[Libguestfs] [PATCH nbdkit v3 5/6] server: debug: Fix error handling

[Eric Blake]

On Tue, May 09, 2023 at 03:51:20PM +0100, Richard W.M. Jones wrote:
> Preserve errno even if the function fails.  We need to set
> errno = err again on every exit path out of the function.
> 
> Check if close_memstream failed and go to the error path if so.  This
> is most important on Windows where close_memstream is what actually
> allocates the memory.  But even on Unix, close_memstream (which is an
> alias for fclose) can return an error.
> 
> Move the error-handling code to the end of the function.
> 
> I added a comment above the 'fputs' explaining why exactly we're using
> memstream in the first place -- because it is more atomic than using
> multiple fprintf.
> 
> See: https://listman.redhat.com/archives/libguestfs/2023-May/031456.html
> Thanks: Laszlo Ersek
> ---
>  server/debug.c | 28 ++++++++++++++++------------
>  1 file changed, 16 insertions(+), 12 deletions(-)

Reviewed-by: Eric Blake <eblake at redhat.com>

> + fail:
> +  /* Try to emit what we can. */
> +  errno = err;
> +  vfprintf (stderr, fs, args);
> +  fprintf (stderr, "\n");
>    errno = err;

It might make more sense if the first line were written:

  errno = err; /* In case fs uses %m */

so that no one tries to "optimize" out what looks like a redundant
assignment when both are really required.

-- 
Eric Blake, Principal Software Engineer
Red Hat, Inc.           +1-919-301-3266
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