crates/clawhdf5-format/src/chunk_cache.rs

@@ -223,13 +223,32 @@ pub const DEFAULT_CACHE_BYTES: usize = 16 * 1024 * 1024; // 16 MiB
 /// coordinate map and reduces collision chains compared to power-of-two sizes.
 pub const DEFAULT_MAX_SLOTS: usize = 521;
 
+/// Most datasets whose chunk index a [`ChunkCache`] keeps at once.
+pub const MAX_INDEXED_DATASETS: usize = 64;
+
+/// Most chunk-index entries, summed over all datasets, a [`ChunkCache`] keeps.
+/// Least-recently-used datasets' indexes are dropped past this (the dataset
+/// being read is always kept), so a file with many or huge chunked datasets
+/// cannot grow the cache without bound.
+pub const MAX_INDEXED_CHUNKS: usize = 1 << 20;
+
+/// The dataset key the address-less (legacy) methods use when
+/// [`ChunkCache::ensure_dataset`] has not been called.
+#[cfg(feature = "std")]
+const UNBOUND_DATASET: u64 = u64::MAX;
+
 // ---------------------------------------------------------------------------
 // LRU entry
 // ---------------------------------------------------------------------------
 
+/// Decompressed chunks are keyed by dataset *and* coordinate: every chunked
+/// dataset has a chunk at (0, 0, ...), so the coordinate alone is ambiguous.
+#[cfg(feature = "std")]
+type SlotKey = (u64, ChunkCoord);
+
 #[cfg(feature = "std")]
 struct CachedChunk {
-    coord: ChunkCoord,
+    key: SlotKey,
     /// Shared so a cache hit is a refcount bump, not a copy of the whole
     /// (potentially large) decompressed chunk.
     data: Arc<CacheAlignedBuffer>,
@@ -237,21 +256,48 @@ struct CachedChunk {
     last_access: u64,
 }
 
+/// Per-dataset index state.
+#[cfg(feature = "std")]
+#[derive(Default)]
+struct DatasetEntry {
+    /// Chunk coordinate -> ChunkInfo (offset + size in file).
+    index: Option<Arc<HashMap<ChunkCoord, ChunkInfo>>>,
+    /// Pre-built chunk index for O(1) coordinate lookups.
+    chunk_index: Option<Arc<ChunkIndex>>,
+    /// Pre-computed chunk layout for fast assembly.
+    chunk_layout: Option<Arc<ChunkLayout>>,
+    /// Tick of the last use, for dropping the least recently used dataset.
+    last_used: u64,
+}
+
+#[cfg(feature = "std")]
+impl DatasetEntry {
+    fn weight(&self) -> usize {
+        self.index.as_ref().map_or(0, |m| m.len())
+            + self.chunk_index.as_ref().map_or(0, |c| c.num_chunks())
+    }
+}
+
 // ---------------------------------------------------------------------------
 // ChunkCache
 // ---------------------------------------------------------------------------
 
-/// A per-dataset chunk cache with hash-based index and LRU eviction.
+/// A per-file chunk cache: chunk indexes per dataset, plus an LRU of
+/// decompressed chunks, all keyed by dataset.
 ///
-/// # Usage
+/// A dataset is identified by the address of its chunk index (B-tree, fixed
+/// or extensible array, ...), which is unique within a file. Every method
+/// that takes an `addr` works on that dataset only, so threads reading
+/// different datasets through one shared cache never see each other's
+/// chunks. The address-less methods (`has_index`, `populate_index`,
+/// `get_decompressed`, ...) act on the dataset last bound with
+/// [`Self::ensure_dataset`]; that binding is shared state, so concurrent
+/// readers must use the `*_in` / `*_for` methods instead (the chunked
+/// readers in [`crate::chunked_read`] do).
 ///
-/// ```ignore
-/// let cache = ChunkCache::new();
-/// // Pass &cache to read_chunked_data — it will populate the index lazily.
-/// ```
-///
-/// The cache is wrapped in `Mutex` internally so it can be mutated through
-/// shared references (thread-safe).
+/// Memory is bounded: decompressed data by `max_bytes`/`max_slots` across
+/// all datasets, indexes by [`MAX_INDEXED_DATASETS`] and
+/// [`MAX_INDEXED_CHUNKS`].
 ///
 /// Only available with the `std` feature because it requires `std::sync::Mutex`.
 #[cfg(feature = "std")]
@@ -261,26 +307,20 @@ pub struct ChunkCache {
 
 #[cfg(feature = "std")]
 struct CacheInner {
-    /// Hash index: chunk coordinate -> ChunkInfo (offset + size in file).
-    /// Populated once per dataset on first access.
-    index: Option<HashMap<ChunkCoord, ChunkInfo>>,
+    /// Per-dataset chunk indexes, keyed by chunk-index address.
+    datasets: HashMap<u64, DatasetEntry>,
 
-    /// Address of the dataset (its chunk-index base address) that the cached
-    /// index, chunk index, layout, and decompressed slots currently belong to.
-    /// The cache is shared per file across datasets, so every cached-read entry
-    /// checks this and resets the per-dataset state when the dataset changes —
-    /// otherwise one dataset's chunk index (with its own rank) would be reused
-    /// for another, corrupting reads.
-    index_addr: Option<u64>,
+    /// Dataset the address-less methods act on (see `ensure_dataset`).
+    current: Option<u64>,
 
     /// LRU cache of decompressed chunk data.
     slots: Vec<CachedChunk>,
 
-    /// Coordinate -> index into `slots`, for O(1) lookup instead of a linear
+    /// Key -> index into `slots`, for O(1) lookup instead of a linear
     /// scan. Kept in sync with `slots` on every insert/evict/clear — in
     /// particular, `slots.swap_remove(i)` moves the last element into slot
     /// `i`, so the moved element's index entry must be updated too.
-    slot_index: HashMap<ChunkCoord, usize>,
+    slot_index: HashMap<SlotKey, usize>,
 
     /// Current total bytes of cached decompressed data.
     current_bytes: usize,
@@ -294,17 +334,145 @@ struct CacheInner {
     /// Monotonic counter for LRU ordering.
     tick: u64,
 
-    /// Last accessed chunk coordinate (for sequential detection).
-    last_coord: Option<ChunkCoord>,
+    /// Last accessed chunk (for sequential detection).
+    last_coord: Option<SlotKey>,
 
