A transient chromadb exception inside `_get_collection` was swallowed by
the bare `except Exception: return None`, leaving every subsequent tool
call hitting the same poisoned cache silently. The fix wraps the body
in a `for attempt in range(2)` loop: on attempt 0 failure, log via
`logger.exception(...)` and clear `_client_cache` / `_collection_cache`
/ `_metadata_cache` so the next iteration forces `_get_client()` to
rebuild from scratch — that path now re-runs `quarantine_stale_hnsw`
(per #1322), so the second attempt heals the common stale-handle case
automatically. If both attempts fail, return `None` (matches the prior
contract for permanent failures).
Two new tests in `tests/test_mcp_server.py::TestCacheInvalidation`:
- `test_get_collection_retries_once_on_exception` — first attempt raises
via a monkeypatched `_get_client`, second attempt succeeds; assert the
caller gets the collection back, not None.
- `test_get_collection_returns_none_after_two_failures` — both attempts
fail, assert we exhaust the loop and return None (no infinite retry).
Surgical extraction from PR #1286, which carried the same fix idea
(plus a fork-sync bundle that couldn't be merged); credit to the
original author below.
Co-authored-by: Jeffrey Hein <jp@jphein.com>
Igor Lins e Silva
e334e257bf