Hi all,
While working with pg_rewind, I noticed that it can sometimes request a rewind even when no actual changes exist after a failover.
Problem:
Currently, pg_rewind determines the end-of-WAL on the target by using the last shutdown checkpoint (or minRecoveryPoint for a standby). This creates a false positive scenario:
1)Suppose a standby is promoted to become the new primary.
2)Later, the old primary is cleanly shut down.
3)The only WAL record generated on the old primary after divergence is a shutdown checkpoint.
At this point, the old primary and new primary contain identical data. However, since the shutdown checkpoint extends the WAL past the divergence point, pg_rewind concludes:
if (target_wal_endrec > divergerec)
rewind_needed = true;
That forces a rewind even though there are no meaningful changes.
To reproduce this scenario use the below attached script.
Fix:
The attached patch changes the logic so that pg_rewind no longer treats shutdown checkpoints as meaningful records when determining the end-of-WAL. Instead, we scan backward from the last checkpoint until we find the most recent valid WAL record that is not a shutdown-only related record.
This ensures rewind is only triggered when there are actual modifications after divergence, avoiding unnecessary rewinds in clean failover scenarios.
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