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  fix(mcp): serialize local SQLite writes to end "database is locked" under concurrent stores

  Ghost released this 2026-06-19 06:06:09 +00:00 | 0 commits to master since this release

  Under heavy concurrent memory_store (many subagents/sessions writing close
  together) the local SQLite layer raced on the single SQLite writer and surfaced
  sqlite3.OperationalError: database is locked, which made memory tools slow and
  eventually dropped whole sessions. Two root causes:

  • The MCP server (mcp_server.py) and the background SyncEngine (sync.py) each
    opened a SEPARATE connection to the same SQLite file. WAL allows one writer;
    when the sync writer held the lock across a resync, a concurrent store blew
    past busy_timeout and raised "database is locked".
  • mcp_server's connection was opened WITHOUT check_same_thread=False, so the
    moment two requests were handled on different threads every local store/recall
    raised ProgrammingError "SQLite objects created in a thread...".

  Fix: a single process-wide serialized writer.

  • New LocalStore (local_store.py) owns ONE connection (check_same_thread=False)
    guarded by ONE re-entrant lock, keeps WAL, and wraps writes in
    transaction()/write() with bounded exponential-backoff retry on the rare
    residual lock (e.g. another OS process) instead of failing the call.
  • MemoryServer builds the LocalStore and SHARES it with the SyncEngine, so the
    sync writer no longer opens a second connection — the two-connections race is
    eliminated. All server reads/writes go through the shared lock; stores stay
    snappy (enqueue-local + async sync).
  • Bound the one genuinely slow path (remote semantic memory_recall) with an
    explicit RECALL_TIMEOUT and, on timeout/unreachable backend, return a clear
    "working / retry" signal instead of hanging silently or crashing. When a
    client supplies _meta.progressToken, emit one notifications/progress so the
    call shows life.

  Ships to users via the plugin (mcp/memory-mcp.json runs src/.../mcp_server.py);
  no server-side/API change needed. TDD: added concurrency tests (many threads +
  sync writer on one file) and recall progress/bounding tests; full gate green
  (ruff + mypy strict + 185 pytest).

  Co-Authored-By: Claude Opus 4.8 noreply@anthropic.com