feat(format): ObjectHeader::object_class, libhdf5's object classification

libhdf5 decides what an object header is in a fixed order
(H5O__obj_class_real): a group if it has a Symbol Table or Link Info
message, a dataset if it has a Datatype *and* a Dataspace message, a named
datatype if it has a Datatype message. The conformance probe called any
header with a Data Layout message a dataset, so cve-2024-33874's /Dset1 (a
datatype and a layout, no dataspace), which h5py opens as a named
datatype, was reported as a dataset we failed to read
(MissingMessage(Dataspace)). The probe now classifies with
object_class().

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

crates/clawhdf5-format/src/object_header.rs

@@ -75,7 +75,40 @@ fn read_offset(data: &[u8], pos: usize, size: u8) -> Result<u64, FormatError> {
     })
 }
 
+/// The kind of object an object header describes.
+#[derive(Debug, Clone, Copy, PartialEq, Eq)]
+pub enum ObjectClass {
+    /// A group: the header has a Symbol Table or a Link Info message.
+    Group,
+    /// A dataset: the header has a Datatype and a Dataspace message.
+    Dataset,
+    /// A committed (named) datatype: a Datatype message, no Dataspace.
+    NamedDatatype,
+}
+
 impl ObjectHeader {
+    /// The kind of object this header describes, decided as libhdf5 decides
+    /// it (`H5O__obj_class_real`): group first (a Symbol Table or Link Info
+    /// message), then dataset (a Datatype *and* a Dataspace message — not a
+    /// Data Layout message), then named datatype (a Datatype message).
+    /// `None` when none applies; libhdf5 then cannot open the object
+    /// ("unable to determine object type").
+    ///
+    /// A header with a Datatype and a Data Layout message but no Dataspace
+    /// is a named datatype to libhdf5, not a dataset.
+    pub fn object_class(&self) -> Option<ObjectClass> {
+        let has = |t: MessageType| self.messages.iter().any(|m| m.msg_type == t);
+        if has(MessageType::SymbolTable) || has(MessageType::LinkInfo) {
+            Some(ObjectClass::Group)
+        } else if has(MessageType::Datatype) && has(MessageType::Dataspace) {
+            Some(ObjectClass::Dataset)
+        } else if has(MessageType::Datatype) {
+            Some(ObjectClass::NamedDatatype)
+        } else {
+            None
+        }
+    }
+
     /// Parse an object header at the given offset in the data buffer.
     ///
     /// `offset_size` and `length_size` come from the superblock.
@@ -662,6 +695,54 @@ fn check_message(
 mod tests {
     use super::*;
 
+    fn header_with(types: &[MessageType]) -> ObjectHeader {
+        ObjectHeader {
+            version: 2,
+            messages: types
+                .iter()
+                .map(|&msg_type| HeaderMessage {
+                    msg_type,
+                    size: 0,
+                    flags: 0,
+                    creation_order: None,
+                    data: Vec::new(),
+                })
+                .collect(),
+            reference_count: None,
+            flags: 0,
+            access_time: None,
+            modification_time: None,
+            change_time: None,
+            birth_time: None,
+        }
+    }
+
+    #[test]
+    fn object_class_follows_libhdf5() {
+        use MessageType::*;
+        let class = |t: &[MessageType]| header_with(t).object_class();
+        assert_eq!(
+            class(&[Datatype, Dataspace, DataLayout]),
+            Some(ObjectClass::Dataset)
+        );
+        // A Data Layout message does not make a dataset without a dataspace
+        // (cve-2024-33874 `/Dset1`: h5py opens it as a named datatype).
+        assert_eq!(
+            class(&[Datatype, DataLayout]),
+            Some(ObjectClass::NamedDatatype)
+        );
+        assert_eq!(class(&[Datatype]), Some(ObjectClass::NamedDatatype));
+        // Group messages win over dataset messages.
+        assert_eq!(
+            class(&[Datatype, Dataspace, SymbolTable]),
+            Some(ObjectClass::Group)
+        );
+        assert_eq!(class(&[LinkInfo]), Some(ObjectClass::Group));
+        // Link messages alone are not a group; nothing is not an object.
+        assert_eq!(class(&[Link]), None);
+        assert_eq!(class(&[]), None);
+    }
+
     // Helper: build a v1 object header with given messages
     fn build_v1_header(
         messages: &[(u16, &[u8], u8)], // (type, data, flags)