Addresses remaining PR #976 review items after rebase on develop.
`get_collection(create=False)` previously returned existing collections without
re-applying `hnsw:num_threads=1`, so palaces created before the fix kept the
unsafe parallel-insert path. Add `_pin_hnsw_threads()` helper that calls
`collection.modify(configuration=UpdateCollectionConfiguration(
hnsw=UpdateHNSWConfiguration(num_threads=1)))` best-effort on every
`get_collection` call (including the MCP server's `_get_collection`).
In chromadb 1.5.x the runtime config does not persist to disk across
`PersistentClient` reopens, so the retrofit is re-applied each process start
rather than being a one-shot migration. Fresh palaces keep the metadata-based
pin as primary defense; legacy palaces now also get per-session protection
without requiring `mempalace nuke` + re-mine.
After the rebase on develop, `hook_precompact` delegates to `_mine_sync` and
no longer emits `decision: block`, so the attempt-cap constant was orphaned.
Grep confirms 0 usages in the repo — remove it.
- `_pin_hnsw_threads` retrofits legacy collection (num_threads None -> 1)
- `_pin_hnsw_threads` swallows all errors (never raises)
- `ChromaBackend.get_collection(create=False)` applies retrofit on legacy palace
- 62 tests pass (10 backends + 6 palace locks + 46 hooks_cli)
Felipe Truman
8df944a54d