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Commit497f863

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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 parent52dbd9f commit497f863

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2 files changed

+22
-2
lines changed

2 files changed

+22
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lines changed

‎src/backend/replication/logical/decode.c

Lines changed: 12 additions & 2 deletions
Original file line numberDiff line numberDiff line change
@@ -517,7 +517,7 @@ DecodeLogicalMsgOp(LogicalDecodingContext *ctx, XLogRecordBuffer *buf)
517517
TransactionIdxid=XLogRecGetXid(r);
518518
uint8info=XLogRecGetInfo(r)& ~XLR_INFO_MASK;
519519
RepOriginIdorigin_id=XLogRecGetOrigin(r);
520-
Snapshotsnapshot;
520+
Snapshotsnapshot=NULL;
521521
xl_logical_message*message;
522522

523523
if (info!=XLOG_LOGICAL_MESSAGE)
@@ -547,7 +547,17 @@ DecodeLogicalMsgOp(LogicalDecodingContext *ctx, XLogRecordBuffer *buf)
547547
SnapBuildXactNeedsSkip(builder,buf->origptr)))
548548
return;
549549

550-
snapshot=SnapBuildGetOrBuildSnapshot(builder,xid);
550+
/*
551+
* If this is a non-transactional change, get the snapshot we're expected
552+
* to use. We only get here when the snapshot is consistent, and the
553+
* change is not meant to be skipped.
554+
*
555+
* For transactional changes we don't need a snapshot, we'll use the
556+
* regular snapshot maintained by ReorderBuffer. We just leave it NULL.
557+
*/
558+
if (!message->transactional)
559+
snapshot=SnapBuildGetOrBuildSnapshot(builder,xid);
560+
551561
ReorderBufferQueueMessage(ctx->reorder,xid,snapshot,buf->endptr,
552562
message->transactional,
553563
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
@@ -622,6 +622,13 @@ ReorderBufferQueueMessage(ReorderBuffer *rb, TransactionId xid,
622622

623623
Assert(xid!=InvalidTransactionId);
624624

625+
/*
626+
* We don't expect snapshots for transactional changes - we'll use the
627+
* snapshot derived later during apply (unless the change gets
628+
* skipped).
629+
*/
630+
Assert(!snapshot);
631+
625632
oldcontext=MemoryContextSwitchTo(rb->context);
626633

627634
change=ReorderBufferGetChange(rb);
@@ -640,6 +647,9 @@ ReorderBufferQueueMessage(ReorderBuffer *rb, TransactionId xid,
640647
ReorderBufferTXN*txn=NULL;
641648
volatileSnapshotsnapshot_now=snapshot;
642649

650+
/* Non-transactional changes require a valid snapshot. */
651+
Assert(snapshot_now);
652+
643653
if (xid!=InvalidTransactionId)
644654
txn=ReorderBufferTXNByXid(rb,xid, true,NULL,lsn, true);
645655

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