From clang-13:
nbd/server.c:976:22: error: variable 'bitmaps' set but not used \
    [-Werror,-Wunused-but-set-variable]

Cc: qemu-block@nongnu.org
Cc: Eric Blake <eblake@redhat.com>
Cc: Vladimir Sementsov-Ogievskiy <vsementsov@virtuozzo.com>
Signed-off-by: Richard Henderson <richard.henderson@linaro.org>
---
 nbd/server.c | 4 ----
 1 file changed, 4 deletions(-)

diff --git a/nbd/server.c b/nbd/server.c
index b60ebc3ab6..721349ec00 100644
--- a/nbd/server.c
+++ 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) {
-- 
2.25.1

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]
> 
> Cc: qemu-block@nongnu.org
> Cc: Eric Blake <eblake@redhat.com>
> Cc: Vladimir Sementsov-Ogievskiy <vsementsov@virtuozzo.com>
> Signed-off-by: Richard Henderson <richard.henderson@linaro.org>
> ---
>   nbd/server.c | 4 ----
>   1 file changed, 4 deletions(-)
> 
> diff --git a/nbd/server.c b/nbd/server.c
> index b60ebc3ab6..721349ec00 100644
> --- a/nbd/server.c
> +++ 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.


-- 
Best regards,
Vladimir

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
Virtualization:  qemu.org | libvirt.org

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
Virtualization:  qemu.org | libvirt.org

On 7/13/21 6:14 AM, Eric Blake wrote:
> 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;

That works.  I found

   https://bugs.llvm.org/show_bug.cgi?id=3888

and gave it a nudge.


r~