Ok(())
    }

    #[tokio::test]
    async fn test_reopen_with_existing_eviction_order() -> Result<(), StoreError> {
        let temp = TempDir::new().unwrap();
        let path = temp.path().join("blobs");

        {
            let store = LmdbBlobStore::new(&path)?;
            let h1 = sha256(b"aaaaaaaaaa");
            let h2 = sha256(b"bbbbbbbbbb");
            store.put(h1, b"aaaaaaaaaa".to_vec()).await?;
            store.put(h2, b"bbbbbbbbbb".to_vec()).await?;
        }

        let reopened = LmdbBlobStore::new(&path)?;
        let h3 = sha256(b"cccccccccc");
        assert!(reopened.put(h3, b"cccccccccc".to_vec()).await?);
        assert!(reopened.has(&h3).await?);

        Ok(())
    }

    #[tokio::test]
    async fn test_reopen_with_lower_max_bytes_evicts_existing_blobs() -> Result<(), StoreError> {
        let temp = TempDir::new().unwrap();
        let path = temp.path().join("blobs");

        {
            let store = LmdbBlobStore::new(&path)?;
            let h1 = sha256(b"aaaaaaaaaa");
            let h2 = sha256(b"bbbbbbbbbb");
            let h3 = sha256(b"cccccccccc");
            store.put(h1, b"aaaaaaaaaa".to_vec()).await?;
            store.put(h2, b"bbbbbbbbbb".to_vec()).await?;
            store.put(h3, b"cccccccccc".to_vec()).await?;
        }

        let reopened = LmdbBlobStore::with_max_bytes(&path, 25)?;
        let h1 = sha256(b"aaaaaaaaaa");
        let h2 = sha256(b"bbbbbbbbbb");
        let h3 = sha256(b"cccccccccc");

        assert!(
            !reopened.has(&h1).await?,
            "oldest blob should be evicted when reopening over the new cap"
        );
        assert!(reopened.has(&h2).await?);
        assert!(reopened.has(&h3).await?);

        let stats = reopened.stats()?;
        assert!(
            stats.total_bytes <= 22,
            "reopened store should be reduced to the 90% target"
        );

        Ok(())
    }

    #[test]
    fn test_supports_many_concurrent_readers() -> Result<(), Box<dyn std::error::Error>> {
        const READER_THREADS: usize = 160;

        let temp = TempDir::new()?;
        let store = Arc::new(LmdbBlobStore::new(temp.path().join("blobs"))?);
        let hash = sha256(b"many readers");
        store.put_sync(hash, b"many readers")?;

        let start = Arc::new(Barrier::new(READER_THREADS + 1));
        let release = Arc::new(AtomicBool::new(false));
        let mut handles = Vec::with_capacity(READER_THREADS);

        for _ in 0..READER_THREADS {
            let env = store.env.clone();
            let start = Arc::clone(&start);
            let release = Arc::clone(&release);
            handles.push(std::thread::spawn(move || -> Result<(), String> {
                start.wait();
                let _rtxn = env.read_txn().map_err(|err| err.to_string())?;
                while !release.load(Ordering::Relaxed) {
                    std::thread::sleep(Duration::from_millis(1));
                }
                Ok(())
            }));
        }

        start.wait();
        std::thread::sleep(Duration::from_millis(50));
        release.store(true, Ordering::Relaxed);

        let results: Vec<Result<(), String>> = handles
            .into_iter()
            .map(|handle| handle.join().expect("reader thread panicked"))
            .collect();

        let failures: Vec<String> = results.into_iter().filter_map(Result::err).collect();
        assert!(
            failures.is_empty(),
            "concurrent reader failures: {}",
            failures.join(" | ")
        );
        assert!(store.exists(&hash)?);

        Ok(())
    }

    #[cfg(unix)]
    #[test]
    fn test_reclaims_stale_reader_slots() -> Result<(), Box<dyn std::error::Error>> {
        let temp = TempDir::new()?;
        let path = temp.path().join("blobs");
        let data = b"hello stale readers";
        let hash = sha256(data);

        run_helper("setup", &path, &temp.path().join("setup.marker"));

        for index in 0..TEST_MAX_READERS {
            let marker = temp.path().join(format!("helper-{index}.marker"));
            run_helper("stale", &path, &marker);
        }

        let store = LmdbBlobStore::with_map_size(&path, 1024 * 1024)?;
        assert!(store.exists(&hash)?);

        Ok(())
    }

    #[cfg(unix)]
    #[test]
    fn test_reopens_existing_env_with_larger_map_size() -> Result<(), Box<dyn std::error::Error>> {
        let temp = TempDir::new()?;
        let path = temp.path().join("blobs");
        run_helper("small-map", &path, &temp.path().join("small-map.marker"));

        let reopened = LmdbBlobStore::with_map_size(&path, 8 * 1024 * 1024)?;
        assert!(reopened.map_size_bytes() >= 8 * 1024 * 1024);

