How we tracked down a 16-year-old SQLite bug

Tailscale traced months of control-plane instability and 19 SQLite database corruption incidents to a rare 16-year-old SQLite race condition involving write transactions and manually triggered WAL checkpoints. Its architecture uses one SQLite database per control-plane shard, and corruption could temporarily disable affected tailnets’ administrative services and prevent new devices from joining, although existing peer-to-peer connections, private keys, and traffic were not affected. After extensive forensic logging, automated recovery improvements, transaction replay analysis, and collaboration with SQLite’s developers, Tailscale identified the “WAL-Reset bug,” in which a precisely timed write during checkpointing could cause committed pages to be lost and databases to become corrupt. SQLite released a fix, though an initial version also exposed an unrelated stale expression-index issue that produced false corruption alerts; a revised release addressed the race condition, while later SQLite changes improved index recovery. Tailscale subsequently confirmed that the triggering condition occurred in production without causing corruption and reported four months without further incidents, while noting that its unusually aggressive manual checkpointing increased exposure to an otherwise exceptionally rare bug.

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