Bootstrap WAL to begin at segment logid=0 logseg=1 (000000010000000000000001)

rather than 0/0, so that we can safely use 0/0 as an invalid value. This is a
more future-proof fix for the corner-case bug in streaming replication that
was fixed yesterday. We had a similar corner-case bug with log/seg 0/0 back in
February as well. Avoiding 0/0 as a valid value should prevent bugs like that
in the future. Per Tom Lane's idea.

Back-patch to 9.0. Since this only affects bootstrapping, it makes no
difference to existing installations. We don't need to worry about the
bug in existing installations, because if you've managed to get past the
initial base backup already, you won't hit the bug in the future either.

src/backend/access/transam/xlog.c

@@ -364,7 +364,7 @@ typedef struct XLogCtlData
 	uint32		ckptXidEpoch;	/* nextXID & epoch of latest checkpoint */
 	TransactionId ckptXid;
 	XLogRecPtr	asyncXactLSN; /* LSN of newest async commit/abort */
-	uint32		lastRemovedLog; /* latest removed/recycled XLOG segment + 1 */
+	uint32		lastRemovedLog; /* latest removed/recycled XLOG segment */
 	uint32		lastRemovedSeg;
 
 	/* Protected by WALWriteLock: */
@@ -3218,9 +3218,7 @@ PreallocXlogFiles(XLogRecPtr endptr)
 }
 
 /*
- * Get the log/seg of the first WAL segment that has not been removed or
+ * Get the log/seg of the latest removed or recycled WAL segment.
- * recycled. In other words, the log/seg of the last removed/recycled WAL
- * segment + 1.
  * Returns 0/0 if no WAL segments have been removed since startup.
  */
 void
@@ -3249,7 +3247,6 @@ UpdateLastRemovedPtr(char *filename)
 				seg;
 
 	XLogFromFileName(filename, &tli, &log, &seg);
-	NextLogSeg(log, seg);
 
 	SpinLockAcquire(&xlogctl->info_lck);
 	if (log > xlogctl->lastRemovedLog ||
@@ -4903,9 +4900,15 @@ BootStrapXLOG(void)
 	page = (XLogPageHeader) TYPEALIGN(ALIGNOF_XLOG_BUFFER, buffer);
 	memset(page, 0, XLOG_BLCKSZ);
 
-	/* Set up information for the initial checkpoint record */
+	/*
+	 * Set up information for the initial checkpoint record
+	 *
+	 * The initial checkpoint record is written to the beginning of the
+	 * WAL segment with logid=0 logseg=1. The very first WAL segment, 0/0, is
+	 * not used, so that we can use 0/0 to mean "before any valid WAL segment".
+	 */
 	checkPoint.redo.xlogid = 0;
-	checkPoint.redo.xrecoff = SizeOfXLogLongPHD;
+	checkPoint.redo.xrecoff = XLogSegSize + SizeOfXLogLongPHD;
 	checkPoint.ThisTimeLineID = ThisTimeLineID;
 	checkPoint.nextXidEpoch = 0;
 	checkPoint.nextXid = FirstNormalTransactionId;
@@ -4928,7 +4931,7 @@ BootStrapXLOG(void)
 	page->xlp_info = XLP_LONG_HEADER;
 	page->xlp_tli = ThisTimeLineID;
 	page->xlp_pageaddr.xlogid = 0;
-	page->xlp_pageaddr.xrecoff = 0;
+	page->xlp_pageaddr.xrecoff = XLogSegSize;
 	longpage = (XLogLongPageHeader) page;
 	longpage->xlp_sysid = sysidentifier;
 	longpage->xlp_seg_size = XLogSegSize;
@@ -4954,7 +4957,7 @@ BootStrapXLOG(void)
 
 	/* Create first XLOG segment file */
 	use_existent = false;
-	openLogFile = XLogFileInit(0, 0, &use_existent, false);
+	openLogFile = XLogFileInit(0, 1, &use_existent, false);
 
 	/* Write the first page with the initial record */
 	errno = 0;

src/backend/replication/walsender.c

@@ -654,7 +654,7 @@ XLogRead(char *buf, XLogRecPtr recptr, Size nbytes)
 	XLogGetLastRemoved(&lastRemovedLog, &lastRemovedSeg);
 	XLByteToSeg(startRecPtr, log, seg);
 	if (log < lastRemovedLog ||
-		(log == lastRemovedLog && seg < lastRemovedSeg))
+		(log == lastRemovedLog && seg <= lastRemovedSeg))
 	{
 		char		filename[MAXFNAMELEN];