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

[Richard W.M. Jones]

On Tue, May 09, 2023 at 01:25:18PM -0500, Eric Blake wrote:
> 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.

I actually did that already in my local version!

Rich.

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