        Ok(())
    }

    #[cfg(unix)]
    #[test]
    fn test_reopens_existing_env_with_smaller_requested_map_size(
    ) -> Result<(), Box<dyn std::error::Error>> {
        let temp = TempDir::new()?;
        let path = temp.path().join("blobs");
        let hash = sha256(b"large existing blob");
        run_helper("large-map", &path, &temp.path().join("large-map.marker"));

        let existing_size = std::fs::metadata(path.join("data.mdb"))?.len();
        assert!(
            existing_size > 1024 * 1024,
            "test setup should create an environment larger than the reopen request"
        );

        let reopened = LmdbBlobStore::with_map_size(&path, 1024 * 1024)?;
        assert!(
            reopened.map_size_bytes() as u64 >= existing_size,
            "expected map size to cover existing data.mdb size {existing_size}, got {}",
            reopened.map_size_bytes()
        );
        assert!(reopened.exists(&hash)?);

        Ok(())
    }

    #[cfg(unix)]
    #[test]
    #[ignore = "used as a subprocess helper by test_reclaims_stale_reader_slots"]
    fn lmdb_stale_reader_helper() {
        let Some(db_path) = std::env::var_os(STALE_READER_DB_PATH_ENV) else {
            return;
        };
        let marker_path =
            PathBuf::from(std::env::var_os(STALE_READER_MARKER_PATH_ENV).expect("marker path"));
        std::fs::write(&marker_path, b"started").expect("write helper marker");

        let _env_flag = std::env::var_os(STALE_READER_HELPER_ENV).expect("helper mode enabled");
        let mode = std::env::var(STALE_READER_HELPER_MODE_ENV).expect("helper mode");
        let db_path = PathBuf::from(db_path);
        std::fs::create_dir_all(&db_path).expect("create helper db dir");
        let helper_map_size = if mode == "large-map" {
            8 * 1024 * 1024
        } else {
            1024 * 1024
        };
        let env = unsafe {
            EnvOpenOptions::new()
                .map_size(helper_map_size)
                .max_dbs(DATABASE_COUNT)
                .max_readers(TEST_MAX_READERS)
                .open(&db_path)
                .expect("open lmdb env")
        };
        match mode.as_str() {
            "setup" => {
                let mut wtxn = env.write_txn().expect("open write txn");
                let blobs: Database<Bytes, Bytes> = env
                    .create_database(&mut wtxn, Some("blobs"))
                    .expect("create blobs database");
                let data = b"hello stale readers";
                let hash = sha256(data);
                blobs.put(&mut wtxn, &hash, data).expect("seed blob");
                wtxn.commit().expect("commit setup txn");
                std::process::exit(0);
            }
            "stale" => {
                let _rtxn = env.read_txn().expect("open read txn");
                std::process::exit(0);
            }
            "small-map" => {
                let mut wtxn = env.write_txn().expect("open write txn");
                let _blobs: Database<Bytes, Bytes> = env
                    .create_database(&mut wtxn, Some("blobs"))
                    .expect("create blobs database");
                let _metadata: Database<Bytes, Bytes> = env
                    .create_database(&mut wtxn, Some("metadata"))
                    .expect("create metadata database");
                let _eviction_order: Database<Bytes, Unit> = env
                    .create_database(&mut wtxn, Some("eviction_order"))
                    .expect("create eviction_order database");
                let _pins: Database<Bytes, Bytes> = env
                    .create_database(&mut wtxn, Some("pins"))
                    .expect("create pins database");
                wtxn.commit().expect("commit small-map setup txn");
                std::process::exit(0);
            }
            "large-map" => {
                let mut wtxn = env.write_txn().expect("open write txn");
                let blobs: Database<Bytes, Bytes> = env
                    .create_database(&mut wtxn, Some("blobs"))
                    .expect("create blobs database");
                let _metadata: Database<Bytes, Bytes> = env
                    .create_database(&mut wtxn, Some("metadata"))
                    .expect("create metadata database");
                let _eviction_order: Database<Bytes, Unit> = env
                    .create_database(&mut wtxn, Some("eviction_order"))
                    .expect("create eviction_order database");
                let _pins: Database<Bytes, Bytes> = env
                    .create_database(&mut wtxn, Some("pins"))
                    .expect("create pins database");
                let data = vec![42u8; 3 * 1024 * 1024];
                let hash = sha256(b"large existing blob");
                blobs.put(&mut wtxn, &hash, &data).expect("seed blob");
                wtxn.commit().expect("commit large-map setup txn");
                std::process::exit(0);
            }
            other => panic!("unknown helper mode: {other}"),
        }
    }
}