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

["Daniel P. Berrangé" <berrange@redhat.com>]

On Tue, May 08, 2018 at 05:17:40PM +0200, Paolo Bonzini wrote:
> 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.

Ok, that change looks fine with me assuming it passes your build tests.
In fact that is what I had done in an earlier version.

Regards,
Daniel
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