Post-incident hardening. A WAL-mode DB corrupted (most likely an interrupted
write during a hard restart), and the backup routine made it unrecoverable:
it (a) never checked integrity, so src.backup() faithfully copied the corrupt
pages into every rolling backup, and (b) pruned oldest-by-mtime, so each new
corrupt backup evicted the last good one. Result: all snapshots poisoned.
New core/db_integrity.py (pure, unit-tested):
- quick_check()/is_healthy(): fast read-only PRAGMA quick_check probe.
- safe_backup(): verifies the SOURCE is healthy BEFORE the Online-Backup copy
and the RESULT after; refuses + discards rather than save a corrupt copy.
- prune_backups(): rotation that NEVER deletes the most-recent verified-healthy
backup, even to honor max_keep — so a run of bad backups can't drop your last
good snapshot.
Wired into BOTH backup paths (the /api/database/backup endpoint and the
auto_backup_database automation handler) — they now refuse on integrity failure
(409 / error status, existing backups untouched) and prune safely.
Tests: tests/test_db_integrity.py (8) using REAL temp DBs incl. a physically
corrupted one — proves refuse-corrupt-source, discard-corrupt-result, and the
exact incident scenario (newest backups corrupt -> the older healthy one is
protected from pruning). Existing maintenance-handler backup test still green
(29 passed). compile + ruff clean.
NOTE: this prevents silent backup poisoning; it does NOT stop the underlying
corruption. Follow-ups still worth doing: WAL-checkpoint on clean shutdown +
a periodic live-DB integrity alert (so corruption is caught on day 1).
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