Busy: WAL snapshot is out of date

SQLITE_BUSY_SNAPSHOT (517) is returned in WAL mode when a read transaction sees an older snapshot and a write transaction cannot proceed because a checkpoint would need to overwrite pages that the read transaction is viewing.

1. 1Long-running read transaction in WAL mode is blocking the checkpoint.
2. 2Read connection holds a snapshot that is older than the write connection needs.

WAL-mode database with concurrent long-running readers and active writers.

-- Connection A (read transaction, holds old snapshot):
BEGIN; SELECT * FROM big_table;

-- Connection B (write transaction, blocked):
-- BEGIN; INSERT INTO big_table ... → SQLITE_BUSY_SNAPSHOT

sqlite3.OperationalError: database is locked

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