fix(format): read array members of version-1 compound datatypes

HDF5 1.6 encoded a compound member that is a fixed-size array through
legacy per-member fields (dimensionality, permutation, four dimension
sizes) that the v1 decoder skipped, so a [4] i32 member read as one i32
with the wrong size. Build the array type from those fields as libhdf5
does (ignoring the permutation) and refuse more than four dimensions.

Co-Authored-By: Claude Opus 5.5 (1M context) <[email protected]>

CHANGELOG.md

@@ -273,6 +273,12 @@
 - CI keeps zlib-ng building and tested; the arm64 job no longer needs cmake.
 
 ### Correctness
+- `clawhdf5-format` reader: array members of version-1 compound datatypes
+  (HDF5 1.6-era files, e.g. libhdf5's `tcompound.h5`) were read as a single
+  element: a `[4] i32` member came back as one `i32`, with the wrong size.
+  The legacy per-member dimension fields are now decoded into an array type,
+  as libhdf5 does; more than four dimensions, or a zero-sized one, is an
+  error.
 - `clawhdf5-format` reader — **values returned wrong with no error:**
   - Fixed Array and Extensible Array chunk indexes were laid out by the
     dataset's current shape instead of its max shape (23 libhdf5 test files,

crates/clawhdf5-format/src/datatype.rs

@@ -423,13 +423,44 @@ impl Datatype {
                         ensure_len(data, pos, 4)?;
                         let byte_offset = LittleEndian::read_u32(&data[pos..pos + 4]) as u64;
                         pos += 4;
+                        // v1 members can be fixed-size arrays of the member
+                        // type (libhdf5 builds an array type from these
+                        // fields; the permutation is ignored, as libhdf5
+                        // does). Skipping them read a `[4] i32` member as
+                        // one `i32`.
+                        let mut array_dims = Vec::new();
                         if version == 1 {
                             ensure_len(data, pos, 28)?;
+                            let ndims = data[pos] as usize;
+                            // libhdf5 refuses more than four dimensions and,
+                            // when building the array type, a zero-sized one.
+                            let zero_dim = (0..ndims.min(4)).any(|j| {
+                                let at = pos + 12 + 4 * j;
+                                LittleEndian::read_u32(&data[at..at + 4]) == 0
+                            });
+                            if ndims > 4 || zero_dim {
+                                return Err(FormatError::InvalidDatatypeVersion {
+                                    class: class_id,
+                                    version,
+                                });
+                            }
+                            array_dims = (0..ndims)
+                                .map(|j| {
+                                    let at = pos + 12 + 4 * j;
+                                    LittleEndian::read_u32(&data[at..at + 4])
+                                })
+                                .collect();
                             pos += 28;
                         }
-                        let (member_dt, consumed) =
+                        let (mut member_dt, consumed) =
                             Self::parse_with_depth(&data[pos..], depth + 1)?;
                         pos += consumed;
+                        if !array_dims.is_empty() {
+                            member_dt = Datatype::Array {
+                                base_type: Box::new(member_dt),
+                                dimensions: array_dims,
+                            };
+                        }
                         members.push(CompoundMember {
                             name,
                             byte_offset,
@@ -1336,6 +1367,64 @@ mod tests {
         assert_xyid_compound(dt);
     }
 
+    #[test]
+    fn test_compound_v1_member_array_fields() {
+        // HDF5 1.6 wrote array members of a v1 compound through the legacy
+        // per-member fields (as in libhdf5's tools/test/testfiles/
+        // tcompound.h5 `type2`: `int_array` [4] i32, `float_array` [5][6]
+        // f32). They used to be skipped, reading each member as a scalar.
+        let i32le: [u8; 12] = [
+            0x10, 0x08, 0x00, 0x00, 0x04, 0x00, 0x00, 0x00, 0x00, 0x00, 0x20, 0x00,
+        ];
+        let mut b = vec![0x16, 0x02, 0x00, 0x00, 0x88, 0x00, 0x00, 0x00];
+        for (name, offset, dims) in [
+            (&b"int_array"[..], 0u32, &[4u32][..]),
+            (&b"xy"[..], 16, &[5u32, 6][..]),
+        ] {
+            let mut padded = name.to_vec();
+            padded.resize((name.len() + 1 + 7) & !7, 0);
+            b.extend_from_slice(&padded);
+            b.extend_from_slice(&offset.to_le_bytes());
+            b.push(dims.len() as u8);
+            b.extend_from_slice(&[0u8; 3 + 4 + 4]); // reserved, permutation, reserved
+            for j in 0..4 {
+                b.extend_from_slice(&dims.get(j).copied().unwrap_or(0).to_le_bytes());
+            }
+            b.extend_from_slice(&i32le);
+        }
+        let (dt, consumed) = Datatype::parse(&b).unwrap();
+        assert_eq!(consumed, b.len());
+        let Datatype::Compound { members, .. } = dt else {
+            panic!("expected Compound, got {dt:?}");
+        };
+        let got: Vec<(&str, u64, u32, Option<Vec<u32>>)> = members
+            .iter()
+            .map(|m| {
+                let dims = match &m.datatype {
+                    Datatype::Array { dimensions, .. } => Some(dimensions.clone()),
+                    _ => None,
+                };
+                (m.name.as_str(), m.byte_offset, m.datatype.type_size(), dims)
+            })
+            .collect();
+        assert_eq!(
+            got,
+            vec![
+                ("int_array", 0, 16, Some(vec![4])),
+                ("xy", 16, 120, Some(vec![5, 6])),
+            ]
+        );
+
+        // More than four dimensions cannot be encoded, and libhdf5 refuses a
+        // zero-sized dimension (a fuzzed tcompound.h5, cve-2024-32616.h5).
+        let mut bad = b.clone();
+        bad[8 + 16 + 4] = 5;
+        assert!(Datatype::parse(&bad).is_err());
+        let mut bad = b.clone();
+        bad[8 + 16 + 4] = 2; // [4, 0]
+        assert!(Datatype::parse(&bad).is_err());
+    }
+
     #[test]
     fn test_compound_v1_truncated_is_error_not_panic() {
         let bytes = compound_v1_bytes();

docs/known-issues.md

@@ -74,6 +74,8 @@ the VDS item, which is marked.
   - Hyperslab selection versions 1 and 2 are refused.
 - **Files with a user block:** the base address is not applied.
 - **Old-style shared messages (version 1)** read the wrong address.
+- **Array members of version-1 compound datatypes** (found while fixing the
+  item above) were read as one element. **Fixed 2026-09-25.**
 - **Groups and links:**
   - Groups with a user-defined link type (e.g. 187) cannot be listed.
   - Dense groups with more than about 22 000 links cannot be listed.