In XLogFileInit(), fix *use_existent postcondition to suit callers.

Infrequently, the mismatch caused log_checkpoints messages and
TRACE_POSTGRESQL_CHECKPOINT_DONE() to witness an "added" count too high
by one.  Since that consequence is so minor, no back-patch.

This commit has been applied as of 85656bc305 in v15 and newer
versions.  This is required on stable branches of v13 and v14 to fix a
regression reported by Noah Misch, introduced by 1f95181b44, causing
spurious failures in archive recovery (neither streaming nor archive
recovery) with concurrent restartpoints.  The backpatched versions of
the patches have been aligned on these branches by me, Noah Misch is the
author.  Tests have been conducted by the both of us.

Reported-by: Arun Thirupathi
Author: Noah Misch <noah@leadboat.com>
Discussion: https://postgr.es/m/20210202151416.GB3304930@rfd.leadboat.com
Discussion: https://postgr.es/m/20250306193013.36.nmisch@google.com
Backpatch-through: 13

src/backend/access/transam/xlog.c

@@ -3259,8 +3259,8 @@ XLogNeedsFlush(XLogRecPtr record)
  * logsegno: identify segment to be created/opened.
  *
  * *use_existent: if true, OK to use a pre-existing file (else, any
- * pre-existing file will be deleted).  On return, true if a pre-existing
- * file was used.
+ * pre-existing file will be deleted).  On return, false iff this call added
+ * some segment on disk.
  *
  * Returns FD of opened file.
  *
@@ -3414,8 +3414,10 @@ XLogFileInit(XLogSegNo logsegno, bool *use_existent)
 	 * CheckPointSegments.
 	 */
 	max_segno = logsegno + CheckPointSegments;
-	if (!InstallXLogFileSegment(&installed_segno, tmppath,
-								*use_existent, max_segno))
+	if (InstallXLogFileSegment(&installed_segno, tmppath,
+							   *use_existent, max_segno))
+		*use_existent = false;
+	else
 	{
 		/*
 		 * No need for any more future segments, or InstallXLogFileSegment()
@@ -3425,9 +3427,6 @@ XLogFileInit(XLogSegNo logsegno, bool *use_existent)
 		unlink(tmppath);
 	}
 
-	/* Set flag to tell caller there was no existent file */
-	*use_existent = false;
-
 	/* Now open original target segment (might not be file I just made) */
 	fd = BasicOpenFile(path, O_RDWR | PG_BINARY | get_sync_bit(sync_method));
 	if (fd < 0)