v5-0006-Fix-logging-snapshot-conflict-horizon.patch

From 59f3f80f82ed7a63d86c991d0cb025e4cde2caec Mon Sep 17 00:00:00 2001
From: Heikki Linnakangas <heikki.linnakangas@iki.fi>
Date: Wed, 20 Mar 2024 13:36:41 +0200
Subject: [PATCH v5 06/26] Fix logging snapshot conflict horizon.

- it was accessed without proper alignment, which won't work on
  architectures that are strict about alignment. Use memcpy.

- in heap_xlog_prune_freeze, the code tried to access the xid with
  "(xlhp_conflict_horizon *) (xlrec + SizeOfHeapPrune);" But 'xlrec'
  was "xl_heap_prune *" rather than "char *". That happened to work,
  because sizeof(xl_heap_prune) == 1, but make it more robust by
  adding a cast to char *.

- remove xlhp_conflict_horizon and store a TransactionId directly. A
  separate struct would make sense if we needed to store anything else
  there, but for now it just seems like more code.
---
 src/backend/access/heap/heapam.c       |  6 ++++--
 src/backend/access/heap/pruneheap.c    |  3 +--
 src/backend/access/rmgrdesc/heapdesc.c |  6 ++++--
 src/include/access/heapam_xlog.h       | 12 +++++-------
 4 files changed, 14 insertions(+), 13 deletions(-)

diff --git a/src/backend/access/heap/heapam.c b/src/backend/access/heap/heapam.c
index 69a9aaa501d..b8d21ddd4dd 100644
--- a/src/backend/access/heap/heapam.c
+++ b/src/backend/access/heap/heapam.c
@@ -8628,9 +8628,11 @@ heap_xlog_prune_freeze(XLogReaderState *record)
 	 */
 	if (xlrec->flags & XLHP_HAS_CONFLICT_HORIZON && InHotStandby)
 	{
-		xlhp_conflict_horizon *horizon = (xlhp_conflict_horizon *) (xlrec + SizeOfHeapPrune);
+		TransactionId conflict_xid;
 
-		ResolveRecoveryConflictWithSnapshot(horizon->xid,
+		memcpy(&conflict_xid, ((char *) xlrec) + SizeOfHeapPrune, sizeof(TransactionId));
+
+		ResolveRecoveryConflictWithSnapshot(conflict_xid,
 											xlrec->flags & XLHP_IS_CATALOG_REL,
 											rlocator);
 	}
diff --git a/src/backend/access/heap/pruneheap.c b/src/backend/access/heap/pruneheap.c
index 704604d206a..6482d9d05c1 100644
--- a/src/backend/access/heap/pruneheap.c
+++ b/src/backend/access/heap/pruneheap.c
@@ -1284,7 +1284,6 @@ log_heap_prune_and_freeze(Relation relation, Buffer buffer,
 						  OffsetNumber *unused, int nunused)
 {
 	xl_heap_prune xlrec;
-	xlhp_conflict_horizon horizon;
 	XLogRecPtr	recptr;
 	xlhp_freeze freeze;
 	xlhp_prune_items redirect_items,
@@ -1320,7 +1319,7 @@ log_heap_prune_and_freeze(Relation relation, Buffer buffer,
 	XLogRegisterData((char *) &xlrec, SizeOfHeapPrune);
 
 	if (TransactionIdIsValid(conflict_xid))
-		XLogRegisterData((char *) &horizon, SizeOfSnapshotConflictHorizon);
+		XLogRegisterData((char *) &conflict_xid, sizeof(TransactionId));
 
 	XLogRegisterBuffer(0, buffer, REGBUF_STANDARD);
 
diff --git a/src/backend/access/rmgrdesc/heapdesc.c b/src/backend/access/rmgrdesc/heapdesc.c
index 9ef8a745982..ff238d58279 100644
--- a/src/backend/access/rmgrdesc/heapdesc.c
+++ b/src/backend/access/rmgrdesc/heapdesc.c
@@ -248,10 +248,12 @@ heap2_desc(StringInfo buf, XLogReaderState *record)
 
 		if (xlrec->flags & XLHP_HAS_CONFLICT_HORIZON)
 		{
-			xlhp_conflict_horizon *horizon = (xlhp_conflict_horizon *) (xlrec + SizeOfHeapPrune);
+			TransactionId conflict_xid;
+
+			memcpy(&conflict_xid, rec + SizeOfHeapPrune, sizeof(TransactionId));
 
 			appendStringInfo(buf, "snapshotConflictHorizon: %u",
-							 horizon->xid);
+							 conflict_xid);
 		}
 
 		appendStringInfo(buf, ", isCatalogRel: %c",
diff --git a/src/include/access/heapam_xlog.h b/src/include/access/heapam_xlog.h
index 3d41aeb6d47..f0cbd31189e 100644
--- a/src/include/access/heapam_xlog.h
+++ b/src/include/access/heapam_xlog.h
@@ -247,6 +247,11 @@ typedef struct xl_heap_update
 typedef struct xl_heap_prune
 {
 	uint8		flags;
+
+	/*
+	 * If XLHP_HAS_CONFLICT_HORIZON is set, the conflict XID follows,
+	 * unaligned
+	 */
 } xl_heap_prune;
 
 /* to handle recovery conflict during logical decoding on standby */
@@ -273,13 +278,6 @@ typedef struct xl_heap_prune
 
 #define SizeOfHeapPrune (offsetof(xl_heap_prune, flags) + sizeof(uint8))
 
-typedef struct xlhp_conflict_horizon
-{
-	TransactionId xid;
-} xlhp_conflict_horizon;
-
-#define SizeOfSnapshotConflictHorizon (offsetof(xlhp_conflict_horizon, xid) + sizeof(uint32))
-
 /*
  * This struct represents a 'freeze plan', which describes how to freeze a
  * group of one or more heap tuples (appears in xl_heap_prune's xlhp_freeze
-- 
2.39.2