crates/clawhdf5-format/src/chunked_read.rs

@@ -1491,7 +1491,7 @@ pub fn list_chunks_for_read_in<S: Storage + ?Sized>(
         let chunk_bytes = checked_chunk_byte_len(&chunk_dims, elem_size)?;
         if let Some(c) = chunks
             .iter()
-            .find(|c| c.address != u64::MAX && c.chunk_size as usize != chunk_bytes)
+            .find(|c| c.address != u64::MAX && c.chunk_size != chunk_bytes as u64)
         {
             return Err(FormatError::ChunkedReadError(format!(
                 "incorrect chunk size returned from index for unfiltered chunk at {:?}: \
@@ -2470,7 +2470,7 @@ mod tests {
         // Entries: key[i], child[i] pairs, then final key
         for chunk in chunks {
             // Key: chunk_size(4) + filter_mask(4) + ndims offsets
-            buf.extend_from_slice(&chunk.chunk_size.to_le_bytes());
+            buf.extend_from_slice(&(chunk.chunk_size as u32).to_le_bytes());
             buf.extend_from_slice(&chunk.filter_mask.to_le_bytes());
             for d in 0..ndims {
                 let off = if d < chunk.offsets.len() {

crates/clawhdf5-format/src/chunked_write.rs

@@ -2263,7 +2263,7 @@ mod tests {
         for i in 0..n {
             let (offsets, chunk) = extract_chunk(&data, &shape, &chunks, 2, i);
             assert_eq!(chunk.len(), 2 * 3 * 2 * 2);
-            for (e, pair) in chunk.chunks_exact(2).enumerate() {
+            for (e, pair) in chunk.as_chunks::<2>().0.iter().enumerate() {
                 let c = [e / 6, (e / 2) % 3, e % 2];
                 let g: Vec<u64> = (0..3).map(|d| offsets[d] + c[d] as u64).collect();
                 let expect = if (0..3).all(|d| g[d] < shape[d]) {
@@ -2272,7 +2272,7 @@ mod tests {
                 } else {
                     [0, 0]
                 };
-                assert_eq!(pair, expect, "chunk {i} element {e}");
+                assert_eq!(*pair, expect, "chunk {i} element {e}");
             }
         }
         // Data shorter than the shape: the missing elements stay zero.

crates/clawhdf5-format/src/extensible_array.rs

@@ -946,7 +946,7 @@ mod tests {
         assert_eq!(chunks.len(), 2);
         assert_eq!(chunks[0].address, base_addr);
         assert_eq!(chunks[0].offsets, vec![0]);
-        assert_eq!(chunks[0].chunk_size, chunk_byte_size as u64);
+        assert_eq!(chunks[0].chunk_size, chunk_byte_size);
         assert_eq!(chunks[1].address, base_addr + chunk_byte_size);
         assert_eq!(chunks[1].offsets, vec![20]);
     }

crates/clawhdf5-format/src/filters.rs

@@ -2764,6 +2764,39 @@ mod tests {
         assert_eq!(decompressed, data);
     }
 
+    /// A chunk's size bounds an LZ4 chunk, not the 256 MiB ceiling for an
+    /// unknown size: a 300 MiB chunk was refused ("declared size exceeds
+    /// limit").
+    #[test]
+    #[cfg(feature = "lz4")]
+    fn lz4_chunks_over_256_mib_decode() {
+        let mut data = vec![0u8; 300 << 20];
+        data[12345] = 7;
+        let compressed = lz4_compress(&data, &[]).unwrap();
+        let decompressed = lz4_decompress(&compressed, data.len()).unwrap();
+        assert!(decompressed == data);
+        // Without a chunk size the ceiling still applies.
+        assert!(lz4_decompress(&compressed, 0).is_err());
+    }
+
+    /// A chunk of 4 GiB or more is always in the registered framing, whose
+    /// big-endian size then does not start with four zero bytes: its size
+    /// is read whole (here larger than the chunk, so refused before any
+    /// allocation), not taken for a legacy 4-byte size.
+    #[test]
+    #[cfg(all(feature = "lz4", target_pointer_width = "64"))]
+    fn lz4_chunks_of_4_gib_use_the_registered_framing() {
+        let chunk = (1usize << 32) + 8;
+        let mut data = ((chunk + 8) as u64).to_be_bytes().to_vec();
+        data.extend_from_slice(&(1u32 << 30).to_be_bytes());
+        data.extend_from_slice(&[0; 8]);
+        let err = lz4_decompress(&data, chunk).unwrap_err();
+        assert!(
+            matches!(&err, FormatError::DecompressionError(m) if m.contains("exceeds chunk size")),
+            "{err:?}"
+        );
+    }
+
     #[test]
     #[cfg(feature = "lz4")]
     fn pipeline_lz4_only() {

