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Commit4df581f

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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 parent906356c commit4df581f

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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
@@ -514,7 +514,7 @@ DecodeLogicalMsgOp(LogicalDecodingContext *ctx, XLogRecordBuffer *buf)
514514
TransactionIdxid=XLogRecGetXid(r);
515515
uint8info=XLogRecGetInfo(r)& ~XLR_INFO_MASK;
516516
RepOriginIdorigin_id=XLogRecGetOrigin(r);
517-
Snapshotsnapshot;
517+
Snapshotsnapshot=NULL;
518518
xl_logical_message*message;
519519

520520
if (info!=XLOG_LOGICAL_MESSAGE)
@@ -544,7 +544,17 @@ DecodeLogicalMsgOp(LogicalDecodingContext *ctx, XLogRecordBuffer *buf)
544544
SnapBuildXactNeedsSkip(builder,buf->origptr)))
545545
return;
546546

547-
snapshot=SnapBuildGetOrBuildSnapshot(builder,xid);
547+
/*
548+
* If this is a non-transactional change, get the snapshot we're expected
549+
* to use. We only get here when the snapshot is consistent, and the
550+
* change is not meant to be skipped.
551+
*
552+
* For transactional changes we don't need a snapshot, we'll use the
553+
* regular snapshot maintained by ReorderBuffer. We just leave it NULL.
554+
*/
555+
if (!message->transactional)
556+
snapshot=SnapBuildGetOrBuildSnapshot(builder,xid);
557+
548558
ReorderBufferQueueMessage(ctx->reorder,xid,snapshot,buf->endptr,
549559
message->transactional,
550560
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
@@ -672,6 +672,13 @@ ReorderBufferQueueMessage(ReorderBuffer *rb, TransactionId xid,
672672

673673
Assert(xid!=InvalidTransactionId);
674674

675+
/*
676+
* We don't expect snapshots for transactional changes - we'll use the
677+
* snapshot derived later during apply (unless the change gets
678+
* skipped).
679+
*/
680+
Assert(!snapshot);
681+
675682
oldcontext=MemoryContextSwitchTo(rb->context);
676683

677684
change=ReorderBufferGetChange(rb);
@@ -690,6 +697,9 @@ ReorderBufferQueueMessage(ReorderBuffer *rb, TransactionId xid,
690697
ReorderBufferTXN*txn=NULL;
691698
volatileSnapshotsnapshot_now=snapshot;
692699

700+
/* Non-transactional changes require a valid snapshot. */
701+
Assert(snapshot_now);
702+
693703
if (xid!=InvalidTransactionId)
694704
txn=ReorderBufferTXNByXid(rb,xid, true,NULL,lsn, true);
695705

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