TLA+ Caught a Silent Data Divergence Bug in Postgres's pg_rewind

11 hours ago

Postgres streaming replication enables high availability by replicating write-ahead logs (WAL) to standby servers, allowing promotion when the primary fails.
pg_rewind solves data divergence by comparing the old primary's data with the new primary's, rewinding the old primary to a common point.
Timeline IDs (TLI) track promotions, with history files recording lineage, but identical TLI and LSN can mislead pg_rewind.
A bug occurs when two crashes cause independent promotions to the same TLI, leading pg_rewind to incorrectly skip rewinding.
This results in silent data divergence, where a standby retains unreplicated writes, risking phantom data reappearance.
TLA+ modeling identified this bug by exhaustively checking states, confirming the violation of storage consistency invariants.
The fix introduces promotion UUIDs (UUIDv7) in timeline history files to uniquely identify each promotion.
UUID comparison in pg_rewind detects independent promotions, forcing rewinds when UUIDs differ, even with matching TLIs.
Backward compatibility is maintained, with missing UUIDs treated as compatible, ensuring graceful degradation.
Demonstration shows the bug and fix via a test setup, highlighting how UUIDs prevent silent data loss.

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