From 8de91ebf2ac1e9922214bf2976a2fcc5c045c169 Mon Sep 17 00:00:00 2001 From: Tomas Vondra Date: Wed, 22 Feb 2023 15:24:09 +0100 Subject: [PATCH] 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 --- src/backend/replication/logical/decode.c | 14 ++++++++++++-- src/backend/replication/logical/reorderbuffer.c | 10 ++++++++++ 2 files changed, 22 insertions(+), 2 deletions(-) diff --git a/src/backend/replication/logical/decode.c b/src/backend/replication/logical/decode.c index dc83743c384..6e66ef6180a 100644 --- a/src/backend/replication/logical/decode.c +++ b/src/backend/replication/logical/decode.c @@ -518,7 +518,7 @@ 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) @@ -548,7 +548,17 @@ 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 diff --git a/src/backend/replication/logical/reorderbuffer.c b/src/backend/replication/logical/reorderbuffer.c index b72f43678bd..cdbb3c801a9 100644 --- a/src/backend/replication/logical/reorderbuffer.c +++ b/src/backend/replication/logical/reorderbuffer.c @@ -614,6 +614,13 @@ 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); @@ -632,6 +639,9 @@ 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); -- 2.39.5