tempfile: avoid "ferror | fclose" trick

The current code wants to record an error condition from
either ferror() or fclose(), but makes sure that we always
call both functions. So it can't use logical-OR "||", which
would short-circuit when ferror() is true. Instead, it uses
bitwise-OR "|" to evaluate both functions and set one or
more bits in the "err" flag if they reported a failure.

Unlike logical-OR, though, bitwise-OR does not introduce a
sequence point, and the order of evaluation for its operands
is unspecified. So a compiler would be free to generate code
which calls fclose() first, and then ferror() on the
now-freed filehandle.

There's no indication that this has happened in practice,
but let's write it out in a way that follows the standard.

Noticed-by: Andreas Schwab <schwab@linux-m68k.org>
Signed-off-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

1 files changed, 2 insertions, 6 deletions

diff --git a/tempfile.c b/tempfile.c
index 2990c92..ffcc272 100644
--- a/tempfile.c
+++ b/tempfile.c
@@ -247,12 +247,8 @@ int close_tempfile(struct tempfile *tempfile)
 	tempfile->fd = -1;
 	if (fp) {
 		tempfile->fp = NULL;
-
-		/*
-		 * Note: no short-circuiting here; we want to fclose()
-		 * in any case!
-		 */
-		err = ferror(fp) | fclose(fp);
+		err = ferror(fp);
+		err |= fclose(fp);
 	} else {
 		err = close(fd);
 	}