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Commit949ac32

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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 parent576b25b commit949ac32

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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
@@ -564,7 +564,7 @@ logicalmsg_decode(LogicalDecodingContext *ctx, XLogRecordBuffer *buf)
564564
TransactionIdxid=XLogRecGetXid(r);
565565
uint8info=XLogRecGetInfo(r)& ~XLR_INFO_MASK;
566566
RepOriginIdorigin_id=XLogRecGetOrigin(r);
567-
Snapshotsnapshot;
567+
Snapshotsnapshot=NULL;
568568
xl_logical_message*message;
569569

570570
if (info!=XLOG_LOGICAL_MESSAGE)
@@ -594,7 +594,17 @@ logicalmsg_decode(LogicalDecodingContext *ctx, XLogRecordBuffer *buf)
594594
SnapBuildXactNeedsSkip(builder,buf->origptr)))
595595
return;
596596

597-
snapshot=SnapBuildGetOrBuildSnapshot(builder,xid);
597+
/*
598+
* If this is a non-transactional change, get the snapshot we're expected
599+
* to use. We only get here when the snapshot is consistent, and the
600+
* change is not meant to be skipped.
601+
*
602+
* For transactional changes we don't need a snapshot, we'll use the
603+
* regular snapshot maintained by ReorderBuffer. We just leave it NULL.
604+
*/
605+
if (!message->transactional)
606+
snapshot=SnapBuildGetOrBuildSnapshot(builder,xid);
607+
598608
ReorderBufferQueueMessage(ctx->reorder,xid,snapshot,buf->endptr,
599609
message->transactional,
600610
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
@@ -828,6 +828,13 @@ ReorderBufferQueueMessage(ReorderBuffer *rb, TransactionId xid,
828828

829829
Assert(xid!=InvalidTransactionId);
830830

831+
/*
832+
* We don't expect snapshots for transactional changes - we'll use the
833+
* snapshot derived later during apply (unless the change gets
834+
* skipped).
835+
*/
836+
Assert(!snapshot);
837+
831838
oldcontext=MemoryContextSwitchTo(rb->context);
832839

833840
change=ReorderBufferGetChange(rb);
@@ -846,6 +853,9 @@ ReorderBufferQueueMessage(ReorderBuffer *rb, TransactionId xid,
846853
ReorderBufferTXN*txn=NULL;
847854
volatileSnapshotsnapshot_now=snapshot;
848855

856+
/* Non-transactional changes require a valid snapshot. */
857+
Assert(snapshot_now);
858+
849859
if (xid!=InvalidTransactionId)
850860
txn=ReorderBufferTXNByXid(rb,xid, true,NULL,lsn, true);
851861

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