[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.

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