Re: [PATCH] Don't fflush(stdout) when it's not helpful

From: Linus Torvalds <torvalds@linux-foundation.org>
Date: Jun 30, 2007, 04:24 UTC
Message-ID: <alpine.LFD.0.98.0706292114350.8675@woody.linux-foundation.org>
In-Reply-To: <7vlke2dw6w.fsf@assigned-by-dhcp.pobox.com>

On Fri, 29 Jun 2007, Junio C Hamano wrote:

Show 20 quoted lines

> 
> Do you mean this part?
> 
> +	/* Somebody closed stdout? */
> +	if (fstat(fileno(stdout), &st))
> +		return 0;
> +	/* Ignore write errors for pipes and sockets.. */
> +	if (S_ISFIFO(st.st_mode) || S_ISSOCK(st.st_mode))
> +		return 0;
> +
> +	/* Check for ENOSPC and EIO errors.. */
> +	if (ferror(stdout))
> +		die("write failure on standard output");
> +	if (fflush(stdout) || fclose(stdout))
> +		die("write failure on standard output: %s", strerror(errno));
> +
> +	return 0;
> +}
> 
> I was planning to push this out to 'master' this weekend.

I think that code is fine, but switching the order around could probably make it less likely that stdio loses the errno for us.

So doing the last part in a different order, and making it say

	/* Check for ENOSPC and EIO errors.. */
	if (fflush(stdout))
		die("write failure on standard output: %s", strerror(errno));
	if (ferror(stdout))
		die("unknown write failure on standard output");
	if (fclose(stdout))
		die("close failed on standard output: %s", strerror(errno));
	return 0;

may recover at least non-transient errors.

It's still not perfect. As I've been harping on, stdio simply isn't very good for error reporting. For example, if an IO error happened, you'd want to see EIO, wouldn't you? And yes, that's what the kernel would return. However, with buffered stdio (and flushing outside of our control), what would likely happen is that some intermediate error return _does_ return EIO, but then the kernel might decide to re-mount the filesystem read-only due to the error, and the actual *report* for us might be

	"write failure on standard output: read-only filesystem"

which lost the EIO.

But even worse, if the output happened to be buffer-aligned, stdio will have thrown the error out entirely, and the "fflush()" will return 0, and then we end up with that "unknown write failure" after all.

Or we might have had a ENOSPC at some point, but removed a temp-file, and the final fflush() doesn't error out at all, so now the incomplete write got done (with one or more buffer chunks missing), and we get "unknown write failure" again, because we again lost the ENOSPC.

So you basically cannot get "perfect" with stdio. It's impossible. But the above re-ordering will at least get you _closer_, and *most* of the time you'll get exactly the error you'd expect.

(I'm not a huge fan of "most of the time it works", but that's stdio for you).

		Linus