Re: [Qemu-devel] [PATCH 3/3] glib: enforce the minimum required version and warn about old APIs

[Paolo Bonzini]

On 04/05/2018 18:00, Daniel P. Berrangé wrote:
> There are two useful macros that can be defined before including
> glib.h that are related to the min required glib version
> 
>  - GLIB_VERSION_MIN_REQUIRED
> 
>    When this is defined, if code uses an API that was deprecated
>    in this version, or older, a compiler warning will be emitted.
>    This alerts maintainers to update their code to whatever new
>    replacement API is now recommended best practice.
> 
>  - GLIB_VERSION_MAX_ALLOWED
> 
>    When this is defined, if code uses an API that was introduced
>    in a version that is newer than the declared version, a compiler
>    warning will be emitted. This alerts maintainers if new code
>    accidentally uses functionality that won't be available on some
>    supported platforms.
> 
> The GLIB_VERSION_MAX_ALLOWED constant makes it a bit harder to opt
> in to using specific new APIs with a GLIB_CHECK_VERSION conditional.
> To workaround this Pragmas can be used to temporarily turn off the
> -Wdeprecated-declarations compiler warning, while a static inline
> compat function is implemented. Nothing needs that at this time,
> so just illustrate it in a comment for benefit of future changes.

Unfortunately there is one need, and it's a bug in glib itself:

In file included from /usr/lib64/glib-2.0/include/glibconfig.h:9:0,
                 from /usr/include/glib-2.0/glib/gtypes.h:32,
                 from /usr/include/glib-2.0/glib/galloca.h:32,
                 from /usr/include/glib-2.0/glib.h:30,
                 from 
/home/pbonzini/work/upstream/qemu/include/glib-compat.h:29,
                 from 
/home/pbonzini/work/upstream/qemu/include/qemu/osdep.h:107,
                 from /home/pbonzini/work/upstream/qemu/disas/arm-a64.cc:21:
/usr/include/glib-2.0/glib/glib-autocleanups.h: In function ‘void 
glib_autoptr_cleanup_GOptionGroup(GOptionGroup**)’:
/usr/include/glib-2.0/glib/glib-autocleanups.h:63:45: error: ‘void 
g_option_group_unref(GOptionGroup*)’ is deprecated (declared at 
/usr/include/glib-2.0/glib/goption.h:372): Not available before 2.44 
[-Werror=deprecated-declarations]
 G_DEFINE_AUTOPTR_CLEANUP_FUNC(GOptionGroup, g_option_group_unref)
                                             ^
/usr/include/glib-2.0/glib/gmacros.h:444:88: note: in definition of macro 
‘G_DEFINE_AUTOPTR_CLEANUP_FUNC’
   static inline void _GLIB_AUTOPTR_FUNC_NAME(TypeName) (TypeName **_ptr) { if 
(*_ptr) (func) (*_ptr); }         \
                                                                                
        ^
/usr/include/glib-2.0/glib/gmacros.h:444:100: error: ‘void 
g_option_group_unref(GOptionGroup*)’ is deprecated (declared at 
/usr/include/glib-2.0/glib/goption.h:372): Not available before 2.44 
[-Werror=deprecated-declarations]
   static inline void _GLIB_AUTOPTR_FUNC_NAME(TypeName) (TypeName **_ptr) { if 
(*_ptr) (func) (*_ptr); }         \
                                                                                
                    ^
/usr/include/glib-2.0/glib/glib-autocleanups.h:63:1: note: in expansion of 
macro ‘G_DEFINE_AUTOPTR_CLEANUP_FUNC’
 G_DEFINE_AUTOPTR_CLEANUP_FUNC(GOptionGroup, g_option_group_unref)

(This is RHEL7 glib2-2.50.3-3.el7.x86_64).

Luckily you suggested the solution yourself---this seems to work:

diff --git a/include/glib-compat.h b/include/glib-compat.h
index d066f6d..ef07489 100644
--- a/include/glib-compat.h
+++ b/include/glib-compat.h
@@ -26,6 +26,8 @@
  */
 #define GLIB_VERSION_MAX_ALLOWED GLIB_VERSION_2_42
 
+_Pragma("GCC diagnostic push")
+_Pragma("GCC diagnostic ignored \"-Wdeprecated-declarations\"")
 #include <glib.h>
 
 /*
@@ -42,8 +44,6 @@
  * signature that does what we need, but with a "_qemu"
  * suffix eg
  *
- * _Pragma("GCC diagnostic push")
- * _Pragma("GCC diagnostic ignored \"-Wdeprecated-declarations\"")
  * static inline void g_foo_qemu(const char *wibble)
  * {
  *     #if GLIB_CHECK_VERSION(X, Y, 0)
@@ -52,12 +52,10 @@
  *        g_something_equivalent_in_older_glib(wibble);
  *     #endif
  * }
- * _Pragma("GCC diagnostic pop")
  *
- * The Pragma calls turn off -Wdeprecated-declarations,
- * ensuring this wrapper function impl doesn't trigger the
- * compiler warning about using too new glib APIs. Finally
- * we can do
+ * The _Pragma at the top of this file turns off -Wdeprecated-declarations,
+ * ensuring this wrapper function impl doesn't trigger the compiler warning
+ * about using too new glib APIs. Finally we can do
  *
  *   #define g_foo(a) g_foo_qemu(a)
  *
@@ -94,4 +92,6 @@ gint g_poll_fixed(GPollFD *fds, guint nfds, gint timeout);
     } while (0)
 #endif
 
+_Pragma("GCC diagnostic pop")
+
 #endif

I'll give it a shot on clang too.

Paolo