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Commit8b9cbd4

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Fix snapshot handling in logicalmsg_decode
Whe decoding a transactional logical message, logicalmsg_decode calledSnapBuildGetOrBuildSnapshot. But we may not have a consistent snapshotyet at that point. We don't actually need the snapshot in this case(during replay we'll have the snapshot from the transaction), so inpractice this is harmless. But in assert-enabled build this crashes.Fixed by requesting the snapshot only in non-transactional case, wherewe are guaranteed to have SNAPBUILD_CONSISTENT.Backpatch to 11. The issue exists since 9.6.Backpatch-through: 11Reviewed-by: Andres FreundDiscussion:https://postgr.es/m/84d60912-6eab-9b84-5de3-41765a5449e8@enterprisedb.com
1 parent482ab3e commit8b9cbd4

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2 files changed

+22
-2
lines changed

2 files changed

+22
-2
lines changed

‎src/backend/replication/logical/decode.c‎

Lines changed: 12 additions & 2 deletions
Original file line numberDiff line numberDiff line change
@@ -628,7 +628,7 @@ DecodeLogicalMsgOp(LogicalDecodingContext *ctx, XLogRecordBuffer *buf)
628628
TransactionIdxid=XLogRecGetXid(r);
629629
uint8info=XLogRecGetInfo(r)& ~XLR_INFO_MASK;
630630
RepOriginIdorigin_id=XLogRecGetOrigin(r);
631-
Snapshotsnapshot;
631+
Snapshotsnapshot=NULL;
632632
xl_logical_message*message;
633633

634634
if (info!=XLOG_LOGICAL_MESSAGE)
@@ -658,7 +658,17 @@ DecodeLogicalMsgOp(LogicalDecodingContext *ctx, XLogRecordBuffer *buf)
658658
SnapBuildXactNeedsSkip(builder,buf->origptr)))
659659
return;
660660

661-
snapshot=SnapBuildGetOrBuildSnapshot(builder,xid);
661+
/*
662+
* If this is a non-transactional change, get the snapshot we're expected
663+
* to use. We only get here when the snapshot is consistent, and the
664+
* change is not meant to be skipped.
665+
*
666+
* For transactional changes we don't need a snapshot, we'll use the
667+
* regular snapshot maintained by ReorderBuffer. We just leave it NULL.
668+
*/
669+
if (!message->transactional)
670+
snapshot=SnapBuildGetOrBuildSnapshot(builder,xid);
671+
662672
ReorderBufferQueueMessage(ctx->reorder,xid,snapshot,buf->endptr,
663673
message->transactional,
664674
message->message,/* first part of message is

‎src/backend/replication/logical/reorderbuffer.c‎

Lines changed: 10 additions & 0 deletions
Original file line numberDiff line numberDiff line change
@@ -821,6 +821,13 @@ ReorderBufferQueueMessage(ReorderBuffer *rb, TransactionId xid,
821821

822822
Assert(xid!=InvalidTransactionId);
823823

824+
/*
825+
* We don't expect snapshots for transactional changes - we'll use the
826+
* snapshot derived later during apply (unless the change gets
827+
* skipped).
828+
*/
829+
Assert(!snapshot);
830+
824831
oldcontext=MemoryContextSwitchTo(rb->context);
825832

826833
change=ReorderBufferGetChange(rb);
@@ -839,6 +846,9 @@ ReorderBufferQueueMessage(ReorderBuffer *rb, TransactionId xid,
839846
ReorderBufferTXN*txn=NULL;
840847
volatileSnapshotsnapshot_now=snapshot;
841848

849+
/* Non-transactional changes require a valid snapshot. */
850+
Assert(snapshot_now);
851+
842852
if (xid!=InvalidTransactionId)
843853
txn=ReorderBufferTXNByXid(rb,xid, true,NULL,lsn, true);
844854

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