fix(local): publish DBM migrations only after successful conversion

[Kuang-xianxin]

If DBM-to-SQLite migration fails after opening the destination, it leaves storage.sqlite behind. The next collection open treats that file as a completed migration and silently skips the points still in storage.dbm.

Build the SQLite database in a temporary directory beside the destination, commit and close both stores, then publish the completed file with Path.replace. The legacy store is opened read-only and removed only after publication. Exceptions close the handles and remove the staging files, allowing a later open to retry.

The regression uses real DBM and SQLite files: a malformed legacy key aborts migration; after repairing that key, opening the store again recovers every valid point. It fails on the unmodified dev implementation. This addresses the recovery limitation noted in #1335; its separate logging-format change remains outside this PR.

Validation on Windows / Python 3.13.14: the regression and 10 other applicable persistence/in-memory tests pass. The two complete files report 11 passed and 4 failures; the same four failures reproduce on unmodified da05a2b (existing Windows collection-deletion/open-handle behavior). Scoped Ruff lint/format, source Mypy, and git diff --check pass. No server-backed integration suite was run.

All Submissions

• Based on and targets dev.
• Read the available repository contribution guidance and PR template.
• Checked open and merged PRs for duplicate migration work, including fix(local): pass the migration error as a logging format arg #1335.

Changes to Core Features

• Explained the failure and the fix.
• Added a regression test.
• Ran relevant local tests; existing failures are described above.

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ℹ️ Recent review info

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Reviewing files that changed from the base of the PR and between da05a2b and 20ff5c4.

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• qdrant_client/local/persistence.py
• tests/test_local_persistence.py

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📝 Walkthrough

Walkthrough

The migration now writes SQLite data in a temporary directory and publishes the completed database by replacement. It opens the legacy DBM store in read-only mode and closes resources through context managers. Failed migrations leave the legacy store available for retry and do not create a partial destination. A new test covers failure, retry, point loading, and legacy-file cleanup.

Area	Change
Migration	Added staged SQLite creation and atomic publication.
Tests	Added coverage for failed and retried migrations.
Suggested reviewers:	joein

Priority: ⬇️ Low

Estimated code review effort: 3 (Moderate) | ~20 minutes

Change: Bug fix

Merge Risk: ⚪ Minimal · up to 20ff5

The migration preserves the legacy store until a completed SQLite database is published, and failed conversions can be retried without exposing a partial destination.

🚥 Pre-merge checks | ✅ 4 | ❌ 1

❌ Failed checks (1 warning)

Check name	Status	Explanation	Resolution
Docstring Coverage	⚠️ Warning	Docstring coverage is 0.00% which is insufficient. The required threshold is 80.00%. Docstring coverage is scoped to functions touched by this diff. Analyzed 3 functions across 2 files.	Write docstrings for the functions missing them to satisfy the coverage threshold.

✅ Passed checks (4 passed)

Check name	Status	Explanation
Description check	✅ Passed	The description clearly explains the DBM-to-SQLite migration failure, the staging and publication fix, the regression test, and validation results.
Title check	✅ Passed	The title clearly and concisely summarizes the main change: publishing DBM migrations only after successful conversion.
Linked Issues check	✅ Passed	Check skipped because no linked issues were found for this pull request.
Out of Scope Changes check	✅ Passed	Check skipped because no linked issues were found for this pull request.

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• Create PR with unit tests

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