Clang has previously had trouble with G_DEFINE_AUTOPTR_CLEANUP_FUNC
generated code, thinking it was unused. We turn off -Wunused-function
to avoid tripping up on that with CLang.

New Clang has started having trouble with g_autoptr now too. In usage
scenarios where the variable is set, but never again read, it thinks
it is unused not realizing the destructor has useful side effects.
For this we have to skip -Wunused-but-set-variable on CLang.

Reviewed-by: Pavel Hrdina <phrdina@redhat.com>
Signed-off-by: Daniel P. Berrangé <berrange@redhat.com>
---
 build-aux/syntax-check.mk |  2 +-
 meson.build               | 21 +++++++++++++++++++++
 2 files changed, 22 insertions(+), 1 deletion(-)

diff --git a/build-aux/syntax-check.mk b/build-aux/syntax-check.mk
index 2058af0b77..5c5a2a8771 100644
--- a/build-aux/syntax-check.mk
+++ b/build-aux/syntax-check.mk
@@ -1589,7 +1589,7 @@ exclude_file_name_regexp--sc_prohibit_canonicalize_file_name = \
   ^(build-aux/syntax-check\.mk|tests/virfilemock\.c)$$
 
 exclude_file_name_regexp--sc_prohibit_raw_allocation = \
-  ^(docs/advanced-tests\.rst|src/util/viralloc\.[ch]|examples/.*|tests/(securityselinuxhelper|(vircgroup|nss)mock|commandhelper)\.c|tools/wireshark/src/packet-libvirt\.c|tools/nss/libvirt_nss(_leases|_macs)?\.c)$$
+  ^(meson\.build|docs/advanced-tests\.rst|src/util/viralloc\.[ch]|examples/.*|tests/(securityselinuxhelper|(vircgroup|nss)mock|commandhelper)\.c|tools/wireshark/src/packet-libvirt\.c|tools/nss/libvirt_nss(_leases|_macs)?\.c)$$
 
 exclude_file_name_regexp--sc_prohibit_readlink = \
   ^src/(util/virutil|lxc/lxc_container)\.c$$
diff --git a/meson.build b/meson.build
index 5af09d319a..ae2e672c1d 100644
--- a/meson.build
+++ b/meson.build
@@ -470,6 +470,27 @@ if get_option('warning_level') == '2'
     supported_cc_flags += [ '-Wno-unused-function' ]
   endif
 
+  if supported_cc_flags.contains('-Wunused-but-set-variable')
+    # Clang complains about unused variables in many scenarios related
+    # to attribute((cleanup)) aka g_auto*
+    w_unused_but_set_var_args = [ '-Wunused-but-set-variable', '-Werror' ]
+    w_unused_but_set_var_code = '''
+      static inline void free_pointer (void *p) {
+        void **pp = (void**)p;
+        free (*pp);
+      }
+
+      int main(void) {
+        __attribute__((cleanup(free_pointer))) char *buffer = 0x0;
+        buffer = 0x1;
+      }
+    '''
+    # We previously turned on unused-but-set-variable, so we must turn
+    # it off again explicitly now.
+    if not cc.compiles(w_unused_but_set_var_code, args: w_unused_but_set_var_args)
+      supported_cc_flags += [ '-Wno-unused-but-set-variable' ]
+    endif
+  endif
 endif
 add_project_arguments(supported_cc_flags, language: 'c')
 
-- 
2.31.1

On Wed, Sep 01, 2021 at 01:29:25PM +0100, Daniel P. Berrangé wrote:
> Clang has previously had trouble with G_DEFINE_AUTOPTR_CLEANUP_FUNC
> generated code, thinking it was unused. We turn off -Wunused-function
> to avoid tripping up on that with CLang.
> 
> New Clang has started having trouble with g_autoptr now too. In usage
> scenarios where the variable is set, but never again read, it thinks
> it is unused not realizing the destructor has useful side effects.
> For this we have to skip -Wunused-but-set-variable on CLang.
> 
> Reviewed-by: Pavel Hrdina <phrdina@redhat.com>
> Signed-off-by: Daniel P. Berrangé <berrange@redhat.com>
> ---
>  build-aux/syntax-check.mk |  2 +-
>  meson.build               | 21 +++++++++++++++++++++
>  2 files changed, 22 insertions(+), 1 deletion(-)

Reviewed-by: Pavel Hrdina <phrdina@redhat.com>