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

[Eric Blake]

On Tue, Jul 13, 2021 at 08:01:34AM -0500, Eric Blake wrote:
> > > @@ -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};
> > 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.

Hmm; in glib 2.68.2 (on Fedora 34), g_autofree does NOT include an
attribute unused.  Thus, does this silence the compiler?  (Even cooler
would be making the comment a link to an actual bug in the clang
database, but I couldn't quickly find one)

diff --git i/nbd/server.c w/nbd/server.c
index b60ebc3ab6ac..393cbd81c57a 100644
--- i/nbd/server.c
+++ w/nbd/server.c
@@ -973,7 +973,8 @@ static int nbd_negotiate_meta_queries(NBDClient *client,
 {
     int ret;
     g_autofree char *export_name = NULL;
-    g_autofree bool *bitmaps = NULL;
+    /* G_GNUC_UNUSED needed to work around a clang bug */
+    g_autofree G_GNUC_UNUSED bool *bitmaps = NULL;
     NBDExportMetaContexts local_meta = {0};
     uint32_t nb_queries;
     size_t i;


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