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

On Fri, Feb 17, 2017 at 03:54:42PM -0500, Jeff King wrote:

So here's that patch with a justification.

At this point, this snippet of code would be appropriate to pull into xfclose() if we wanted. But possibly that is the wrong direction, as it encourages callers to do:

  if (xfclose(fp))
	err = error_errno("failure writing to ...");

when they could do:

  if (ferror(fp))
	err = error("failure writing to ...");
  if (fclose(fp))
        err = error_errno("failure writing to ...");

While longer, it's arguably better for them to distinguish the two cases. It's only worth doing the errno magic when the close is deep inside a callstack, and passing out the two cases is awkward.

-- >8 --
Subject: tempfile: set errno to a known value before calling ferror()

In close_tempfile(), we return an error if ferror() indicated a previous failure, or if fclose() failed. In the latter case, errno is set and it is useful for callers to report it.

However, if _only_ ferror() triggers, then the value of errno is based on whatever syscall happened to last fail, which may not be related to our filehandle at all. A caller cannot tell the difference between the two cases, and may use "die_errno()" or similar to report a nonsense errno value.

One solution would be to actually pass back separate return values for the two cases, so a caller can write a more appropriate message for each case. But that makes the interface clunky.

Instead, let's just set errno to the generic EIO in this case. That's not as descriptive as we'd like, but at least it's predictable. So it's better than the status quo in all cases but one: when the last syscall really did involve a failure on our filehandle, we'll be wiping that out. But that's a fragile thing for us to rely on.

In any case, we'll let the errno result from fclose() take precedence over our value, as we know that's recent and accurate (and many I/O errors will persist through the fclose anyway).

Signed-off-by: Jeff King <peff@peff.net>
---
 tempfile.c | 9 +++++++--
 1 file changed, 7 insertions(+), 2 deletions(-)