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-tools/tests/edit_interop.rs

@@ -1315,3 +1315,90 @@ fn optional_filters_that_fail_are_skipped() {
         assert!(o.status.success(), "h5rs check --data {p}:\n{}", text(&o));
     }
 }
+
+/// libhdf5 counts a version-2 object header's attributes through its
+/// Attribute Info message and reports none without one. The editor adds
+/// that message, as `H5O__attr_create` does, when it gives a version-2
+/// header (h5py `libver='v110'`/`'latest'`) its first attribute: afterwards
+/// h5py lists, counts and reads every attribute, new and existing, h5dump
+/// agrees, and h5py `r+` can add more (past the compact limit, into dense
+/// storage) with the count still right.
+#[test]
+fn attribute_count_in_version_2_headers() {
+    if !tools_ok() {
+        return;
+    }
+    for (lv, dump) in [("'v110'", true), ("'latest'", false)] {
+        let dir = tmpdir();
+        let path = dir.path().join("acount.h5");
+        let p = path.to_str().unwrap();
+        py(&format!(
+            "import h5py, numpy as np\n\
+             with h5py.File({p:?}, 'w', libver={lv}) as f:\n\
+             \x20   f.create_group('g')\n\
+             \x20   f.create_group('has').attrs['old'] = 7\n\
+             \x20   f.create_dataset('d', data=np.arange(3, dtype='<i4'))\n\
+             \x20   f.create_dataset('e', data=np.arange(3, dtype='<i4')).attrs['old'] = 7\n"
+        ));
+        let mut ed = FileEditor::open(&path).unwrap();
+        for obj in ["/", "g", "has", "d", "e"] {
+            for i in 0..3 {
+                ed.set_attr(obj, &format!("a{i}"), &AttrValue::I64(i))
+                    .unwrap();
+            }
+            // Replacing one keeps the count.
+            ed.set_attr(obj, "a1", &AttrValue::F64(1.5)).unwrap();
+        }
+        drop(ed);
+        check_tools(&path, dump);
+        let check = |extra: usize| {
+            py(&format!(
+                "import h5py\n\
+                 f = h5py.File({p:?}, 'r')\n\
+                 for o in ['/', 'g', 'has', 'd', 'e']:\n\
+                 \x20   a = f[o].attrs\n\
+                 \x20   want = {{'a0': 0, 'a1': 1.5, 'a2': 2}}\n\
+                 \x20   if o in ('has', 'e'): want['old'] = 7\n\
+                 \x20   for i in range({extra}): want[f'h{{i}}'] = i\n\
+                 \x20   assert len(a) == len(want), (o, len(a), list(a))\n\
+                 \x20   assert h5py.h5o.get_info(f[o].id).num_attrs == len(want), o\n\
+                 \x20   assert sorted(a) == sorted(want), (o, list(a))\n\
+                 \x20   assert {{k: a[k] for k in a}} == want, (o, dict(a))\n"
+            ));
+            let f = File::open(&path).unwrap();
+            for (o, n) in [("/", 3), ("g", 3), ("has", 4), ("d", 3), ("e", 4)] {
+                let attrs = match o {
+                    "/" => f.root().attrs().unwrap(),
+                    "d" | "e" => f.dataset(o).unwrap().attrs().unwrap(),
+                    _ => f.group(o).unwrap().attrs().unwrap(),
+                };
+                assert_eq!(attrs.len(), n + extra, "{o}");
+                assert!(matches!(attrs.get("a1"), Some(AttrValue::F64(x)) if *x == 1.5));
+            }
+            if dump {
+                // Every object's attributes: 3 set by the editor on each of
+                // the five, 2 existing, `extra` from h5py on each.
+                let out = Command::new("h5dump").args(["-A", p]).output().unwrap();
+                assert!(out.status.success(), "h5dump -A:\n{}", text(&out));
+                let s = String::from_utf8_lossy(&out.stdout);
+                assert_eq!(
+                    s.matches("ATTRIBUTE \"").count(),
+                    5 * (3 + extra) + 2,
+                    "h5dump -A:\n{s}"
+                );
+            }
+        };
+        check(0);
+        // libhdf5 adds more: to 7 or 8 (compact), then past its limit.
+        for extra in [4usize, 10] {
+            py(&format!(
+                "import h5py\n\
+                 with h5py.File({p:?}, 'r+') as f:\n\
+                 \x20   for o in ['/', 'g', 'has', 'd', 'e']:\n\
+                 \x20       for i in range({extra}): f[o].attrs[f'h{{i}}'] = i\n"
+            ));
+            check(extra);
+            check_tools(&path, dump);
+        }
+    }
+}