     /// Access pattern statistics.
     stats: AccessStats,
+}
 
-    /// Pre-built chunk index for O(1) coordinate lookups.
-    chunk_index: Option<ChunkIndex>,
+#[cfg(feature = "std")]
+impl CacheInner {
+    fn current(&self) -> u64 {
+        self.current.unwrap_or(UNBOUND_DATASET)
+    }
 
-    /// Pre-computed chunk layout for fast assembly.
-    chunk_layout: Option<ChunkLayout>,
+    fn touch(&mut self, addr: u64) -> &mut DatasetEntry {
+        self.tick += 1;
+        let tick = self.tick;
+        let entry = self.datasets.entry(addr).or_default();
+        entry.last_used = tick;
+        entry
+    }
+
+    fn entry(&self, addr: u64) -> Option<&DatasetEntry> {
+        self.datasets.get(&addr)
+    }
+
+    /// Drop least-recently-used datasets' indexes (never `keep`'s) until the
+    /// dataset and chunk-entry budgets hold.
+    fn trim_datasets(&mut self, keep: u64) {
+        loop {
+            let total: usize = self.datasets.values().map(DatasetEntry::weight).sum();
+            if self.datasets.len() <= MAX_INDEXED_DATASETS && total <= MAX_INDEXED_CHUNKS {
+                return;
+            }
+            let victim = self
+                .datasets
+                .iter()
+                .filter(|(a, _)| **a != keep)
+                .min_by_key(|(_, e)| e.last_used)
+                .map(|(a, _)| *a);
+            match victim {
+                Some(a) => {
+                    self.datasets.remove(&a);
+                }
+                None => return,
+            }
+        }
+    }
+
+    fn get_decompressed(&mut self, addr: u64, coord: &[u64]) -> Option<Arc<CacheAlignedBuffer>> {
+        self.tick += 1;
+        let tick = self.tick;
+
+        // Track sequential vs random access
+        let is_sequential = self.last_coord.as_ref().is_some_and(|(prev_addr, prev)| {
+            // Sequential if exactly one dimension changed
+            let changes: usize = prev
+                .iter()
+                .zip(coord.iter())
+                .filter(|(a, b)| a != b)
+                .count();
+            *prev_addr == addr && changes <= 1
+        });
+        if is_sequential {
+            self.stats.sequential_count += 1;
+        } else if self.last_coord.is_some() {
+            self.stats.random_count += 1;
+        }
+        let key: SlotKey = (addr, coord.to_vec());
+        let found = if let Some(&idx) = self.slot_index.get(&key) {
+            self.slots[idx].last_access = tick;
+            Some(Arc::clone(&self.slots[idx].data))
+        } else {
+            None
+        };
+        self.last_coord = Some(key);
+        if let Some(ref data) = found {
+            self.stats.hits += 1;
+            self.stats.bytes_read += data.len() as u64;
+        } else {
+            self.stats.misses += 1;
+        }
+        found
+    }
+
+    fn put_decompressed(
+        &mut self,
+        key: SlotKey,
+        data: Arc<CacheAlignedBuffer>,
+    ) -> Arc<CacheAlignedBuffer> {
+        let data_len = data.len();
+
+        // Don't cache if single chunk exceeds budget — still return the data
+        // to the caller, just don't retain it.
+        if data_len > self.max_bytes {
+            return data;
+        }
+
+        // Check if already present
+        self.tick += 1;
+        let tick = self.tick;
+        if let Some(&idx) = self.slot_index.get(&key) {
+            self.slots[idx].last_access = tick;
+            return Arc::clone(&self.slots[idx].data); // already cached
+        }
+
+        // Evict until we have room
+        while self.slots.len() >= self.max_slots
+            || (self.current_bytes + data_len > self.max_bytes && !self.slots.is_empty())
+        {
+            // Find LRU slot
+            let lru_idx = self
+                .slots
+                .iter()
+                .enumerate()
+                .min_by_key(|(_, s)| s.last_access)
+                .map(|(i, _)| i)
+                .unwrap();
+            let removed = self.slots.swap_remove(lru_idx);
+            self.slot_index.remove(&removed.key);
+            // swap_remove moved the former last element into `lru_idx` (unless
+            // it *was* the last element) — fix up that element's index entry.
+            if lru_idx < self.slots.len() {
+                let moved_key = self.slots[lru_idx].key.clone();
+                self.slot_index.insert(moved_key, lru_idx);
+            }
+            self.current_bytes -= removed.data.len();
+            self.stats.evictions += 1;
+        }
+
+        self.current_bytes += data_len;
+        let new_idx = self.slots.len();
+        self.slot_index.insert(key.clone(), new_idx);
+        self.slots.push(CachedChunk {
+            key,
+            data: Arc::clone(&data),
+            last_access: tick,
+        });
+        data
+    }
 }
 
 /// Access pattern statistics tracked by the chunk cache.
@@ -356,8 +524,8 @@ impl ChunkCache {
     pub fn with_capacity(max_bytes: usize, max_slots: usize) -> Self {
         Self {
             inner: std::sync::Mutex::new(CacheInner {
-                index: None,
-                index_addr: None,
+                datasets: HashMap::new(),
+                current: None,
                 slots: Vec::with_capacity(max_slots.min(64)),
                 slot_index: HashMap::with_capacity(max_slots.min(64)),
                 current_bytes: 0,
@@ -366,340 +534,331 @@ impl ChunkCache {
                 tick: 0,
                 last_coord: None,
                 stats: AccessStats::default(),
-                chunk_index: None,
-                chunk_layout: None,
             }),
         }
     }
 
-    // ----- Index operations -----
+    fn lock(&self) -> std::sync::MutexGuard<'_, CacheInner> {
+        self.inner.lock().unwrap_or_else(|e| e.into_inner())
+    }
 
     /// The most decompressed bytes this cache will hold.
     pub fn max_bytes(&self) -> usize {
-        self.inner.lock().map(|g| g.max_bytes).unwrap_or(0)
+        self.lock().max_bytes
     }
 
