Fix snapshot handling in logicalmsg_decode

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

1 files changed, 10 insertions, 0 deletions

diff --git a/src/backend/replication/logical/reorderbuffer.c b/src/backend/replication/logical/reorderbuffer.c
index d5f90a5f5d2..2d17c551a80 100644
--- a/src/backend/replication/logical/reorderbuffer.c
+++ b/src/backend/replication/logical/reorderbuffer.c
@@ -856,6 +856,13 @@ 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(!snap);
+
 		oldcontext = MemoryContextSwitchTo(rb->context);
 
 		change = ReorderBufferGetChange(rb);
@@ -874,6 +881,9 @@ ReorderBufferQueueMessage(ReorderBuffer *rb, TransactionId xid,
 		ReorderBufferTXN *txn = NULL;
 		volatile Snapshot snapshot_now = snap;
 
+		/* Non-transactional changes require a valid snapshot. */
+		Assert(snapshot_now);
+
 		if (xid != InvalidTransactionId)
 			txn = ReorderBufferTXNByXid(rb, xid, true, NULL, lsn, true);