[LIghtJUNction]

Motivation

• Prevent external content pins from accumulating when store.package_deposit durably pins blobs before the enclosing metadata transaction commits, which allows repeated failed deposits to exhaust storage.

Description

• Add a rollback-compensation hook on_rollback to the MetadataSession contract so handlers can register async cleanup work to run when a transaction or savepoint rolls back (src/msg/core/contracts.py).
• Implement rollback compensation bookkeeping and execution in SqliteSession by tracking rollback_effects, exposing on_rollback, and running compensations on rollback and nested savepoint aborts (src/msg/storage/sqlite.py).
• Mirror nested savepoint rollback compensation and post-commit cleanup invocation in the Postgres transaction wrapper so PostgreSQL-backed sessions also run compensations on rollback (src/msg/storage/postgres.py).
• Register an idempotent unpin callback immediately before creating durable pins in store.package_deposit so the content store is unpinned if the metadata transaction aborts (src/msg/plugins/store.py).
• Add a regression test test_failed_deposit_projection_removes_external_pins that replays an invalid-projection deposit and asserts no new pins or package rows accumulate, and extend the test request helper to allow exercising return_fields (tests/test_store_foundation.py, tests/test_service.py).

Testing

• Ran a focused transaction rollback unit test via PYTHONPATH=src uv run --python 3.14 --no-project --with pytest --with pytest-asyncio pytest -q -c /dev/null --confcutdir=/tmp /tmp/test_rollback_effects.py, which passed and verified nested rollback-effect ordering.
• Ran a bytecode/compile check via uv run --python 3.14 --no-project -m compileall -q src/msg tests/test_service.py tests/test_store_foundation.py, which succeeded.
• Ran repository checks (git diff --check) which reported no issues.
• Attempted to run pytest -q tests/test_store_foundation.py to exercise database-backed tests; collection and one independent test passed but five PostgreSQL-backed tests could not initialize in this environment because initdb (PostgreSQL) is not available.

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Codex Review Summary

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Review	Status	Commit	Review trigger
🔒 Security Review	✅ Completed 2026-09-28T11:46:56.469142Z	c2e3205	PR opened

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[LIghtJUNction]

Superseded by merged #142 (edf0d55). This branch targets the pre-integration tree and would duplicate or conflict with the security repairs already on main.