-    /// Bind the cache to the dataset at chunk-index address `addr`.
+    // ----- Dataset-keyed operations (safe to use concurrently) -----
+
+    /// The chunk list of the dataset whose chunk index is at `addr`.
     ///
-    /// The cache is shared per file across all of its datasets. If the cache
-    /// currently holds state for a different dataset, all per-dataset state
-    /// (chunk index, chunk-index map, layout, and decompressed slots) is
-    /// dropped so the next access rebuilds it for this dataset. Reading the
-    /// same dataset again is a no-op, preserving the cache's benefit for
-    /// repeated/sequential access. Returns `true` if a reset occurred.
+    /// On the first call for a dataset, `build` scans its chunk index; the
+    /// result is kept (offsets truncated to `rank` for the lookup key), so
+    /// later calls skip the scan. `build` runs without the cache lock held;
+    /// if two threads race to build the same dataset's index, the first
+    /// stored one wins and both return equivalent lists.
+    pub fn chunks_for<E>(
+        &self,
+        addr: u64,
+        rank: usize,
+        build: impl FnOnce() -> Result<Vec<ChunkInfo>, E>,
+    ) -> Result<Vec<ChunkInfo>, E> {
+        Ok(self
+            .index_for(addr, rank, build)?
+            .values()
+            .cloned()
+            .collect())
+    }
+
+    fn index_for<E>(
+        &self,
+        addr: u64,
+        rank: usize,
+        build: impl FnOnce() -> Result<Vec<ChunkInfo>, E>,
+    ) -> Result<Arc<HashMap<ChunkCoord, ChunkInfo>>, E> {
+        if let Some(index) = self.lock().touch(addr).index.clone() {
+            return Ok(index);
+        }
+        let chunks = build()?;
+        let map: HashMap<ChunkCoord, ChunkInfo> = chunks
+            .into_iter()
+            .map(|ci| (ci.offsets.iter().take(rank).copied().collect(), ci))
+            .collect();
+        let mut inner = self.lock();
+        let entry = inner.touch(addr);
+        let index = Arc::clone(entry.index.get_or_insert_with(|| Arc::new(map)));
+        inner.trim_datasets(addr);
+        Ok(index)
+    }
+
+    /// The pre-computed assembly layout of the dataset at `addr`, building
+    /// its chunk index (via `build`, as in [`Self::chunks_for`]) and layout on
+    /// first use.
+    pub fn chunk_layout_for<E>(
+        &self,
+        addr: u64,
+        rank: usize,
+        build: impl FnOnce() -> Result<Vec<ChunkInfo>, E>,
+        ds_dims: &[usize],
+        chunk_dims: &[usize],
+        elem_size: usize,
+    ) -> Result<Arc<ChunkLayout>, E> {
+        let (layout, chunk_index) = {
+            let mut inner = self.lock();
+            let entry = inner.touch(addr);
+            (entry.chunk_layout.clone(), entry.chunk_index.clone())
+        };
+        if let Some(layout) = layout {
+            return Ok(layout);
+        }
+        let chunk_index = match chunk_index {
+            Some(ci) => ci,
+            None => {
+                let index = self.index_for(addr, rank, build)?;
+                let chunks: Vec<ChunkInfo> = index.values().cloned().collect();
+                Arc::new(ChunkIndex::build(&chunks, rank))
+            }
+        };
+        let layout = ChunkLayout::build(&chunk_index, ds_dims, chunk_dims, elem_size);
+        let mut inner = self.lock();
+        let entry = inner.touch(addr);
+        entry.chunk_index.get_or_insert(chunk_index);
+        let layout = Arc::clone(entry.chunk_layout.get_or_insert_with(|| Arc::new(layout)));
+        inner.trim_datasets(addr);
+        Ok(layout)
+    }
+
+    /// Cached decompressed chunk at `coord` of the dataset at `addr`.
+    ///
+    /// O(1) lookup; the clone is an `Arc` refcount bump, not a copy of the
+    /// underlying decompressed data.
+    pub fn get_decompressed_in(&self, addr: u64, coord: &[u64]) -> Option<Arc<CacheAlignedBuffer>> {
+        self.lock().get_decompressed(addr, coord)
+    }
+
+    /// Cache decompressed chunk data for `coord` of the dataset at `addr`.
+    /// Returns the `Arc`-shared buffer now cached (or already cached).
+    pub fn put_decompressed_in(
+        &self,
+        addr: u64,
+        coord: ChunkCoord,
+        data: Vec<u8>,
+    ) -> Arc<CacheAlignedBuffer> {
+        self.put_decompressed_aligned_in(addr, coord, CacheAlignedBuffer::from_vec(data))
+    }
+
+    /// [`Self::put_decompressed_in`] for an already-aligned buffer.
+    pub fn put_decompressed_aligned_in(
+        &self,
+        addr: u64,
+        coord: ChunkCoord,
+        data: CacheAlignedBuffer,
+    ) -> Arc<CacheAlignedBuffer> {
+        let data = Arc::new(data);
+        self.lock().put_decompressed((addr, coord), data)
+    }
+
+    /// Record that the given chunk coordinates of the dataset at `addr` are
+    /// predicted to be accessed soon (bookkeeping only).
+    ///
+    /// This does **not** prefetch or pre-decompress anything — it only
+    /// checks whether each coordinate is already in the chunk index and
+    /// updates access-pattern stats accordingly.
+    pub fn prefetch_hint_in(&self, addr: u64, next_coords: &[ChunkCoord]) {
+        let mut inner = self.lock();
+        let Some(index) = inner.entry(addr).and_then(|e| e.index.clone()) else {
+            return;
+        };
+        let known = next_coords
+            .iter()
+            .filter(|c| index.contains_key(*c))
+            .count();
+        inner.stats.sequential_count += known as u64;
+    }
+
+    // ----- Address-less operations on the bound dataset -----
+
+    /// Bind the address-less methods to the dataset at chunk-index address
+    /// `addr`. Returns `true` if this changed the bound dataset.
+    ///
+    /// Each dataset's state is kept separately, so switching loses nothing
+    /// and never exposes one dataset's index or chunks to another. The
+    /// binding itself is shared, though: concurrent readers should use the
+    /// `addr`-taking methods rather than bind and then call these.
     pub fn ensure_dataset(&self, addr: u64) -> bool {
-        let mut inner = self.inner.lock().unwrap_or_else(|e| e.into_inner());
-        if inner.index_addr == Some(addr) {
-            return false;
-        }
-        inner.index = None;
-        inner.chunk_index = None;
-        inner.chunk_layout = None;
-        inner.slots.clear();
-        inner.slot_index.clear();
-        inner.current_bytes = 0;
-        inner.last_coord = None;
-        inner.index_addr = Some(addr);
-        true
+        let mut inner = self.lock();
+        let changed = inner.current != Some(addr);
+        inner.current = Some(addr);
+        changed
     }
 
