Replace XLogRecPtr struct with a 64-bit integer.

This simplifies code that needs to do arithmetic on XLogRecPtrs.

To avoid changing on-disk format of data pages, the LSN on data pages is
still stored in the old format. That should keep pg_upgrade happy. However,
we have XLogRecPtrs embedded in the control file, and in the structs that
are sent over the replication protocol, so this changes breaks compatibility
of pg_basebackup and server. I didn't do anything about this in this patch,
per discussion on -hackers, the right thing to do would to be to change the
replication protocol to be architecture-independent, so that you could use
a newer version of pg_receivexlog, for example, against an older server
version.

Author: hlinnaka

contrib/pageinspect/rawpage.c

@@ -206,7 +206,8 @@ page_header(PG_FUNCTION_ARGS)
 	/* Extract information from the page header */
 
 	lsn = PageGetLSN(page);
-	snprintf(lsnchar, sizeof(lsnchar), "%X/%X", lsn.xlogid, lsn.xrecoff);
+	snprintf(lsnchar, sizeof(lsnchar), "%X/%X",
+			 (uint32) (lsn >> 32), (uint32) lsn);
 
 	values[0] = CStringGetTextDatum(lsnchar);
 	values[1] = UInt16GetDatum(PageGetTLI(page));

src/backend/access/gist/gist.c

@@ -197,7 +197,7 @@ gistplacetopage(Relation rel, Size freespace, GISTSTATE *giststate,
 		SplitedPageLayout *dist = NULL,
 				   *ptr;
 		BlockNumber oldrlink = InvalidBlockNumber;
-		GistNSN		oldnsn = {0, 0};
+		GistNSN		oldnsn = 0;
 		SplitedPageLayout rootpg;
 		BlockNumber blkno = BufferGetBlockNumber(buffer);
 		bool		is_rootsplit;
@@ -488,7 +488,7 @@ gistdoinsert(Relation r, IndexTuple itup, Size freespace, GISTSTATE *giststate)
 
 	/* Start from the root */
 	firststack.blkno = GIST_ROOT_BLKNO;
-	firststack.lsn.xrecoff = 0;
+	firststack.lsn = 0;
 	firststack.parent = NULL;
 	firststack.downlinkoffnum = InvalidOffsetNumber;
 	state.stack = stack = &firststack;

src/backend/access/gist/gistutil.c

@@ -706,13 +706,7 @@ gistoptions(PG_FUNCTION_ARGS)
 XLogRecPtr
 GetXLogRecPtrForTemp(void)
 {
-	static XLogRecPtr counter = {0, 1};
-
-	counter.xrecoff++;
-	if (counter.xrecoff == 0)
-	{
-		counter.xlogid++;
-		counter.xrecoff++;
-	}
+	static XLogRecPtr counter = 1;
+	counter++;
 	return counter;
 }

src/backend/access/transam/transam.c

@@ -24,9 +24,6 @@
 #include "access/transam.h"
 #include "utils/snapmgr.h"
 
-/* Handy constant for an invalid xlog recptr */
-const XLogRecPtr InvalidXLogRecPtr = {0, 0};
-
 /*
  * Single-item cache for results of TransactionLogFetch.  It's worth having
  * such a cache because we frequently find ourselves repeatedly checking the

src/backend/access/transam/twophase.c

@@ -333,8 +333,7 @@ MarkAsPreparing(TransactionId xid, const char *gid,
 
 	gxact->prepared_at = prepared_at;
 	/* initialize LSN to 0 (start of WAL) */
-	gxact->prepare_lsn.xlogid = 0;
-	gxact->prepare_lsn.xrecoff = 0;
+	gxact->prepare_lsn = 0;
 	gxact->owner = owner;
 	gxact->locking_xid = xid;
 	gxact->valid = false;

src/backend/access/transam/xact.c

@@ -951,7 +951,7 @@ RecordTransactionCommit(void)
 	if (XLogStandbyInfoActive())
 		nmsgs = xactGetCommittedInvalidationMessages(&invalMessages,
 													 &RelcacheInitFileInval);
-	wrote_xlog = (XactLastRecEnd.xrecoff != 0);
+	wrote_xlog = (XactLastRecEnd != 0);
 
 	/*
 	 * If we haven't been assigned an XID yet, we neither can, nor do we want
@@ -1200,7 +1200,7 @@ RecordTransactionCommit(void)
 		SyncRepWaitForLSN(XactLastRecEnd);
 
 	/* Reset XactLastRecEnd until the next transaction writes something */
-	XactLastRecEnd.xrecoff = 0;
+	XactLastRecEnd = 0;
 
 cleanup:
 	/* Clean up local data */
@@ -1402,7 +1402,7 @@ RecordTransactionAbort(bool isSubXact)
 	{
 		/* Reset XactLastRecEnd until the next transaction writes something */
 		if (!isSubXact)
-			XactLastRecEnd.xrecoff = 0;
+			XactLastRecEnd = 0;
 		return InvalidTransactionId;
 	}
 
@@ -1501,7 +1501,7 @@ RecordTransactionAbort(bool isSubXact)
 
 	/* Reset XactLastRecEnd until the next transaction writes something */
 	if (!isSubXact)
-		XactLastRecEnd.xrecoff = 0;
+		XactLastRecEnd = 0;
 
 	/* And clean up local data */
 	if (rels)
@@ -2170,7 +2170,7 @@ PrepareTransaction(void)
 	 */
 
 	/* Reset XactLastRecEnd until the next transaction writes something */
-	XactLastRecEnd.xrecoff = 0;
+	XactLastRecEnd = 0;
 
 	/*
 	 * Let others know about no transaction in progress by me.	This has to be