Fix confusion on the padding of GIDs in on commit and abort records.

Review of commit 1eb6d652: It's pointless to add padding to the GID fields,
when the code that follows assumes that there is no alignment, and uses
memcpy(). Remove the pointless padding.

Update comments to note the new fields in the WAL records.

Reviewed-by: Michael Paquier
Discussion: https://www.postgresql.org/message-id/33b787bf-dc20-1161-54e9-3f3b607bf59d%40iki.fi

src/backend/access/rmgrdesc/xactdesc.c

@@ -104,18 +104,18 @@ ParseCommitRecord(uint8 info, xl_xact_commit *xlrec, xl_xact_parsed_commit *pars
 
 		if (parsed->xinfo & XACT_XINFO_HAS_GID)
 		{
-			int gidlen;
 			strlcpy(parsed->twophase_gid, data, sizeof(parsed->twophase_gid));
-			gidlen = strlen(data) + 1;
-			data += MAXALIGN(gidlen);
+			data += strlen(data) + 1;
 		}
 	}
 
+	/* Note: no alignment is guaranteed after this point */
+
 	if (parsed->xinfo & XACT_XINFO_HAS_ORIGIN)
 	{
 		xl_xact_origin xl_origin;
 
-		/* we're only guaranteed 4 byte alignment, so copy onto stack */
+		/* no alignment is guaranteed, so copy onto stack */
 		memcpy(&xl_origin, data, sizeof(xl_origin));
 
 		parsed->origin_lsn = xl_origin.origin_lsn;
@@ -188,18 +188,18 @@ ParseAbortRecord(uint8 info, xl_xact_abort *xlrec, xl_xact_parsed_abort *parsed)
 
 		if (parsed->xinfo & XACT_XINFO_HAS_GID)
 		{
-			int gidlen;
 			strlcpy(parsed->twophase_gid, data, sizeof(parsed->twophase_gid));
-			gidlen = strlen(data) + 1;
-			data += MAXALIGN(gidlen);
+			data += strlen(data) + 1;
 		}
 	}
 
+	/* Note: no alignment is guaranteed after this point */
+
 	if (parsed->xinfo & XACT_XINFO_HAS_ORIGIN)
 	{
 		xl_xact_origin xl_origin;
 
-		/* we're only guaranteed 4 byte alignment, so copy onto stack */
+		/* no alignment is guaranteed, so copy onto stack */
 		memcpy(&xl_origin, data, sizeof(xl_origin));
 
 		parsed->origin_lsn = xl_origin.origin_lsn;

src/backend/access/transam/twophase.c

@@ -1129,9 +1129,11 @@ EndPrepare(GlobalTransaction gxact)
 	gxact->prepare_end_lsn = XLogInsert(RM_XACT_ID, XLOG_XACT_PREPARE);
 
 	if (replorigin)
+	{
 		/* Move LSNs forward for this replication origin */
 		replorigin_session_advance(replorigin_session_origin_lsn,
 								   gxact->prepare_end_lsn);
+	}
 
 	XLogFlush(gxact->prepare_end_lsn);
 

src/backend/access/transam/xact.c

@@ -5245,9 +5245,7 @@ XactLogCommitRecord(TimestampTz commit_time,
 	xl_xact_invals xl_invals;
 	xl_xact_twophase xl_twophase;
 	xl_xact_origin xl_origin;
-
 	uint8		info;
-	int			gidlen = 0;
 
 	Assert(CritSectionCount > 0);
 
@@ -5313,10 +5311,7 @@ XactLogCommitRecord(TimestampTz commit_time,
 		Assert(twophase_gid != NULL);
 
 		if (XLogLogicalInfoActive())
-		{
 			xl_xinfo.xinfo |= XACT_XINFO_HAS_GID;
-			gidlen = strlen(twophase_gid) + 1; /* include '\0' */
-		}
 	}
 
 	/* dump transaction origin information */
@@ -5370,12 +5365,7 @@ XactLogCommitRecord(TimestampTz commit_time,
 	{
 		XLogRegisterData((char *) (&xl_twophase), sizeof(xl_xact_twophase));
 		if (xl_xinfo.xinfo & XACT_XINFO_HAS_GID)
-		{
-			static const char zeroes[MAXIMUM_ALIGNOF] = { 0 };
-			XLogRegisterData((char*) twophase_gid, gidlen);
-			if (MAXALIGN(gidlen) != gidlen)
-				XLogRegisterData((char*) zeroes, MAXALIGN(gidlen) - gidlen);
-		}
+			XLogRegisterData((char *) twophase_gid, strlen(twophase_gid));
 	}
 
 	if (xl_xinfo.xinfo & XACT_XINFO_HAS_ORIGIN)
@@ -5409,7 +5399,6 @@ XactLogAbortRecord(TimestampTz abort_time,
 	xl_xact_origin xl_origin;
 
 	uint8		info;
-	int			gidlen = 0;
 