-    /// Returns `true` if the chunk index has been built.
+    /// Returns `true` if the bound dataset's chunk index has been built.
     pub fn has_index(&self) -> bool {
-        self.inner
-            .lock()
-            .unwrap_or_else(|e| e.into_inner())
-            .index
-            .is_some()
+        let inner = self.lock();
+        inner
+            .entry(inner.current())
+            .is_some_and(|e| e.index.is_some())
     }
 
-    /// Build the chunk index from a pre-collected list of `ChunkInfo`.
+    /// Build the bound dataset's chunk index from a pre-collected list of
+    /// `ChunkInfo`.
     ///
     /// The `rank` parameter is used to truncate offsets to spatial dims only
     /// (B-tree v1 stores rank+1 offsets).
     pub fn populate_index(&self, chunks: &[ChunkInfo], rank: usize) {
-        let mut inner = self.inner.lock().unwrap_or_else(|e| e.into_inner());
-        if inner.index.is_some() {
-            return; // already populated
-        }
-        let mut map = HashMap::with_capacity(chunks.len());
-
-        for ci in chunks {
-            let coord: ChunkCoord = ci.offsets.iter().take(rank).copied().collect();
-            map.insert(coord, ci.clone());
-        }
-        inner.index = Some(map);
+        let addr = self.lock().current();
+        let _ = self.index_for::<core::convert::Infallible>(addr, rank, || Ok(chunks.to_vec()));
     }
 
-    /// Look up a chunk by its spatial coordinate in the index.
+    /// Look up a chunk by its spatial coordinate in the bound dataset's index.
     pub fn lookup_index(&self, coord: &[u64]) -> Option<ChunkInfo> {
-        let inner = self.inner.lock().unwrap_or_else(|e| e.into_inner());
-        inner.index.as_ref()?.get(coord).cloned()
+        let inner = self.lock();
+        inner
+            .entry(inner.current())?
+            .index
+            .as_ref()?
+            .get(coord)
+            .cloned()
     }
 
-    /// Return all indexed chunks as a `Vec<ChunkInfo>` (order unspecified).
+    /// Return all of the bound dataset's indexed chunks (order unspecified).
     pub fn all_indexed_chunks(&self) -> Option<Vec<ChunkInfo>> {
-        let inner = self.inner.lock().unwrap_or_else(|e| e.into_inner());
-        inner.index.as_ref().map(|m| m.values().cloned().collect())
+        let inner = self.lock();
+        let index = inner.entry(inner.current())?.index.as_ref()?;
+        Some(index.values().cloned().collect())
     }
 
-    // ----- Chunk index (pre-built coordinate → ChunkInfo map) -----
-
-    /// Returns `true` if the chunk B-tree index has been built.
+    /// Returns `true` if the bound dataset's `ChunkIndex` has been built.
     pub fn has_chunk_index(&self) -> bool {
-        self.inner
-            .lock()
-            .unwrap_or_else(|e| e.into_inner())
-            .chunk_index
-            .is_some()
+        let inner = self.lock();
+        inner
+            .entry(inner.current())
+            .is_some_and(|e| e.chunk_index.is_some())
     }
 
-    /// Build and store the chunk B-tree index from a pre-collected list of `ChunkInfo`.
+    /// Build and store the bound dataset's `ChunkIndex`.
     pub fn populate_chunk_index(&self, chunks: &[ChunkInfo], rank: usize) {
-        let mut inner = self.inner.lock().unwrap_or_else(|e| e.into_inner());
-        if inner.chunk_index.is_some() {
-            return;
-        }
-        inner.chunk_index = Some(ChunkIndex::build(chunks, rank));
+        let built = Arc::new(ChunkIndex::build(chunks, rank));
+        let mut inner = self.lock();
+        let addr = inner.current();
+        inner.touch(addr).chunk_index.get_or_insert(built);
+        inner.trim_datasets(addr);
     }
 
-    // ----- Chunk layout (pre-computed assembly plan) -----
-
-    /// Returns `true` if the chunk layout has been computed.
+    /// Returns `true` if the bound dataset's chunk layout has been computed.
     pub fn has_chunk_layout(&self) -> bool {
-        self.inner
-            .lock()
-            .unwrap_or_else(|e| e.into_inner())
-            .chunk_layout
-            .is_some()
+        let inner = self.lock();
+        inner
+            .entry(inner.current())
+            .is_some_and(|e| e.chunk_layout.is_some())
     }
 
-    /// Build and store the pre-computed chunk layout for fast assembly.
+    /// Build and store the bound dataset's chunk layout (needs its
+    /// `ChunkIndex`; does nothing without one).
     pub fn populate_chunk_layout(&self, ds_dims: &[usize], chunk_dims: &[usize], elem_size: usize) {
-        let mut inner = self.inner.lock().unwrap_or_else(|e| e.into_inner());
-        if inner.chunk_layout.is_some() {
+        let mut inner = self.lock();
+        let addr = inner.current();
+        let entry = inner.touch(addr);
+        if entry.chunk_layout.is_some() {
             return;
         }
-        if let Some(ref idx) = inner.chunk_index {
-            inner.chunk_layout = Some(ChunkLayout::build(idx, ds_dims, chunk_dims, elem_size));
+        if let Some(idx) = entry.chunk_index.clone() {
+            entry.chunk_layout = Some(Arc::new(ChunkLayout::build(
+                &idx, ds_dims, chunk_dims, elem_size,
+            )));
         }
     }
 
-    /// Execute a function with a reference to the chunk layout.
-    ///
-    /// Returns `None` if the layout hasn't been computed yet.
+    /// Execute a function with a reference to the bound dataset's chunk
+    /// layout. Returns `None` if the layout hasn't been computed yet.
     pub fn with_chunk_layout<F, R>(&self, f: F) -> Option<R>
     where
         F: FnOnce(&ChunkLayout) -> R,
     {
-        let inner = self.inner.lock().unwrap_or_else(|e| e.into_inner());
-        inner.chunk_layout.as_ref().map(f)
+        let layout = {
+            let inner = self.lock();
+            inner.entry(inner.current())?.chunk_layout.clone()?
+        };
+        Some(f(&layout))
     }
 
