fix: recover from malformed Hugging Face cache metadata

[hulkbig]

Summary

Recover from malformed files_metadata.json without discarding intact cached model files. Both metadata read paths now catch only json.JSONDecodeError and UnicodeDecodeError, treating malformed JSON or undecodable metadata bytes with the existing missing-metadata behavior.

• Offline loading can still resolve the cached snapshot and use the existing legacy-cache fallback.
• When the online download helper is reached, it collects metadata for the requested revision, validates file sizes, and rebuilds the metadata while allowing Hugging Face to reuse cached files. This does not add force_download.
• The download_model offline fast path can reuse an intact cache without rewriting the malformed JSON; repair happens when the online helper is needed.
• Valid metadata size/revision checks, unrelated I/O and Hub errors, and behavior for valid JSON with the wrong root type are unchanged.

Regression coverage

The regression tests first create an intact dummy model cache with valid metadata, then replace the metadata with truncated/empty JSON or invalid UTF-8 bytes. The initial JSON regression suite reported 11 failures and 10 passes before the fix, and 21 passes after it. Five added byte-corruption cases fail before the encoding follow-up fix and pass after it. The current focused suite passes all 26 cases. Tests also preserve propagation of permission errors, unrelated ValueError/UnicodeError/UnicodeEncodeError, and Hub failures. Hub calls are mocked; no model weights were downloaded.

Validation

Validated at 83dc69bb60d40371f03ea9a2bacad0e20320f031 with the installed FastEmbed package and real dependencies, without import shims: Python 3.12.14, Linux, huggingface-hub 1.33.0.

• Focused suite: 26 passed.
• Selected offline suite below: 53 passed, including the 26 focused tests.
• Ruff 0.3.4 (the repository pre-commit pin): repository Python lint and changed-file formatting passed.
• compileall passed.
• The initial selected suite had 48 passes and one environment failure: tests/test_parallel_processor.py::test_closing_partially_consumed_iterator_stops_workers cannot create an AF_UNIX socket (PermissionError: [Errno 1] Operation not permitted). It failed identically on untouched base 3c267018 and the initial candidate, and remains excluded from the runnable subset.
• Full model-download/inference suites, other OS/Python/Hub versions, and type-checker jobs were not run locally.

Commands

export HF_HUB_OFFLINE=1 HF_HUB_DISABLE_TELEMETRY=1 DO_NOT_TRACK=1

.venv/bin/python -m pytest -q tests/test_model_management.py tests/test_model_metadata_recovery.py

.venv/bin/python -m pytest -q \
  tests/test_model_management.py tests/test_model_metadata_recovery.py \
  tests/test_common.py tests/test_image_transform.py \
  tests/test_postprocess.py::test_empty_multivectors_raise_value_error \
  tests/test_postprocess.py::test_muvera_fills_from_nearest_occupied_cluster \
  tests/test_postprocess.py::test_muvera_fills_match_full_matrix_reference \
  tests/test_custom_models.py::test_mock_add_custom_models \
  tests/test_custom_models.py::test_custom_text_model_lookup_is_case_insensitive \
  tests/test_custom_models.py::test_do_not_add_existing_model \
  tests/test_custom_models.py::test_do_not_add_existing_cross_encoder

.venv/bin/ruff check fastembed tests
.venv/bin/ruff format --check fastembed/common/model_management.py tests/test_model_metadata_recovery.py

All Submissions

• Followed the contributing guidelines.
• Checked current open pull requests for the same change.

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📥 Commits

Reviewing files that changed from the base of the PR and between a3808bc and 83dc69b.

📒 Files selected for processing (2)

• fastembed/common/model_management.py
• tests/test_model_metadata_recovery.py

🚧 Files skipped from review as they are similar to previous changes (2)

• fastembed/common/model_management.py
• tests/test_model_metadata_recovery.py

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📝 Walkthrough

Walkthrough

The metadata reader logs malformed or undecodable JSON and returns empty metadata. Offline and online cache verification use this reader. Tests cover snapshot reuse, metadata reconstruction and size validation, legacy-cache fallback, and propagation of unrelated errors.

Priority: ⬇️ Low

Estimated code review effort: 2 (Simple) | ~12 minutes

Change: Bug fix

Merge Risk: ⚪ Minimal · up to 83dc6

The change has no identified issue that should block merging after normal checks.

🚥 Pre-merge checks | ✅ 4 | ❌ 1

❌ Failed checks (1 warning)

Check name	Status	Explanation	Resolution
Docstring Coverage	⚠️ Warning	Docstring coverage is 7.69% which is insufficient. The required threshold is 80.00%. Docstring coverage is scoped to functions touched by this diff. Analyzed 13 functions across 2 files.	Write docstrings for the functions missing them to satisfy the coverage threshold.

✅ Passed checks (4 passed)

Check name	Status	Explanation
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.
Title check	✅ Passed	The title clearly and concisely describes the primary change: recovery from malformed Hugging Face cache metadata.
Description check	✅ Passed	The description directly explains the metadata recovery behavior, affected paths, regression tests, and validation results.

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Actionable comments posted: 1

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• 🪄 Fix CodeRabbit comments on this PR

🤖 Prompt to fix review comments

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Inline comments:
Review comments at @fastembed/common/model_management.py:
- Around line 295-296: Update the metadata loading exception handling around
read_text() and json.loads() to treat UnicodeDecodeError as malformed metadata
alongside JSONDecodeError, so existing offline recovery and online metadata
rebuilding can proceed. Add a recovery test covering invalid UTF-8 bytes in
files_metadata.json.

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ℹ️ Review info

⚙️ Run configuration

• Configuration used: Organization UI
• Review profile: CHILL
• Plan: Advanced
• Run ID: ffac6701-8f96-4436-ad9b-67a9b7698e24

📥 Commits

Reviewing files that changed from the base of the PR and between 539499b and a3808bc.

📒 Files selected for processing (2)

• fastembed/common/model_management.py
• tests/test_model_metadata_recovery.py

Included review availability: This review used your included allowance. Your plan provides up to 8 included reviews per hour; 7 remain after this review.