crates/clawhdf5-format/src/parallel_read.rs

@@ -255,7 +255,7 @@ pub fn decompress_chunks_lane_partitioned_in<S: Storage + ?Sized>(
                     for &local in &indices {
                         let index = batch.start + local;
                         let chunk_info = &chunks[index];
-                        let size = chunk_info.chunk_size as usize;
+                        let size = crate::addr::saturating_usize(chunk_info.chunk_size);
                         let raw_chunk = raw_bytes.get(index, &reqs[index])?;
 
                         let decompressed = decompress_chunk_exact(

crates/clawhdf5-format/src/partial_read.rs

@@ -368,7 +368,10 @@ pub fn read_selection_filled_in<S: Storage + ?Sized>(
     fill: Option<&[u8]>,
 ) -> Result<Option<Vec<u8>>, FormatError> {
     let dims = &dataspace.dimensions;
-    if dims.is_empty() || elem_size == 0 {
+    // A fill value that is not one element's bytes is the full path's to
+    // interpret.
+    let odd_fill = fill.is_some_and(|f| !f.is_empty() && f.len() != elem_size);
+    if dims.is_empty() || elem_size == 0 || odd_fill {
         return Ok(None);
     }
     let total = dataspace.checked_num_elements()?;

crates/clawhdf5-format/tests/raw_fetch_bounds.rs

@@ -151,7 +151,7 @@ fn crafted() -> (Vec<u8>, Chunked, Vec<ChunkInfo>) {
         // v1 B-tree key (size, filter mask, offsets + 0) then the child
         // address.
         let mut pat = Vec::new();
-        pat.extend_from_slice(&c.chunk_size.to_le_bytes());
+        pat.extend_from_slice(&(c.chunk_size as u32).to_le_bytes());
         pat.extend_from_slice(&c.filter_mask.to_le_bytes());
         // The key holds one offset per dimension plus the element offset
         // (0); `offsets` may or may not list that last one.

crates/clawhdf5-tools/src/check.rs

@@ -916,7 +916,7 @@ impl Checker<'_> {
                 bad.push("has size 0".into());
             } else if !filtered
                 && let Some(cb) = chunk_bytes
-                && u64::from(c.chunk_size) != cb
+                && c.chunk_size != cb
             {
                 bad.push(format!(
                     "is {} bytes; an unfiltered chunk is {cb}",
@@ -933,7 +933,7 @@ impl Checker<'_> {
                     );
                 }
             }
-            self.extent(c.address, u64::from(c.chunk_size), path);
+            self.extent(c.address, c.chunk_size, path);
         }
         if reported > 50 {
             self.problem(

crates/clawhdf5-tools/src/info.rs

@@ -224,7 +224,7 @@ pub fn allocated_bytes(h5: &H5, info: &DsInfo) -> Result<u64> {
             let ds = info.ds.as_ref().map_err(Clone::clone)?;
             chunks(h5, layout, ds, dt)?
                 .iter()
-                .map(|c| u64::from(c.chunk_size))
+                .map(|c| c.chunk_size)
                 .sum()
         }
         DataLayout::Virtual { .. } => 0,

crates/clawhdf5/src/edit/mod.rs

@@ -684,7 +684,7 @@ impl<'t> ChunkedEdit<'t> {
             }
             _ => return Err(Error::Unsupported("chunk index missing".into())),
         }
-        img.free(info.address, u64::from(info.chunk_size));
+        img.free(info.address, info.chunk_size);
         Ok(())
     }
 
