Re: [PATCH 01/11] nbd/server: Remove unused variable

[Eric Blake]

On Tue, Jul 13, 2021 at 12:27:48PM +0300, Vladimir Sementsov-Ogievskiy wrote:
> 13.07.2021 00:55, Richard Henderson wrote:
> >  From clang-13:
> > nbd/server.c:976:22: error: variable 'bitmaps' set but not used \
> >      [-Werror,-Wunused-but-set-variable]
> > 

> > +++ b/nbd/server.c
> > @@ -973,7 +973,6 @@ static int nbd_negotiate_meta_queries(NBDClient *client,
> >   {
> >       int ret;
> >       g_autofree char *export_name = NULL;
> > -    g_autofree bool *bitmaps = NULL;
> >       NBDExportMetaContexts local_meta = {0};
> >       uint32_t nb_queries;
> >       size_t i;
> > @@ -1007,9 +1006,6 @@ static int nbd_negotiate_meta_queries(NBDClient 
> > *client,
> >                               "export '%s' not present", sane_name);
> >       }
> >       meta->bitmaps = g_new0(bool, meta->exp->nr_export_bitmaps);
> > -    if (client->opt == NBD_OPT_LIST_META_CONTEXT) {
> > -        bitmaps = meta->bitmaps;
> > -    }
> >       ret = nbd_opt_read(client, &nb_queries, sizeof(nb_queries), false, 
> > errp);
> >       if (ret <= 0) {
> > 
> 
> 
> Hm. I'm afraid, this way meta->bitmaps will be leaked in 
> NBD_OPT_LIST_META_CONTEXT case.
> 
> Actually, "bitmaps" _is_ used, in cleanup handler, setup by g_autofree. So 
> it's a false positive.
>

Correct; this patch is wrong, and would cause a memory leak. This is a
false positive in clang, and a known issue that clang is in general
unable to see that g_autofree variables are used, sometimes for their
intentional side effects such as easier memory cleanup as done here.

I suspect that the definition of g_autofree already uses
__attribute__((unused)) to work around clang's oddities, which means
I'm not sure how to silence clang on this one.

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