-    // ----- Decompressed data cache (LRU) -----
-
-    /// Try to get cached decompressed data for a chunk coordinate.
+    /// Try to get cached decompressed data for a chunk of the bound dataset.
     ///
-    /// O(1) lookup. Returns an owned copy for API compatibility with callers
-    /// that need a `Vec<u8>`; prefer [`Self::get_decompressed_aligned`] when
-    /// an `Arc`-shared buffer works for the caller, since that avoids the
-    /// copy entirely.
+    /// Returns an owned copy; prefer [`Self::get_decompressed_aligned`] when
+    /// an `Arc`-shared buffer works for the caller.
     pub fn get_decompressed(&self, coord: &[u64]) -> Option<Vec<u8>> {
         self.get_decompressed_aligned(coord)
             .map(|arc| arc.as_slice().to_vec())
     }
 
-    /// Try to get a reference-counted clone of the aligned buffer for a chunk.
-    ///
-    /// O(1) index lookup; the clone is an `Arc` refcount bump, not a copy of
-    /// the underlying decompressed data.
+    /// Reference-counted cached buffer for a chunk of the bound dataset.
     pub fn get_decompressed_aligned(&self, coord: &[u64]) -> Option<Arc<CacheAlignedBuffer>> {
-        let mut inner = self.inner.lock().unwrap_or_else(|e| e.into_inner());
-        inner.tick += 1;
-        let tick = inner.tick;
-
-        // Track sequential vs random access
-        let is_sequential = inner.last_coord.as_ref().is_some_and(|prev| {
-            // Sequential if exactly one dimension changed
-            let changes: usize = prev
-                .iter()
-                .zip(coord.iter())
-                .filter(|(a, b)| a != b)
-                .count();
-            changes <= 1
-        });
-        if is_sequential {
-            inner.stats.sequential_count += 1;
-        } else if inner.last_coord.is_some() {
-            inner.stats.random_count += 1;
-        }
-        inner.last_coord = Some(coord.to_vec());
-
-        let found = if let Some(&idx) = inner.slot_index.get(coord) {
-            inner.slots[idx].last_access = tick;
-            Some(Arc::clone(&inner.slots[idx].data))
-        } else {
-            None
-        };
-        if let Some(ref data) = found {
-            inner.stats.hits += 1;
-            inner.stats.bytes_read += data.len() as u64;
-        } else {
-            inner.stats.misses += 1;
-        }
-        found
+        let mut inner = self.lock();
+        let addr = inner.current();
+        inner.get_decompressed(addr, coord)
     }
 
-    /// Insert decompressed chunk data into the LRU cache.
-    ///
-    /// The data is stored in a [`CacheAlignedBuffer`] so subsequent reads
-    /// return cache-line-aligned memory. Returns the `Arc`-shared buffer that
-    /// is now cached (or already was), so the caller can reuse it directly
-    /// instead of holding a separate copy of the same data.
+    /// Insert decompressed chunk data for the bound dataset into the LRU
+    /// cache, returning the `Arc`-shared buffer now cached.
     pub fn put_decompressed(&self, coord: ChunkCoord, data: Vec<u8>) -> Arc<CacheAlignedBuffer> {
-        let aligned = CacheAlignedBuffer::from_vec(data);
-        self.put_decompressed_aligned(coord, aligned)
+        self.put_decompressed_aligned(coord, CacheAlignedBuffer::from_vec(data))
     }
 
-    /// Insert an already-aligned buffer into the LRU cache.
-    ///
-    /// Returns the `Arc`-shared buffer now held by the cache (the one just
-    /// inserted, or the existing cached copy if `coord` was already present).
+    /// Insert an already-aligned buffer for the bound dataset.
     pub fn put_decompressed_aligned(
         &self,
         coord: ChunkCoord,
         data: CacheAlignedBuffer,
     ) -> Arc<CacheAlignedBuffer> {
         let data = Arc::new(data);
-        let mut inner = self.inner.lock().unwrap_or_else(|e| e.into_inner());
-        let data_len = data.len();
-
-        // Don't cache if single chunk exceeds budget — still return the data
-        // to the caller, just don't retain it.
-        if data_len > inner.max_bytes {
-            return data;
+        let mut inner = self.lock();
+        let addr = inner.current();
+        inner.put_decompressed((addr, coord), data)
     }
 
-        // Check if already present
-        inner.tick += 1;
-        let tick = inner.tick;
-        if let Some(&idx) = inner.slot_index.get(&coord) {
-            inner.slots[idx].last_access = tick;
-            return Arc::clone(&inner.slots[idx].data); // already cached
+    /// [`Self::prefetch_hint_in`] for the bound dataset.
+    pub fn prefetch_hint(&self, next_coords: &[ChunkCoord]) {
+        let addr = self.lock().current();
+        self.prefetch_hint_in(addr, next_coords);
     }
 
-        // Evict until we have room
-        while inner.slots.len() >= inner.max_slots
-            || (inner.current_bytes + data_len > inner.max_bytes && !inner.slots.is_empty())
-        {
-            // Find LRU slot
-            let lru_idx = inner
-                .slots
-                .iter()
-                .enumerate()
-                .min_by_key(|(_, s)| s.last_access)
-                .map(|(i, _)| i)
-                .unwrap();
-            let removed = inner.slots.swap_remove(lru_idx);
-            inner.slot_index.remove(&removed.coord);
-            // swap_remove moved the former last element into `lru_idx` (unless
-            // it *was* the last element) — fix up that element's index entry.
-            if lru_idx < inner.slots.len() {
-                let moved_coord = inner.slots[lru_idx].coord.clone();
-                inner.slot_index.insert(moved_coord, lru_idx);
-            }
-            inner.current_bytes -= removed.data.len();
-            inner.stats.evictions += 1;
-        }
+    // ----- Whole-cache operations -----
 
