fix(peewee): enable WAL journal mode for SQLite

[ErikBjare]

Summary

• Enable WAL (Write-Ahead Logging) journal mode for PeeweeStorage's SQLite database
• SqliteStorage already had WAL enabled, but PeeweeStorage (the default backend) did not

Problem

Without WAL mode, SQLite uses DELETE journal mode which requires exclusive locks for writes. When aw-server's threaded Flask handles concurrent heartbeats from multiple watchers, this causes frequent sqlite3.OperationalError: database is locked errors.

On one system, this resulted in:

• 3,984 "database is locked" errors in a 21-day session
• 1,992 HTTP 500 responses to watcher heartbeats
• Watchers retrying failed heartbeats, creating a cascade of load
• 2060 CPU-minutes accumulated by the aw-server process

Fix

WAL mode allows concurrent readers and a single writer without blocking, dramatically reducing contention. This is a one-line change to pass pragmas={"journal_mode": "wal"} to SqliteExtDatabase.init().

Test plan

• Existing tests pass
• Verify WAL mode is active: sqlite3 <db-path> "PRAGMA journal_mode;" should return wal
• Monitor for "database is locked" errors with multiple active watchers

Related: ActivityWatch/aw-server#151

[greptile-apps]

Greptile Summary

This PR enables WAL journal mode for PeeweeStorage's SQLite database by passing pragmas={"journal_mode": "wal", "wal_autocheckpoint": 100} to SqliteExtDatabase.init(), bringing it in line with SqliteStorage which already set WAL via PRAGMA journal_mode=WAL. The fix directly addresses the database is locked errors under concurrent Flask-threaded writes.

Confidence Score: 5/5

Safe to merge — minimal, well-targeted change with no correctness risks.

The only finding is a P2 style suggestion: auto_migrate opens its own connection and won't inherit the per-connection wal_autocheckpoint setting (though journal_mode=WAL persists at the file level, so correctness is unaffected). No P0/P1 issues exist.

No files require special attention.

Important Files Changed

Filename	Overview
aw_datastore/storages/peewee.py	Adds journal_mode=WAL and wal_autocheckpoint=100 pragmas to the SqliteExtDatabase.init() call, matching the WAL configuration already present in SqliteStorage; one minor note that auto_migrate's separate connection doesn't inherit the per-connection wal_autocheckpoint setting.

Sequence Diagram

sequenceDiagram
    participant Flask Thread 1
    participant Flask Thread 2
    participant PeeweeStorage
    participant SQLite (WAL)

    Flask Thread 1->>PeeweeStorage: heartbeat (insert event)
    Flask Thread 2->>PeeweeStorage: heartbeat (insert event)
    PeeweeStorage->>SQLite (WAL): BEGIN WRITE (Thread 1)
    PeeweeStorage->>SQLite (WAL): BEGIN READ (Thread 2)
    Note over SQLite (WAL): WAL allows concurrent reader + writer
    SQLite (WAL)-->>PeeweeStorage: OK (Thread 2 reads)
    SQLite (WAL)-->>PeeweeStorage: OK (Thread 1 writes)
    Note over SQLite (WAL): Checkpoint every 100 pages

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Comments Outside Diff (1)

1. aw_datastore/storages/peewee.py, line 47-60 (link)

   auto_migrate connection bypasses wal_autocheckpoint

   auto_migrate opens its own SqliteExtDatabase(path) without any pragmas. journal_mode=WAL is a persistent database property and will still be in effect, but wal_autocheckpoint is a per-connection setting — it resets to the SQLite default of 1000 during every migration run. If checkpointing behaviour during migration matters, the pragmas should be passed here too.

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