refactor(backends/chroma): coerce None metadatas to {} at backend boundary (closes #1020)

[jphein]

Summary

Fixes #1020. Moves the None-metadata coercion out of per-read-site defensive guards (meta = meta or {} scattered across searcher.py, layers.py, palace_graph.py, miner.py, etc.) and into the single place the typed contract is produced: ChromaCollection.query() and ChromaCollection.get() in backends/chroma.py.

Why

ChromaDB 1.5.x can return None inside the metadatas list even when the write stored a dict. Three prior PRs already responded to this:

• fix(searcher): guard against None metadata in CLI print path #999 — guards in searcher.py (CLI + API + closet-boost), miner.status(), and 4 mcp_server.py handlers
• fix: guard Layer3.search_raw against None doc/meta from ChromaDB (#1011) #1013 — guard in Layer3.search_raw()
• Various other paths picked up the same defensive pattern organically

That scatter works but violates the typed contract QueryResult and GetResult declare: both advertise metadatas: list[dict], never list[Optional[dict]]. Every call site that forgets the guard is a latent AttributeError: 'NoneType' object has no attribute 'get'.

A boundary coercion is the right architectural fix: one place to handle the backend's quirk, downstream code gets the shape its type annotations promise.

What changed

• mempalace/backends/chroma.py: added inner-list None-coercion inside ChromaCollection.query() (alongside the existing _none_list_to_empty outer coercion) and inside ChromaCollection.get() (after the existing {}-padding step). The guarantee becomes: whenever metadatas is requested, every element is a dict — never None.
• tests/test_backends.py: +6 tests covering the shapes the production code actually sees.

What did NOT change

• The existing per-site meta = meta or {} guards stay in place as defensive belt-and-suspenders. They're redundant after this coercion but removing them risks regressing any call path that bypasses the typed wrappers. A follow-up cleanup PR can remove them once we're confident nothing else reaches chromadb directly.
• No behavior change on the write side. Writes still get whatever metadata the caller provides; this PR is strictly read-side.
• No change to QueryResult / GetResult shape. Same fields, same types — just guaranteed honored at the boundary instead of honored-with-asterisks.

Test plan

• pytest tests/test_backends.py — 35 passed (6 new, 29 existing)
• pytest tests/ full suite — 1072 passed, 106 deselected
• ruff check on modified files — clean
• ruff format --check with pinned 0.4.10 — clean

New test cases specifically target:

Test	Exercises
test_chroma_collection_query_coerces_none_metadatas_to_empty_dict	mixed None/dict in a single-query batch
test_chroma_collection_query_coerces_all_none_inner_metadatas	degenerate: every metadata is None
test_chroma_collection_query_coerces_none_across_multiple_queries	None dicts spread across two queries in one call
test_chroma_collection_get_coerces_none_metadatas_to_empty_dict	mixed None/dict on get() path
test_chroma_collection_get_coerces_padding_remains_dict	regression guard: short metas list → {} pads stay {}
test_chroma_collection_get_without_metadatas_requested_stays_empty	when metadatas not in include=, don't fabricate

Context

I'd committed to driving this on #1020 on 2026-04-18; #1021's merge this week cleared the blocker. @bensig — happy to split into two PRs (the coercion itself and a separate guard-removal follow-up) if you'd prefer landing them serially, but as written this PR keeps behavior strictly monotonic: everything that worked before still works, plus one class of latent None-metadata bugs can't fire anymore.

[Copilot]

Pull request overview

Refactors the ChromaDB adapter boundary to guarantee typed QueryResult / GetResult contracts by coercing None entries inside returned metadatas into {}, preventing downstream meta.get(...) crashes caused by ChromaDB 1.5.x.

Changes:

• Coerce inner-list None metadata dicts to {} in ChromaCollection.query() and ChromaCollection.get().
• Add backend-level regression tests covering mixed/degenerate None metadata shapes and include= behavior.

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated 1 comment.

File	Description
mempalace/backends/chroma.py	Normalizes metadatas so callers always receive list[dict] (no inner None).
tests/test_backends.py	Adds adapter-level tests validating the new coercion behavior for query() and get().

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[Copilot]

This test’s comment says ids/docs/distances are unaffected, but it never asserts result.documents. Adding a documents assertion would make the test actually cover that invariant and catch regressions in the query() return-shaping logic.

Suggested change

	assert result.ids == [["a", "b", "c"]]
	assert result.ids == [["a", "b", "c"]]
	assert result.documents == [["da", "db", "dc"]]

[jphein]

Done in bf91b27 — added assert result.documents == [["da", "db", "dc"]] so the comment's claim is actually pinned by the test. 12/12 backend tests pass.