-        inner.current_bytes += data_len;
-        let new_idx = inner.slots.len();
-        inner.slot_index.insert(coord.clone(), new_idx);
-        inner.slots.push(CachedChunk {
-            coord,
-            data: Arc::clone(&data),
-            last_access: tick,
-        });
-        data
-    }
-
-    /// Clear the entire cache (index + decompressed data).
+    /// Clear the entire cache (indexes + decompressed data + stats).
     pub fn clear(&self) {
-        let mut inner = self.inner.lock().unwrap_or_else(|e| e.into_inner());
-        inner.index = None;
-        inner.index_addr = None;
+        let mut inner = self.lock();
+        inner.datasets.clear();
+        inner.current = None;
         inner.slots.clear();
         inner.slot_index.clear();
         inner.current_bytes = 0;
         inner.tick = 0;
         inner.last_coord = None;
         inner.stats = AccessStats::default();
-        inner.chunk_index = None;
-        inner.chunk_layout = None;
-    }
-
-    /// Record that the given chunk coordinates are predicted to be accessed
-    /// soon (bookkeeping only).
-    ///
-    /// This does **not** prefetch or pre-decompress anything — it only
-    /// checks whether each coordinate is already in the chunk index and
-    /// updates access-pattern stats accordingly. Real prefetching (e.g.
-    /// background pre-decompression) is not implemented.
-    pub fn prefetch_hint(&self, next_coords: &[ChunkCoord]) {
-        let inner = self.inner.lock().unwrap_or_else(|e| e.into_inner());
-        if inner.index.is_none() {
-            return;
-        }
-        drop(inner);
-        // For each predicted coordinate, verify it exists in the index.
-        // The index is already populated, so this is a no-op for known chunks.
-        // The purpose is to signal intent — callers can pre-decompress if needed.
-        // We touch the stats to record that prefetch hints were issued.
-        let mut inner = self.inner.lock().unwrap_or_else(|e| e.into_inner());
-        for coord in next_coords {
-            let exists = inner
-                .index
-                .as_ref()
-                .map(|idx| idx.contains_key(coord))
-                .unwrap_or(false);
-            if exists {
-                inner.stats.sequential_count += 1;
-            }
-        }
     }
 
     /// Return the current access pattern statistics.
     pub fn access_stats(&self) -> AccessStats {
-        self.inner
-            .lock()
-            .unwrap_or_else(|e| e.into_inner())
-            .stats
-            .clone()
+        self.lock().stats.clone()
     }
 
     /// Update the sweep direction label in the access stats.
     pub fn set_sweep_direction(&self, direction: &'static str) {
-        self.inner
-            .lock()
-            .unwrap_or_else(|e| e.into_inner())
-            .stats
-            .sweep_direction = Some(direction);
+        self.lock().stats.sweep_direction = Some(direction);
     }
 
-    /// Number of decompressed chunks currently cached.
+    /// Number of decompressed chunks currently cached (all datasets).
     pub fn cached_chunk_count(&self) -> usize {
-        self.inner
-            .lock()
-            .unwrap_or_else(|e| e.into_inner())
-            .slots
-            .len()
+        self.lock().slots.len()
     }
 
-    /// Total bytes of decompressed data currently cached.
+    /// Total bytes of decompressed data currently cached (all datasets).
     pub fn cached_bytes(&self) -> usize {
-        self.inner
-            .lock()
-            .unwrap_or_else(|e| e.into_inner())
-            .current_bytes
+        self.lock().current_bytes
+    }
+
+    /// Number of datasets whose chunk index is currently kept.
+    pub fn indexed_dataset_count(&self) -> usize {
+        self.lock().datasets.len()
     }
 }
 
@@ -808,6 +967,92 @@ mod tests {
         assert_eq!(cache.cached_bytes(), 0);
     }
 
+    #[test]
+    fn datasets_sharing_coordinates_stay_separate() {
+        let cache = ChunkCache::new();
+        let a = vec![make_chunk(vec![0, 0], 0x100, 8)];
+        let b = vec![make_chunk(vec![0, 0], 0x900, 8)];
+        let got_a = cache.chunks_for::<()>(1, 1, || Ok(a.clone())).unwrap();
+        let got_b = cache.chunks_for::<()>(2, 1, || Ok(b.clone())).unwrap();
+        assert_eq!(got_a[0].address, 0x100);
+        assert_eq!(got_b[0].address, 0x900);
+        // Built once per dataset: a second lookup doesn't call the builder.
+        let again = cache
+            .chunks_for::<()>(1, 1, || panic!("index rebuilt"))
+            .unwrap();
+        assert_eq!(again[0].address, 0x100);
+
+        cache.put_decompressed_in(1, vec![0], vec![1; 4]);
+        cache.put_decompressed_in(2, vec![0], vec![2; 4]);
+        assert_eq!(
+            cache.get_decompressed_in(1, &[0]).unwrap().as_slice(),
+            &[1; 4]
+        );
+        assert_eq!(
+            cache.get_decompressed_in(2, &[0]).unwrap().as_slice(),
+            &[2; 4]
+        );
+        assert!(cache.get_decompressed_in(3, &[0]).is_none());
+        assert_eq!(cache.cached_chunk_count(), 2);
+
+        // The bound-dataset methods see only the bound dataset.
+        cache.ensure_dataset(2);
+        assert_eq!(cache.lookup_index(&[0]).unwrap().address, 0x900);
+        assert_eq!(cache.get_decompressed(&[0]).unwrap(), vec![2; 4]);
+    }
+
+    #[test]
+    fn dataset_indexes_are_bounded() {
+        let cache = ChunkCache::new();
+        for addr in 0..(MAX_INDEXED_DATASETS as u64 + 10) {
+            cache
+                .chunks_for::<()>(addr, 1, || Ok(vec![make_chunk(vec![0], addr, 8)]))
+                .unwrap();
+        }
+        assert_eq!(cache.indexed_dataset_count(), MAX_INDEXED_DATASETS);
+
+        // One huge index evicts the others but is itself kept.
+        let huge: Vec<ChunkInfo> = (0..MAX_INDEXED_CHUNKS as u64)
+            .map(|i| make_chunk(vec![i], i, 8))
+            .collect();
+        let got = cache.chunks_for::<()>(9999, 1, || Ok(huge)).unwrap();
+        assert_eq!(got.len(), MAX_INDEXED_CHUNKS);
+        assert_eq!(cache.indexed_dataset_count(), 1);
+    }
+
+    #[test]
+    fn concurrent_readers_of_different_datasets_see_their_own_chunks() {
+        let cache = std::sync::Arc::new(ChunkCache::with_capacity(1 << 20, 64));
+        let handles: Vec<_> = (0..8u64)
+            .map(|t| {
+                let cache = std::sync::Arc::clone(&cache);
+                std::thread::spawn(move || {
+                    for round in 0..500u64 {
+                        let addr = (t + round) % 16;
+                        let coord = vec![round % 4];
+                        let chunks = cache
+                            .chunks_for::<()>(addr, 1, || {
+                                Ok((0..4).map(|c| make_chunk(vec![c], addr, 8)).collect())
+                            })
+                            .unwrap();
+                        assert!(chunks.iter().all(|c| c.address == addr));
+                        let want = vec![addr as u8; 8];
+                        let got = match cache.get_decompressed_in(addr, &coord) {
+                            Some(hit) => hit.to_vec(),
+                            None => cache
+                                .put_decompressed_in(addr, coord, want.clone())
+                                .to_vec(),
+                        };
+                        assert_eq!(got, want);
+                    }
+                })
+            })
+            .collect();
+        for h in handles {
+            h.join().unwrap();
+        }
+    }
+
     #[test]
     fn duplicate_insert_is_noop() {
         let cache = ChunkCache::new();

