Re: [PATCH] Fix pg_rewind false positives caused by shutdown-only WAL

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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Thanks,
Srinath Reddy Sadipiralla
EDB: https://www.enterprisedb.com/