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

diff --git a/src/backend/replication/logical/decode.c b/src/backend/replication/logical/decode.c
index a53e23c679d3a5b42715355897d8d519d35b3d07..8fe7bb65f1f18365b64c6faa7d5cfffc2d277d8f 100644 (file)
--- a/src/backend/replication/logical/decode.c
+++ b/src/backend/replication/logical/decode.c
@@ -564,7 +564,7 @@ logicalmsg_decode(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)
@@ -594,7 +594,17 @@ logicalmsg_decode(LogicalDecodingContext *ctx, XLogRecordBuffer *buf)
                          SnapBuildXactNeedsSkip(builder, buf->origptr)))
                return;
 
-       snapshot = SnapBuildGetOrBuildSnapshot(builder);
+       /*
+        * 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);
+
        ReorderBufferQueueMessage(ctx->reorder, xid, snapshot, buf->endptr,
                                                          message->transactional,
                                                          message->message, /* first part of message is

diff --git a/src/backend/replication/logical/reorderbuffer.c b/src/backend/replication/logical/reorderbuffer.c
index d5f90a5f5d2de15b575034841e26f561f709256a..2d17c551a80f8190df869f03ee8801c4dcf6226f 100644 (file)
--- 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);