Re: Assertion failure in SnapBuildInitialSnapshot() - Mailing list pgsql-hackers

I've been investigating the assert failure in ProcArraySetReplicationSlotXmin() and would like to share my approach and get feedback. Instead of inverting the locks and what robert shared before [1].
Instead of unconditionally updating procArray->replication_slot_xmin in ProcArraySetReplicationSlotXmin() in procarray.c, I made the updates conditional:
1) Only update if the incoming xmin is valid
2) Only update if it's older than the currently stored xmin
3) Do the same for procArray->replication_slot_catalog_xmin

void ProcArraySetReplicationSlotXmin(TransactionId xmin, TransactionId catalog_xmin, bool already_locked)
{
       Assert(!already_locked || LWLockHeldByMe(ProcArrayLock));

       if (!already_locked)
           LWLockAcquire(ProcArrayLock, LW_EXCLUSIVE);

       if (TransactionIdIsValid(xmin))
      {
           if (!TransactionIdIsValid(procArray->replication_slot_xmin) || TransactionIdPrecedes(xmin, procArray->replication_slot_xmin))
                 procArray->replication_slot_xmin = xmin;
      }
      if (TransactionIdIsValid(catalog_xmin))
      {
             if (!TransactionIdIsValid(procArray->replication_slot_catalog_xmin) || TransactionIdPrecedes(catalog_xmin, procArray->replication_slot_catalog_xmin))
                   procArray->replication_slot_catalog_xmin = catalog_xmin;
      }
      if (!already_locked)
          LWLockRelease(ProcArrayLock);

     elog(DEBUG1, "xmin required by slots: data %u, catalog %u", xmin, catalog_xmin);
}

In above block of code ensures we always track the minimum xmin across all active replication slots without losing data. And also no need to worry about locks. And also while reproducing this issue [2] In SnapBuildInitialSnapshot() while we computing safexid by calling GetOldestSafeDecodingTransactionId(false) will enters into first case and update the oldestSafeXid = procArray->replication_slot_xmin. So it won't return nextXid. And also it solves this issue [2].

[1] https://www.postgresql.org/message-id/CA+TgmoYLzJxCEa0aCan3KR7o_25G52cbqw-90Q0VGRmV3a8XGQ@mail.gmail.com
[2] https://www.postgresql.org/message-id/CAA4eK1KDFeh=ZbvSWPx=ir2QOXBxJbH0K8YqifDtG3xJENLR+w@mail.gmail.com