Restrict backend key unpickling

[dipeshbabu]

What

Route synchronous and asynchronous LSH key serialization through a restricted loader that rejects global construction opcodes, persistent IDs, and non-hashable results. Validate keys before insertion and document the supported key types and migration impact.

Why

Redis and other external backends can return tampered bytes. Passing those bytes to the standard pickle loader can invoke attacker-selected global callables during a query.

Impact

Primitive built-in keys continue to round-trip. Custom class keys now fail before insertion, and existing indexes using them must migrate to supported built-in values. Malicious or corrupt backend values raise an unpickling error without executing their payload.

Root cause

Backend data was treated as trusted and decoded with unrestricted pickle.loads calls.

Checks

• uv run pytest -q test/test_lsh.py test/test_lshensemble.py test/test_integration.py test/aio/test_lsh.py (33 passed, 94 backend-service tests skipped locally)
• executable-payload regression confirms no side effect occurs
• uv run ruff check on changed Python files
• uvx pyright (0 errors)
• uv run python -m compileall -q datasketch
• git diff --check

[codecov-commenter]

⚠️ Please install the to ensure uploads and comments are reliably processed by Codecov.

Codecov Report

✅ All modified and coverable lines are covered by tests.
⚠️ Please upload report for BASE (master@3da96a5). Learn more about missing BASE report.
❗ Your organization needs to install the Codecov GitHub app to enable full functionality.

Additional details and impacted files

@@            Coverage Diff            @@
##             master     #329   +/-   ##
=========================================
  Coverage          ?   72.94%           
=========================================
  Files             ?       22           
  Lines             ?     2828           
  Branches          ?        0           
=========================================
  Hits              ?     2063           
  Misses            ?      765           
  Partials          ?        0           

Flag	Coverage Δ
unittests	72.94% <100.00%> (?)

Flags with carried forward coverage won't be shown. Click here to find out more.

☔ View full report in Codecov by Harness.
📢 Have feedback on the report? Share it here.

🚀 New features to boost your workflow:

• ❄️ Test Analytics: Detect flaky tests, report on failures, and find test suite problems.