 	Assert(CritSectionCount > 0);
 
@@ -5448,10 +5437,7 @@ XactLogAbortRecord(TimestampTz abort_time,
 		Assert(twophase_gid != NULL);
 
 		if (XLogLogicalInfoActive())
-		{
 			xl_xinfo.xinfo |= XACT_XINFO_HAS_GID;
-			gidlen = strlen(twophase_gid) + 1; /* include '\0' */
-		}
 	}
 
 	if (TransactionIdIsValid(twophase_xid) && XLogLogicalInfoActive())
@@ -5487,7 +5473,6 @@ XactLogAbortRecord(TimestampTz abort_time,
 	if (xl_xinfo.xinfo & XACT_XINFO_HAS_DBINFO)
 		XLogRegisterData((char *) (&xl_dbinfo), sizeof(xl_dbinfo));
 
-
 	if (xl_xinfo.xinfo & XACT_XINFO_HAS_SUBXACTS)
 	{
 		XLogRegisterData((char *) (&xl_subxacts),
@@ -5508,12 +5493,7 @@ XactLogAbortRecord(TimestampTz abort_time,
 	{
 		XLogRegisterData((char *) (&xl_twophase), sizeof(xl_xact_twophase));
 		if (xl_xinfo.xinfo & XACT_XINFO_HAS_GID)
-		{
-			static const char zeroes[MAXIMUM_ALIGNOF] = { 0 };
-			XLogRegisterData((char*) twophase_gid, gidlen);
-			if (MAXALIGN(gidlen) != gidlen)
-				XLogRegisterData((char*) zeroes, MAXALIGN(gidlen) - gidlen);
-		}
+			XLogRegisterData((char *) twophase_gid, strlen(twophase_gid) + 1);
 	}
 
 	if (xl_xinfo.xinfo & XACT_XINFO_HAS_ORIGIN)

src/include/access/xact.h

@@ -269,6 +269,7 @@ typedef struct xl_xact_commit
 	/* xl_xact_relfilenodes follows if XINFO_HAS_RELFILENODES */
 	/* xl_xact_invals follows if XINFO_HAS_INVALS */
 	/* xl_xact_twophase follows if XINFO_HAS_TWOPHASE */
+	/* twophase_gid follows if XINFO_HAS_GID. As a null-terminated string. */
 	/* xl_xact_origin follows if XINFO_HAS_ORIGIN, stored unaligned! */
 } xl_xact_commit;
 #define MinSizeOfXactCommit (offsetof(xl_xact_commit, xact_time) + sizeof(TimestampTz))
@@ -278,11 +279,13 @@ typedef struct xl_xact_abort
 	TimestampTz xact_time;		/* time of abort */
 
 	/* xl_xact_xinfo follows if XLOG_XACT_HAS_INFO */
-	/* No db_info required */
+	/* xl_xact_dbinfo follows if XINFO_HAS_DBINFO */
 	/* xl_xact_subxacts follows if HAS_SUBXACT */
 	/* xl_xact_relfilenodes follows if HAS_RELFILENODES */
 	/* No invalidation messages needed. */
 	/* xl_xact_twophase follows if XINFO_HAS_TWOPHASE */
+	/* twophase_gid follows if XINFO_HAS_GID. As a null-terminated string. */
+	/* xl_xact_origin follows if XINFO_HAS_ORIGIN, stored unaligned! */
 } xl_xact_abort;
 #define MinSizeOfXactAbort sizeof(xl_xact_abort)
 

src/include/access/xlog_internal.h

@@ -31,7 +31,7 @@
 /*
  * Each page of XLOG file has a header like this:
  */
-#define XLOG_PAGE_MAGIC 0xD097	/* can be used as WAL version indicator */
+#define XLOG_PAGE_MAGIC 0xD098	/* can be used as WAL version indicator */
 
 typedef struct XLogPageHeaderData
 {