Vector: HNSW+PQ init failure is silent — segment stays unquantized, stats() shows None with no error

[EnRaiha]

Version / build tested against

origin/main @ e235fe5 (2026-09-29)

Deployment mode

Origin — single node (local)

Engine(s) involved

Vector

Summary

An operator requesting HNSW PQ compression can silently receive an unquantized segment: build_pq_for_index returns None on train/encode failure or bad shape, sealed_segment_from then attaches no codec, complete_build still returns true, and only a tracing::warn is emitted. stats().quantization becomes None — no error, no metric.

Steps to reproduce

Create an hnsw_pq index whose training cannot succeed (e.g. pq_m not dividing dim, or an empty live set), build it, then check stats(): quantization is None, complete_build returned true, log shows PQ training refused; segment stays unquantized (warn only).
Existing target: nodedb-vector/tests/vector_suite/cases/collection_pq_config.rs.
(static verification at the pin; runtime repro pending)

Expected behavior

If PQ init fails, build_pq_for_index / complete_build fail fast with a typed init error; the operator is told the codec did not build.

Actual behavior

Silent degradation to an unquantized graph; stats().quantization = None; only a warn log.

What actually happened? (severity facts)

• Acknowledged/committed data was lost, corrupted, or silently wrong
• The server crashed, hung, or failed to start
• A security or isolation boundary was crossed
• Core functionality is broken with no acceptable workaround
• A workaround exists (check stats() after build or avoid hnsw_pq)

Proposed severity

SEV-3 — Medium: requested compression silently not applied; query results still correct, memory not as expected.

Reproducibility

Always — every attempt (deterministic on the failure conditions)

Last known-good version / commit (if a regression)

(unknown / not a regression)

Environment & logs

Linux x86_64; Verified by static code reading at the pin above; runtime reproduction pending.
Code references:

• (see prior-art line below)

Before submitting

• I searched existing issues and this is not a duplicate. (#50/fix(vector): resolve 7 correctness, memory-safety, and quantization bugs (#50) #65 fixed the original wave; residual unraised)
• Reproduced on a released tag or current main build (pending) — code path verified at e235fe55c.
• This is not a security vulnerability.

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Additional evidence (origin/main @ e235fe55c)

• What: build_pq_for_index returns None on invalid pq_m, no live vectors, or train/encode error. sealed_segment_from (build.rs:194-198) then attaches no codec, and complete_build returns true (build.rs:130). Only tracing::warn is emitted (quantize.rs:120,133). stats().quantization becomes None, not Pq.
• Where: nodedb-vector/src/collection/build.rs:191-201; nodedb-vector/src/collection/quantize.rs:91-138; nodedb-vector/src/collection/stats.rs:40-46.
• Evidence: quotes above; test doc comment collection_pq_config.rs:8-12 claims an SQ8 fallback (wrong).
• Impact: an operator requesting 8-16× PQ compression gets no error and no metric signal when the codec fails to train/encode; the query still returns results from the unquantized graph.
• Fix (claim direction): make codec init strict. On HnswPq, return a typed init error from build_pq_for_index / complete_build instead of logging and returning None.
• Prior-art: #50/PR [#65](fix(vector): resolve 7 correctness, memory-safety, and quantization bugs (#50) #65) fixed the original vector bug wave; the silent-failure residual is unraised. gh issue list --search "PQ quantization" --state open -> [].
  Why: an operator requesting PQ compression gets no error and no metric when codec init fails; the segment silently stays unquantized (stats().quantization = None) and complete_build still returns true.
  Steps to verify: build an hnsw_pq index whose training fails (pq_m not dividing dim, or empty live set) and assert a typed init error is returned (today: only tracing::warn + complete_build == true); existing test target nodedb-vector/tests/vector_suite/cases/collection_pq_config.rs.