[PATCH] tempfile: avoid "ferror | fclose" trick

[Jeff King <peff@peff.net>]

On Thu, Feb 16, 2017 at 11:43:59AM -0500, Jeff King wrote:

> On Thu, Feb 16, 2017 at 11:10:18AM +0100, Andreas Schwab wrote:
> 
> > >> 	int xfclose(FILE *fp)
> > >> 	{
> > >> 		return ferror(fp) | fclose(fp);
> > >> 	}
> > >
> > > Yes, that's exactly what I had in mind (might be worth calling out the
> > > bitwise-OR, though, just to make it clear it's not a typo).
> > 
> > Since the order of evaluation is unspecified, it would be better to
> > force sequencing ferror before fclose.
> 
> Good point. Arguably the call in tempfile.c is buggy.

Here's a fix.

I think close_tempfile() suffers from the same errno problem discussed
earlier in this thread (i.e., that after calling it, you may get an
error return with a random, unrelated errno value if ferror() failed but
fclose() did not).

-- >8 --
Subject: [PATCH] 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>
---
 tempfile.c | 8 ++------
 1 file changed, 2 insertions(+), 6 deletions(-)

diff --git a/tempfile.c b/tempfile.c
index 2990c9242..ffcc27237 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);
 	}
-- 
2.12.0.rc1.559.gd292418bf