v10-0006-Remove-some-dead-code-related-to-handling-member.patch

From b35b0aa849a16a9f71ba615c8134fe74db37d088 Mon Sep 17 00:00:00 2001
From: Heikki Linnakangas <heikki.linnakangas@iki.fi>
Date: Wed, 18 Dec 2024 11:57:06 +0200
Subject: [PATCH v10 06/14] Remove some dead code related to handling members
 wraparound

Now that offsets are 64-bit, we assume members never wrap around.

Start the offset counter from 1 so that we don't need the special case
for starting from 0.
---
 src/backend/access/transam/multixact.c | 42 +++-----------------------
 src/backend/access/transam/xlog.c      |  2 +-
 2 files changed, 5 insertions(+), 39 deletions(-)

diff --git a/src/backend/access/transam/multixact.c b/src/backend/access/transam/multixact.c
index 3d27995a299..737154814a8 100644
--- a/src/backend/access/transam/multixact.c
+++ b/src/backend/access/transam/multixact.c
@@ -147,19 +147,6 @@ MultiXactIdToOffsetSegment(MultiXactId multi)
 #define MULTIXACT_MEMBERS_PER_PAGE	\
 	(MULTIXACT_MEMBERGROUPS_PER_PAGE * MULTIXACT_MEMBERS_PER_MEMBERGROUP)
 
-/*
- * Because the number of items per page is not a divisor of the last item
- * number (member 0xFFFFFFFF), the last segment does not use the maximum number
- * of pages, and moreover the last used page therein does not use the same
- * number of items as previous pages.  (Another way to say it is that the
- * 0xFFFFFFFF member is somewhere in the middle of the last page, so the page
- * has some empty space after that item.)
- *
- * This constant is the number of members in the last page of the last segment.
- */
-#define MAX_MEMBERS_IN_LAST_MEMBERS_PAGE \
-		((uint32) ((0xFFFFFFFF % MULTIXACT_MEMBERS_PER_PAGE) + 1))
-
 /* page in which a member is to be found */
 static inline int64
 MXOffsetToMemberPage(MultiXactOffset offset)
@@ -1140,18 +1127,10 @@ GetNewMultiXactId(int nmembers, MultiXactOffset *offset)
 	ExtendMultiXactOffset(result);
 
 	/*
-	 * Reserve the members space, similarly to above.  Also, be careful not to
-	 * return zero as the starting offset for any multixact. See
-	 * GetMultiXactIdMembers() for motivation.
+	 * Reserve the members space, similarly to above.
 	 */
 	nextOffset = MultiXactState->nextOffset;
-	if (nextOffset == 0)
-	{
-		*offset = 1;
-		nmembers++;				/* allocate member slot 0 too */
-	}
-	else
-		*offset = nextOffset;
+	*offset = nextOffset;
 
 	ExtendMultiXactMember(nextOffset, nmembers);
 
@@ -2537,22 +2516,9 @@ ExtendMultiXactMember(MultiXactOffset offset, int nmembers)
 		}
 
 		/*
-		 * Compute the number of items till end of current page.  Careful: if
-		 * addition of unsigned ints wraps around, we're at the last page of
-		 * the last segment; since that page holds a different number of items
-		 * than other pages, we need to do it differently.
+		 * Compute the number of items till end of current page.
 		 */
-		if (offset + MAX_MEMBERS_IN_LAST_MEMBERS_PAGE < offset)
-		{
-			/*
-			 * This is the last page of the last segment; we can compute the
-			 * number of items left to allocate in it without modulo
-			 * arithmetic.
-			 */
-			difference = MaxMultiXactOffset - offset + 1;
-		}
-		else
-			difference = MULTIXACT_MEMBERS_PER_PAGE - offset % MULTIXACT_MEMBERS_PER_PAGE;
+		difference = MULTIXACT_MEMBERS_PER_PAGE - offset % MULTIXACT_MEMBERS_PER_PAGE;
 
 		/*
 		 * Advance to next page, taking care to properly handle the wraparound
diff --git a/src/backend/access/transam/xlog.c b/src/backend/access/transam/xlog.c
index 6f58412bcab..067cb70938a 100644
--- a/src/backend/access/transam/xlog.c
+++ b/src/backend/access/transam/xlog.c
@@ -5083,7 +5083,7 @@ BootStrapXLOG(uint32 data_checksum_version)
 		FullTransactionIdFromEpochAndXid(0, FirstNormalTransactionId);
 	checkPoint.nextOid = FirstGenbkiObjectId;
 	checkPoint.nextMulti = FirstMultiXactId;
-	checkPoint.nextMultiOffset = 0;
+	checkPoint.nextMultiOffset = 1;
 	checkPoint.oldestXid = FirstNormalTransactionId;
 	checkPoint.oldestXidDB = Template1DbOid;
 	checkPoint.oldestMulti = FirstMultiXactId;
-- 
2.39.5