@@ -1638,7 +1638,7 @@ fn store_chunk(
     let len = bytes.len() as u64;
     let placed = match existing {
         Some(info) if t.pipeline.is_none() => {
-            if u64::from(info.chunk_size) != len {
+            if info.chunk_size != len {
                 return Err(Error::Unsupported(
                     "unfiltered chunk stored at an unexpected size".into(),
                 ));
@@ -1650,11 +1650,10 @@ fn store_chunk(
         // thing in the file (the chunk an append keeps rewriting usually is)
         // and can grow there.
         Some(info)
-            if len <= u64::from(info.chunk_size)
-                || img.grow_tail(info.address, u64::from(info.chunk_size), len)? =>
+            if len <= info.chunk_size || img.grow_tail(info.address, info.chunk_size, len)? =>
         {
             img.write(info.address, &bytes)?;
-            (len != u64::from(info.chunk_size) || info.filter_mask != mask).then_some(Elem {
+            (len != info.chunk_size || info.filter_mask != mask).then_some(Elem {
                 addr: info.address,
                 size: len,
                 mask,
@@ -1664,7 +1663,7 @@ fn store_chunk(
             let a = img.alloc(len)?;
             img.write(a, &bytes)?;
             if let Some(info) = existing {
-                img.free(info.address, u64::from(info.chunk_size));
+                img.free(info.address, info.chunk_size);
             }
             Some(Elem {
                 addr: a,
@@ -1682,14 +1681,16 @@ fn store_chunk(
 fn img_read<'a>(f: &'a File, info: &ChunkInfo) -> Result<&'a [u8], Error> {
     let start = usize::try_from(info.address)
         .map_err(|_| Error::Unsupported("chunk address out of range".into()))?;
-    f.as_bytes()
-        .get(start..start + info.chunk_size as usize)
-        .ok_or_else(|| {
-            Error::Format(clawhdf5_format::error::FormatError::UnexpectedEof {
-                expected: start + info.chunk_size as usize,
-                available: f.as_bytes().len(),
-            })
+    let end = usize::try_from(info.chunk_size)
+        .ok()
+        .and_then(|n| start.checked_add(n))
+        .unwrap_or(usize::MAX);
+    f.as_bytes().get(start..end).ok_or_else(|| {
+        Error::Format(clawhdf5_format::error::FormatError::UnexpectedEof {
+            expected: end,
+            available: f.as_bytes().len(),
         })
+    })
 }
 
 fn decode_chunk(

crates/clawhdf5/tests/huge_chunks_interop.rs

@@ -88,7 +88,9 @@ fn layout_of(
 
 /// The fixture's datasets: name, chunk index type, shape, and the scaled
 /// origins of the chunks libhdf5 wrote.
-const FILTERED: &[(&str, u8, &[u64], &[&[u64]])] = &[
+type Case = (&'static str, u8, &'static [u64], &'static [&'static [u64]]);
+
+const FILTERED: &[Case] = &[
     ("single", 1, &[N], &[&[0]]),
     ("farray", 3, &[N + 10], &[&[0], &[N]]),
     ("earray", 4, &[N + 10], &[&[0], &[N]]),