crates/clawhdf5-format/src/chunked_read.rs

@@ -836,24 +836,20 @@ pub fn read_chunked_data_cached(
         )));
     }
 
-    // The per-file cache is shared across datasets; bind it to this one so a
-    // different dataset's chunk index is never reused for this read.
-    cache.ensure_dataset(addr);
-
-    // Populate chunk index on first access
-    if !cache.has_index() {
-        let (chunks, _) = list_chunks(
+    // The per-file cache is shared across datasets (and threads); every
+    // lookup is keyed by this dataset's chunk-index address, so another
+    // dataset's index or chunks are never used for this read.
+    let chunks = cache.chunks_for(addr, rank, || {
+        list_chunks(
             file_data,
             layout,
             dataspace,
             elem_size,
             offset_size,
             length_size,
-        )?;
-        cache.populate_index(&chunks, rank);
-    }
-
-    let chunks = cache.all_indexed_chunks().unwrap_or_default();
+        )
+        .map(|(chunks, _)| chunks)
+    })?;
 
     // Assemble output
     let total_bytes = checked_byte_len(dataspace.checked_num_elements()?, elem_size)?;
@@ -920,7 +916,7 @@ pub fn read_chunked_data_cached(
             continue;
         }
         let coord: Vec<u64> = chunk_info.offsets.iter().take(rank).copied().collect();
-        match cache.get_decompressed_aligned(&coord) {
+        match cache.get_decompressed_in(addr, &coord) {
             Some(cached) => place(&cached, chunk_info),
             None => misses.push(chunk_info),
         }
@@ -957,7 +953,7 @@ pub fn read_chunked_data_cached(
                 let data = data?;
                 if cache_them {
                     let coord: Vec<u64> = chunk_info.offsets.iter().take(rank).copied().collect();
-                    let cached = cache.put_decompressed(coord, data);
+                    let cached = cache.put_decompressed_in(addr, coord, data);
                     place(&cached, chunk_info);
                 } else {
                     place(&data, chunk_info);
@@ -1161,24 +1157,20 @@ pub fn read_chunked_data_sweep(
         )));
     }
 
-    // The per-file cache is shared across datasets; bind it to this one so a
-    // different dataset's chunk index is never reused for this read.
-    cache.ensure_dataset(addr);
-
-    // Populate chunk index on first access
-    if !cache.has_index() {
-        let (chunks, _) = list_chunks(
+    // The per-file cache is shared across datasets (and threads); every
+    // lookup is keyed by this dataset's chunk-index address, so another
+    // dataset's index or chunks are never used for this read.
+    let chunks = cache.chunks_for(addr, rank, || {
+        list_chunks(
             file_data,
             layout,
             dataspace,
             elem_size,
             offset_size,
             length_size,
-        )?;
-        cache.populate_index(&chunks, rank);
-    }
-
-    let chunks = cache.all_indexed_chunks().unwrap_or_default();
+        )
+        .map(|(chunks, _)| chunks)
+    })?;
 
     // Assemble output
     let total_bytes = checked_byte_len(dataspace.checked_num_elements()?, elem_size)?;
@@ -1209,12 +1201,12 @@ pub fn read_chunked_data_sweep(
 
         // Issue prefetch hint for predicted next chunks
         if !sweep.predicted_next.is_empty() {
-            cache.prefetch_hint(&sweep.predicted_next);
+            cache.prefetch_hint_in(addr, &sweep.predicted_next);
             cache.set_sweep_direction(sweep.direction);
         }
 
         // Try decompressed cache first
-        let decompressed = if let Some(cached) = cache.get_decompressed_aligned(&coord) {
+        let decompressed = if let Some(cached) = cache.get_decompressed_in(addr, &coord) {
             cached
         } else {
             // Decompress from file
@@ -1233,7 +1225,7 @@ pub fn read_chunked_data_sweep(
             } else {
                 raw_chunk.to_vec()
             };
-            cache.put_decompressed(coord, dec)
+            cache.put_decompressed_in(addr, coord, dec)
         };
 
         let chunk_offsets: Vec<usize> = chunk_info
@@ -1317,48 +1309,34 @@ pub fn read_chunked_data_indexed(
         )));
     }
 
-    // The per-file cache is shared across datasets; bind it to this one so a
-    // different dataset's chunk index is never reused for this read.
-    cache.ensure_dataset(addr);
-
-    // Build chunk index on first access
-    if !cache.has_chunk_index() {
-        let (chunks, _) = list_chunks(
+    // Chunk index and assembly plan for this dataset, built on first access
+    // and kept per dataset (keyed by chunk-index address) in the shared cache.
+    let plan = cache.chunk_layout_for(
+        addr,
+        rank,
+        || {
+            list_chunks(
                 file_data,
                 layout,
                 dataspace,
                 elem_size,
                 offset_size,
                 length_size,
+            )
+            .map(|(chunks, _)| chunks)
+        },
+        &ds_dims,
+        &chunk_dims,
+        elem_size,
     )?;
-        cache.populate_chunk_index(&chunks, rank);
-        // Also populate the legacy index for compatibility
-        if !cache.has_index() {
-            cache.populate_index(&chunks, rank);
-        }
-    }
-
-    // Build chunk layout on first access
-    if !cache.has_chunk_layout() {
-        cache.populate_chunk_layout(&ds_dims, &chunk_dims, elem_size);
-    }
-
-    // Get the layout info (mappings, output size, chunk total bytes)
-    let (mappings_info, output_bytes, chunk_total_bytes) = cache
-        .with_chunk_layout(|layout| {
-            let info: Vec<_> = layout
-                .mappings
-                .iter()
-                .map(|m| (m.coord.clone(), m.file_offset, m.file_size, m.filter_mask))
-                .collect();
-            (info, layout.output_bytes, layout.chunk_total_bytes)
-        })
-        .ok_or_else(|| FormatError::ChunkedReadError("chunk layout not available".into()))?;
+    let chunk_total_bytes = plan.chunk_total_bytes;
 
