When fetching WAL for a basebackup, report errors with a sensible TLI.

The previous code used ThisTimeLineID, which need not even be
initialized here, although it usually was in practice, because
pg_basebackup issues IDENTIFY_SYSTEM before calling BASE_BACKUP,
and that initializes ThisTimeLineID as a side effect. That's not
really good enough, though, not only because we shoudn't be counting
on side effects like that, but also because the TLI could change
meanwhile. Fortunately, we have convenient access to more meaningful
TLI values, so use those instead.

Because of the way this logic is coded, the consequences of using
a possibly-incorrect TLI here are no worse than a slightly confusing
error message, I don't want to take any risk here, so no back-patch
at least for now.

Patch by me, reviewed by Kyotaro Horiguchi and Michael Paquier

Discussion: http://postgr.es/m/CA+TgmoZRNWGWYDX9RgTXMG6_nwSdB=PB-PPRUbvMUTGfmL2sHQ@mail.gmail.com

src/backend/replication/basebackup.c

@@ -502,9 +502,9 @@ perform_base_backup(basebackup_options *opt)
 		 * including them.
 		 */
 		XLByteToSeg(startptr, startsegno, wal_segment_size);
-		XLogFileName(firstoff, ThisTimeLineID, startsegno, wal_segment_size);
+		XLogFileName(firstoff, starttli, startsegno, wal_segment_size);
 		XLByteToPrevSeg(endptr, endsegno, wal_segment_size);
-		XLogFileName(lastoff, ThisTimeLineID, endsegno, wal_segment_size);
+		XLogFileName(lastoff, endtli, endsegno, wal_segment_size);
 
 		dir = AllocateDir("pg_wal");
 		while ((de = ReadDir(dir, "pg_wal")) != NULL)
@@ -528,7 +528,7 @@ perform_base_backup(basebackup_options *opt)
 		 * Before we go any further, check that none of the WAL segments we
 		 * need were removed.
 		 */
-		CheckXLogRemoved(startsegno, ThisTimeLineID);
+		CheckXLogRemoved(startsegno, starttli);
 
 		/*
 		 * Sort the WAL filenames.  We want to send the files in order from
@@ -555,7 +555,7 @@ perform_base_backup(basebackup_options *opt)
 		{
 			char		startfname[MAXFNAMELEN];
 
-			XLogFileName(startfname, ThisTimeLineID, startsegno,
+			XLogFileName(startfname, starttli, startsegno,
 						 wal_segment_size);
 			ereport(ERROR,
 					(errmsg("could not find WAL file \"%s\"", startfname)));
@@ -571,8 +571,7 @@ perform_base_backup(basebackup_options *opt)
 			{
 				char		nextfname[MAXFNAMELEN];
 
-				XLogFileName(nextfname, ThisTimeLineID, nextsegno,
-							 wal_segment_size);
+				XLogFileName(nextfname, tli, nextsegno, wal_segment_size);
 				ereport(ERROR,
 						(errmsg("could not find WAL file \"%s\"", nextfname)));
 			}
@@ -581,7 +580,7 @@ perform_base_backup(basebackup_options *opt)
 		{
 			char		endfname[MAXFNAMELEN];
 
-			XLogFileName(endfname, ThisTimeLineID, endsegno, wal_segment_size);
+			XLogFileName(endfname, endtli, endsegno, wal_segment_size);
 			ereport(ERROR,
 					(errmsg("could not find WAL file \"%s\"", endfname)));
 		}