crates/clawhdf5/tests/partial_read_equivalence.rs

@@ -195,3 +195,90 @@ fn out_of_bounds_selections_are_errors() {
         [99]
     );
 }
+
+/// A chunked dataset with a fill value and chunks never written: a
+/// selection is read over a box of fill values from the chunks it touches
+/// (it used to be picked out of a full read), and through positioned reads
+/// an unfiltered chunk is read row by row. Both equal the full read, in
+/// every chunk index h5py writes.
+#[test]
+fn fill_value_selections_match_full_reads() {
+    let python = std::env::var("CLAWHDF5_PYTHON").unwrap_or_else(|_| "python3".into());
+    let has_h5py = std::process::Command::new(&python)
+        .args(["-c", "import h5py"])
+        .output()
+        .is_ok_and(|o| o.status.success());
+    if !has_h5py {
+        assert!(
+            std::env::var("CLAWHDF5_REQUIRE_INTEROP").as_deref() != Ok("1"),
+            "CLAWHDF5_REQUIRE_INTEROP=1 but python3 with h5py is not available"
+        );
+        eprintln!("SKIP: python3 with h5py not available");
+        return;
+    }
+    let dir = tempfile::tempdir().unwrap();
+    let path = dir.path().join("fill.h5");
+    let script = r#"
+import sys, h5py, numpy as np
+with h5py.File(sys.argv[1], "w", libver="latest") as f:
+    for name, maxshape, gz in [
+        ("farray", None, False), ("farray_gz", None, True),
+        ("earray", (None, 23), False), ("earray_gz", (None, 23), True),
+        ("btree2", (None, None), False), ("btree2_gz", (None, None), True),
+    ]:
+        d = f.create_dataset(name, shape=(37, 23), maxshape=maxshape, chunks=(5, 4),
+                             dtype="<i4", fillvalue=-5,
+                             compression="gzip" if gz else None)
+        d[3:17, 2:11] = np.arange(14 * 9, dtype="<i4").reshape(14, 9)
+        d[30, 20] = 7
+"#;
+    let out = std::process::Command::new(&python)
+        .args(["-c", script])
+        .arg(&path)
+        .output()
+        .unwrap();
+    assert!(
+        out.status.success(),
+        "{}",
+        String::from_utf8_lossy(&out.stderr)
+    );
+
+    let dims = [37u64, 23];
+    let mapped = File::open(&path).unwrap();
+    let storage = clawhdf5::FileStorage::open(&path).unwrap();
+    let positioned = File::open_storage(std::sync::Arc::new(storage)).unwrap();
+    let mut rng = Rng(11);
+    for name in [
+        "farray",
+        "farray_gz",
+        "earray",
+        "earray_gz",
+        "btree2",
+        "btree2_gz",
+    ] {
+        let full = mapped.dataset(name).unwrap().read_i32().unwrap();
+        assert_eq!(full[0], -5, "{name}");
+        assert_eq!(full[3 * 23 + 2], 0, "{name}");
+        for case in 0..60 {
+            let selection = if case % 5 == 4 {
+                let points = (0..1 + rng.below(12))
+                    .map(|_| dims.iter().map(|&d| rng.below(d)).collect())
+                    .collect();
+                Selection::Points(points)
+            } else {
+                random_hyperslab(&mut rng, &dims)
+            };
+            let want = reference(&full, &dims, &selection);
+            for (how, file) in [("mapped", &mapped), ("positioned", &positioned)] {
+                assert_eq!(
+                    file.dataset(name)
+                        .unwrap()
+                        .read_i32_selection(&selection)
+                        .unwrap(),
+                    want,
+                    "{name} {how} case {case}: {selection:?}"
+                );
+            }
+        }
+    }
+}

examples/wasm-viewer/test/test.mjs

@@ -500,6 +500,20 @@ async function limitTests() {
     await fails(() => pkg.openUrl("http://huge.invalid/x.h5", { fetch: mockFetch(farBytes, { total }) }),
       total > 2 ** 53 ? /2\^53 - 1/ : /4 GiB/, `length ${total}`);
   }
+
+  // Nor can it hold a chunk of 4 GiB or more (HDF5 2.0, layout message
+  // version 5): the file opens and lists, and reading such a chunk is an
+  // error naming the size, in every chunk index.
+  const hugeChunks = join(import.meta.dirname, "..", "..", "..",
+    "crates/clawhdf5/tests/fixtures/huge_chunks_filtered.h5");
+  const hc = pkg.open(new Uint8Array(readFileSync(hugeChunks)));
+  eq(hc.list("/").map((e) => e.name), ["btree2", "earray", "farray", "single"], "huge chunks: list");
+  for (const name of ["single", "farray", "earray", "btree2"]) {
+    const [start, count] = name === "btree2" ? [[0, 0], [1, 4]] : [[0], [4]];
+    await fails(() => hc.readHyperslab(`/${name}`, start, count), /exceeds the addressable size/,
+      `huge chunk: ${name}`);
+  }
+  hc.free();
 }
 
 // A body of `total` bytes in 64 KiB pieces, made as they are read; `pulled()`