     // Decompress chunks (using LRU cache where possible)
-    let mut chunk_buffers: Vec<Arc<CacheAlignedBuffer>> = Vec::with_capacity(mappings_info.len());
-    for (coord, file_offset, file_size, filter_mask) in &mappings_info {
-        if let Some(cached) = cache.get_decompressed_aligned(coord) {
+    let mut chunk_buffers: Vec<Arc<CacheAlignedBuffer>> = Vec::with_capacity(plan.mappings.len());
+    for m in &plan.mappings {
+        let (coord, file_offset, file_size, filter_mask) =
+            (&m.coord, &m.file_offset, &m.file_size, &m.filter_mask);
+        if let Some(cached) = cache.get_decompressed_in(addr, coord) {
             chunk_buffers.push(cached);
         } else {
             let c_addr = *file_offset as usize;
@@ -1377,17 +1355,15 @@ pub fn read_chunked_data_indexed(
                 raw_chunk.to_vec()
             };
             let aligned = CacheAlignedBuffer::from_vec(decompressed);
-            let arc = cache.put_decompressed_aligned(coord.clone(), aligned);
+            let arc = cache.put_decompressed_aligned_in(addr, coord.clone(), aligned);
             chunk_buffers.push(arc);
         }
     }
 
     // Assemble using pre-computed layout
-    let mut output = vec![0u8; output_bytes];
+    let mut output = vec![0u8; plan.output_bytes];
     let data_refs: Vec<&[u8]> = chunk_buffers.iter().map(|b| b.as_slice()).collect();
-    cache.with_chunk_layout(|layout| {
-        layout.assemble(&data_refs, &mut output);
-    });
+    plan.assemble(&data_refs, &mut output);
 
     Ok(output)
 }
@@ -2307,12 +2283,12 @@ mod tests {
         let datatype = make_f64_type();
         let cache = ChunkCache::new();
 
-        assert!(!cache.has_index());
+        assert_eq!(cache.indexed_dataset_count(), 0);
         let raw = read_chunked_data_cached(
             &file_data, &layout, &dataspace, &datatype, None, 8, 8, &cache,
         )
         .unwrap();
-        assert!(cache.has_index());
+        assert_eq!(cache.indexed_dataset_count(), 1);
         assert_eq!(raw.len(), 20 * 8);
         for i in 0..20 {
             let val = f64::from_le_bytes(raw[i * 8..(i + 1) * 8].try_into().unwrap());
@@ -2334,7 +2310,7 @@ mod tests {
             &file_data, &layout, &dataspace, &datatype, None, 8, 8, &cache,
         )
         .unwrap();
-        assert!(cache.has_index());
+        assert_eq!(cache.indexed_dataset_count(), 1);
         assert_eq!(cache.cached_chunk_count(), 0);
 
         // Second read — reuses the cached index
@@ -2343,6 +2319,7 @@ mod tests {
         )
         .unwrap();
         assert_eq!(raw1, raw2);
+        assert_eq!(cache.indexed_dataset_count(), 1);
     }
 
     #[test]

crates/clawhdf5/tests/concurrent_chunk_cache.rs

@@ -0,0 +1,87 @@
+//! A `File` is `Send + Sync` and keeps one chunk cache for all its datasets.
+//! Threads reading different chunked datasets through the same `File` must
+//! each get their own dataset's data.
+
+use std::sync::Arc;
+
+use clawhdf5::{File, FileBuilder};
+
+const DATASETS: usize = 24;
+const THREADS: usize = 16;
+const ROUNDS: usize = 40;
+
+/// Contents of dataset `k`: distinct from every other dataset's, element for
+/// element, so any chunk served from the wrong dataset shows.
+fn values(k: usize, n: usize) -> Vec<f64> {
+    (0..n).map(|i| (k * 100_000 + i) as f64).collect()
+}
+
+fn build() -> File {
+    let mut b = FileBuilder::new();
+    for k in 0..DATASETS {
+        let ds = b.create_dataset(&format!("d{k:02}"));
+        match k % 3 {
+            // 1-D, compressed: chunk offsets 0, 8, 16, ... in every dataset.
+            0 => {
+                ds.with_f64_data(&values(k, 64)).with_shape(&[64]);
+                ds.with_chunks(&[8]).with_deflate(1);
+            }
+            // 1-D, shuffle + compressed, a different length.
+            1 => {
+                ds.with_f64_data(&values(k, 40)).with_shape(&[40]);
+                ds.with_chunks(&[8]).with_shuffle().with_deflate(1);
+            }
+            // 2-D, compressed: coordinates (0,0), (0,4), (4,0), ... overlap
+            // the other datasets' in the first dimension.
+            _ => {
+                ds.with_f64_data(&values(k, 64)).with_shape(&[8, 8]);
+                ds.with_chunks(&[4, 4]).with_deflate(1);
+            }
+        }
+    }
+    File::from_bytes(b.finish().unwrap()).unwrap()
+}
+
+fn expected(k: usize) -> Vec<f64> {
+    values(k, if k % 3 == 1 { 40 } else { 64 })
+}
+
+#[test]
+fn threads_reading_different_datasets_get_their_own_chunks() {
+    let file = Arc::new(build());
+    // Sequential sanity check first.
+    for k in 0..DATASETS {
+        let got = file.dataset(&format!("d{k:02}")).unwrap().read_f64();
+        assert_eq!(got.unwrap(), expected(k), "sequential d{k:02}");
+    }
+
+    let handles: Vec<_> = (0..THREADS)
+        .map(|t| {
+            let file = Arc::clone(&file);
+            std::thread::spawn(move || {
+                let mut wrong = Vec::new();
+                for round in 0..ROUNDS {
+                    let k = (t * 7 + round * 5) % DATASETS;
+                    let name = format!("d{k:02}");
+                    match file.dataset(&name).unwrap().read_f64() {
+                        Ok(v) if v == expected(k) => {}
+                        Ok(v) => wrong.push(format!("{name}: wrong data, first {:?}", &v[..4])),
+                        Err(e) => wrong.push(format!("{name}: {e}")),
+                    }
+                }
+                wrong
+            })
+        })
+        .collect();
+    let failures: Vec<String> = handles
+        .into_iter()
+        .flat_map(|h| h.join().unwrap())
+        .collect();
+    assert!(
+        failures.is_empty(),
+        "{} of {} concurrent reads were wrong, e.g. {:?}",
+        failures.len(),
+        THREADS * ROUNDS,
+        &failures[..failures.len().min(5)]
+    );
+}