Handle posix_fallocate() errors.

On some platforms, posix_fallocate() is available but may still return
EINVAL if the underlying filesystem does not support it.  So, in case
of an error, fall through to the alternate implementation that just
writes zeros.

Per buildfarm failure and analysis by Tom Lane.

src/backend/access/transam/xlog.c

@@ -2259,6 +2259,7 @@ XLogFileInit(XLogSegNo logsegno, bool *use_existent, bool use_lock)
 	XLogSegNo	installed_segno;
 	int			max_advance;
 	int			fd;
+	bool		zero_fill = true;
 
 	XLogFilePath(path, ThisTimeLineID, logsegno);
 
@@ -2301,24 +2302,18 @@ XLogFileInit(XLogSegNo logsegno, bool *use_existent, bool use_lock)
 				 errmsg("could not create file \"%s\": %m", tmppath)));
 
 #ifdef HAVE_POSIX_FALLOCATE
-	{
-		errno = posix_fallocate(fd, 0, XLogSegSize);
-
-		if (errno)
-		{
-			int errno_saved = errno;
-
-			close(fd);
-			unlink(tmppath);
-			errno = errno_saved;
+	/*
+	 * If posix_fallocate() is available and succeeds, then the file is
+	 * properly allocated and we don't need to zero-fill it (which is less
+	 * efficient).  In case of an error, fall back to writing zeros, because on
+	 * some platforms posix_fallocate() is available but will not always
+	 * succeed in cases where zero-filling will.
+	 */
+	if (posix_fallocate(fd, 0, XLogSegSize) == 0)
+		zero_fill = false;
+#endif /* HAVE_POSIX_FALLOCATE */
 
-			ereport(ERROR,
-					(errcode_for_file_access(),
-					 errmsg("could not allocate space for file \"%s\" using posix_fallocate: %m",
-							tmppath)));
-		}
-	}
-#else /* !HAVE_POSIX_FALLOCATE */
+	if (zero_fill)
 	{
 		/*
 		 * Allocate a buffer full of zeros. This is done before opening the
@@ -2366,7 +2361,6 @@ XLogFileInit(XLogSegNo logsegno, bool *use_existent, bool use_lock)
 		}
 		pfree(zbuffer);
 	}
-#endif /* HAVE_POSIX_FALLOCATE */
 
 	if (pg_fsync(fd) != 0)
 	{