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Fix snapshot handling in logicalmsg_decode

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Whe decoding a transactional logical message, logicalmsg_decode called
SnapBuildGetOrBuildSnapshot. But we may not have a consistent snapshot
yet at that point. We don't actually need the snapshot in this case
(during replay we'll have the snapshot from the transaction), so in
practice this is harmless. But in assert-enabled build this crashes.

Fixed by requesting the snapshot only in non-transactional case, where
we are guaranteed to have SNAPBUILD_CONSISTENT.

Backpatch to 11. The issue exists since 9.6.

Backpatch-through: 11
Reviewed-by: Andres Freund
Discussion: https://postgr.es/m/84d60912-6eab-9b84-5de3-41765a5449e8@enterprisedb.com
This commit is contained in:
Tomas Vondra 2023-02-22 15:24:09 +01:00
parent 906356cf61
commit 4df581fa0f
2 changed files with 22 additions and 2 deletions
Show Stats Download Patch File Download Diff File Expand all files Collapse all files

14
src/backend/replication/logical/decode.c
View File

@ -514,7 +514,7 @@ DecodeLogicalMsgOp(LogicalDecodingContext *ctx, XLogRecordBuffer *buf)
TransactionId xid = XLogRecGetXid(r);
uint8 info = XLogRecGetInfo(r) & ~XLR_INFO_MASK;
RepOriginId origin_id = XLogRecGetOrigin(r);
Snapshot snapshot;
Snapshot snapshot = NULL;
xl_logical_message *message;
if (info != XLOG_LOGICAL_MESSAGE)
@ -544,7 +544,17 @@ DecodeLogicalMsgOp(LogicalDecodingContext *ctx, XLogRecordBuffer *buf)
SnapBuildXactNeedsSkip(builder, buf->origptr)))
return;
snapshot = SnapBuildGetOrBuildSnapshot(builder, xid);
/*
* If this is a non-transactional change, get the snapshot we're expected
* to use. We only get here when the snapshot is consistent, and the
* change is not meant to be skipped.
*
* For transactional changes we don't need a snapshot, we'll use the
* regular snapshot maintained by ReorderBuffer. We just leave it NULL.
*/
if (!message->transactional)
snapshot = SnapBuildGetOrBuildSnapshot(builder, xid);
ReorderBufferQueueMessage(ctx->reorder, xid, snapshot, buf->endptr,
message->transactional,
message->message, /* first part of message is

10
src/backend/replication/logical/reorderbuffer.c
View File

@ -672,6 +672,13 @@ ReorderBufferQueueMessage(ReorderBuffer *rb, TransactionId xid,
Assert(xid != InvalidTransactionId);
/*
* We don't expect snapshots for transactional changes - we'll use the
* snapshot derived later during apply (unless the change gets
* skipped).
*/
Assert(!snapshot);
oldcontext = MemoryContextSwitchTo(rb->context);
change = ReorderBufferGetChange(rb);
@ -690,6 +697,9 @@ ReorderBufferQueueMessage(ReorderBuffer *rb, TransactionId xid,
ReorderBufferTXN *txn = NULL;
volatile Snapshot snapshot_now = snapshot;
/* Non-transactional changes require a valid snapshot. */
Assert(snapshot_now);
if (xid != InvalidTransactionId)
txn = ReorderBufferTXNByXid(rb, xid, true, NULL, lsn, true);