[improve](testing): close maintained error-path gaps toward 90% coverage

[wxyucs]

Kind of improvement

test

Affected module

not applicable (maintained C++ algorithms and error paths)

Affected area

area/testing

Summary

Continue the maintained-code coverage stage of #2884 using the repository-scoped measurement foundation merged in #2887. Keep algorithm/test changes separate from CI infrastructure.

Prioritize deterministic edge/error paths in active MCI recursion, RaBitQ/SIMQ variants, HGraph/IVF serialization compatibility and corruption handling, API/factory validation, lifecycle failures, and IO/cache failures. Audit MCI's unused legacy no-argument enumerator before deciding whether removal merits a separate algorithm change; do not execute dead code merely to increase coverage. Deprecated HNSW and DiskANN are explicit non-goals.

Acceptance:

• Use a completed post-ci: make C++ coverage repository-scoped #2887 full report to prioritize current maintained-code gaps; historical percentages are not the new denominator.
• Add focused deterministic tests that assert behavior on maintained edge/error paths.
• Resolve the unused MCI legacy-path disposition through a reviewed algorithm-focused change if needed.
• Demonstrate at least 90% repository-scoped line coverage on three consecutive full runs, with identical-source variation no greater than 0.1 percentage points, without blanket exclusions.

Expected benefit

Improve regression protection and reach the parent's 90% objective through useful tests and reviewed dead-code decisions, independently of the pre-merge CI rollout.

Benchmark evidence

The historical gap analysis is in #2884. Post-foundation full reports were still pending when this stage was split; no new full-suite percentage is claimed.

References

• Parent: [improve](testing): restore trustworthy C++ coverage and ratchet main to 90% #2884
• Merged collection foundation: ci: make C++ coverage repository-scoped #2887
• Full-run workflow: .github/workflows/coverage.yml:29
• Original MCI evidence and maintained-path priorities: [improve](testing): restore trustworthy C++ coverage and ratchet main to 90% #2884

Related issues

Duplicate search for coverage error paths returned #2884, #1949, #2447, #2582, and #2661. This issue tracks the coverage stage of #2884; dependency removal and new IO/analyzer features are outside scope.

---

Drafted with AI assistant: Codex:GPT-6

[vsag-bot]

No suggested commands.