writer: order dense name indexes by hash, then name

libhdf5 compares the name when two hashes are equal; the writer broke ties by insertion order, and libhdf5 could not find one of two names whose lookup3 hashes collide (k69209 / k155448). Test fails before the fix with h5py's KeyError. Co-Authored-By: Claude Opus 5.5 (1M context) <[email protected]>
2026-09-26 10:03:22 -05:00
parent de2a53f613
commit 7acfb79584
3 changed files with 77 additions and 16 deletions
@@ -2,6 +2,17 @@

 ## Unreleased

+### Writer: large dense indexes (2026-09-26)
+- **Links and attributes whose name hashes collide are found by name.** The
+  dense name indexes (a group's links: B-tree v2 type 5; an object's
+  attributes: type 8) are ordered by the name's lookup3 hash and, when two
+  hashes are equal, by the name itself, as libhdf5 compares them. The writer
+  broke ties by insertion order, so libhdf5 could not open one of two
+  colliding names (`"k69209"` and `"k155448"` share hash `0x3a0b13e6`;
+  collisions are likely from about 77 000 names). Regression test
+  `names_whose_hashes_collide_are_found_by_name` in
+  `crates/clawhdf5/tests/writer_groups_interop.rs`.
+
 ### Concurrent reads (2026-09-26)
 - **Full reads of chunked datasets scale with threads again when rayon's
  pool has one thread.** Each full read handed its chunks to rayon to