clawhdf5: set_attr adds the Attribute Info message a version-2 header needs

libhdf5 counts a version-2 object header's attributes through its Attribute
Info message (0x15) and reports none when the header has none. set_attr gave
v110/latest groups, the root group and datasets without attributes an
attribute message only, so h5py listed the attribute but len(obj.attrs) and
H5Oget_info's num_attrs said 0, and stayed wrong after h5py r+ added more.

Like H5O__attr_create, the edit now adds the message when a version-2 header
lacks it, in the same planned edit: version 0, the header's creation-order
track/index flags, maximum creation index 0, undefined fractal heap and
B-tree addresses, message flag DONTSHARE — byte for byte what libhdf5
writes. It goes before the attribute (libhdf5's order) when free space
holds both, else after it, so a continuation chunk made for the attribute
also takes it.

Test: edit_interop attribute_count_in_version_2_headers — v110 and latest
files, attributes set on the root group, groups and datasets with and
without existing attributes: h5py's len/num_attrs/list/values, h5dump -A
and our reader agree, also after h5py r+ adds attributes up to and past the
compact limit. Fails on the previous editor (h5py len 0).

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

crates/clawhdf5/src/edit/mod.rs

@@ -47,6 +47,8 @@ const MSG_EXTERNAL: u16 = 0x07;
 const MSG_ATTR_INFO: u16 = 0x15;
 /// Message flag: the message is shared (stored elsewhere).
 const MSG_FLAG_SHARED: u8 = 0x02;
+/// Message flag: the message must not be shared (`H5O_MSG_FLAG_DONTSHARE`).
+const MSG_FLAG_DONTSHARE: u8 = 0x04;
 
 /// An HDF5 file opened for in-place modification.
 ///
@@ -815,7 +817,24 @@ impl FileEditor {
             if let Some(i) = existing {
                 hdr.delete(img, i)?;
             }
+            // libhdf5 counts a version-2 header's attributes through its
+            // Attribute Info message and reports none without one; like
+            // H5O__attr_create, add it when missing: before the attribute
+            // when free space holds both (libhdf5's order), else after it,
+            // so that a new continuation chunk made for the attribute has
+            // room for it too.
+            let ainfo = (hdr.version == 2 && hdr.find(MSG_ATTR_INFO).is_none())
+                .then(|| attr_info_message(hdr.flags, img.os));
+            let ainfo_first = ainfo
+                .as_ref()
+                .is_some_and(|a| hdr.has_free(a.len() + hdr.hsize() + body.len()));
+            if let Some(a) = ainfo.as_ref().filter(|_| ainfo_first) {
+                hdr.insert(img, MSG_ATTR_INFO, MSG_FLAG_DONTSHARE, a, None)?;
+            }
             hdr.insert(img, MSG_ATTRIBUTE, 0, &body, None)?;
+            if let Some(a) = ainfo.as_ref().filter(|_| !ainfo_first) {
+                hdr.insert(img, MSG_ATTR_INFO, MSG_FLAG_DONTSHARE, a, None)?;
+            }
             hdr.finish(img)
         })
     }
@@ -900,6 +919,26 @@ fn attr_name(d: &[u8]) -> Result<&[u8], Error> {
     Ok(name.split(|&b| b == 0).next().unwrap_or(name))
 }
 
+/// A new Attribute Info message for a version-2 header with flags
+/// `hdr_flags`, as `H5O__attr_create` makes it: version 0, creation order
+/// tracked / indexed as the header's flags say, maximum creation index 0,
+/// and no dense storage (undefined fractal heap and B-tree addresses).
+fn attr_info_message(hdr_flags: u8, os: u8) -> Vec<u8> {
+    let track = hdr_flags & 0x04 != 0;
+    let index = hdr_flags & 0x08 != 0;
+    let mut b = vec![0u8, u8::from(track) | (u8::from(index) << 1)];
+    if track {
+        b.extend_from_slice(&0u16.to_le_bytes());
+    }
+    let undef_addr = vec![0xffu8; os as usize];
+    b.extend_from_slice(&undef_addr);
+    b.extend_from_slice(&undef_addr);
+    if index {
+        b.extend_from_slice(&undef_addr);
+    }
+    b
+}
+
 /// A version-2 header's limit on compact attributes: stored when its flags
 /// say so, else libhdf5's default of 8.
 fn max_compact_attrs(img: &Image<'_>, hdr: &Header) -> Result<u16, Error> {

crates/clawhdf5/src/edit/ohdr.rs

@@ -326,6 +326,11 @@ impl Header {
             .map(|(i, _)| i)
     }
 
+    /// Whether free space in the header can take a body of `len` bytes.
+    pub(crate) fn has_free(&self, len: usize) -> bool {
+        self.best_nil(self.padded(len)).is_some()
+    }
+
     /// Put a message into slot `i` (a NIL message, or a message being
     /// moved away), splitting off the rest as a NIL message.
     fn place(