[libvirt] [PATCH 6/7] util: introduce compile time API version checking

[Daniel Henrique Barboza]

Just a small nit:

On 1/6/20 2:26 PM, Daniel P. Berrangé wrote:
> GLib header files annotate every API with a version number.
> 
> It is possible to define some constants before including
> glib.h which will result in useful compile time warnings.
> 
> Setting GLIB_VERSION_MIN_REQUIRED will result in a warning
> if libvirt uses an API that was deprecated in the declared
> version, or before. Such API usage should be rewritten to
> use the documented new replacement API.
> 
> Setting GLIB_VERSION_MAX_ALLOWED will result in a warning
> if libvirt uses an API that was not introduced until a
> version of GLib that's newer than our minimum declared
> version. This avoids accidentally using functionality
> that is not available on some supported platforms.
> 
> Signed-off-by: Daniel P. Berrangé <berrange at redhat.com>
> ---
>   config-post.h         | 10 ++++++++++
>   src/util/glibcompat.c | 39 +++++++++++++++++++++++++++++++++++++++
>   2 files changed, 49 insertions(+)
> 
> diff --git a/config-post.h b/config-post.h
> index 415cc8cc72..de007393da 100644
> --- a/config-post.h
> +++ b/config-post.h
> @@ -49,3 +49,13 @@
>   #else
>   # error You either need at least GCC 4.8 or Clang 3.4 or XCode Clang 5.1 to compile libvirt
>   #endif
> +
> +/* Ask for warnings for anything that was marked deprecated in
> + * the defined version, or before. It is a candidate for rewrite.
> + */
> +#define GLIB_VERSION_MIN_REQUIRED GLIB_VERSION_2_48
> +
> +/* Ask for warnings if code tries to use function that did not
> + * exist in the defined version. These risk breaking builds
> + */
> +#define GLIB_VERSION_MAX_ALLOWED GLIB_VERSION_2_48
> diff --git a/src/util/glibcompat.c b/src/util/glibcompat.c
> index c6390c5c2e..06fe5b3d33 100644
> --- a/src/util/glibcompat.c
> +++ b/src/util/glibcompat.c
> @@ -24,6 +24,45 @@
>   
>   #include "glibcompat.h"
>   
> +/*
> + * Note that because of the GLIB_VERSION_MAX_ALLOWED constant in
> + * config-post.h, allowing use of functions from newer GLib via
> + * this compat impl needs a little trickery to prevent warnings
> + * being emitted.
> + *
> + * Consider a function from newer glib-X.Y that we want to use
> + *
> + *    int g_foo(const char *wibble)
> + *
> + * We must define a function with the same signature that does
> + * what we need, but with a "vir_" prefix e.g.
> + *
> + * void vir_g_foo(const char *wibble)
> + * {
> + *     #if GLIB_CHECK_VERSION(X, Y, 0)
> + *        g_foo(wibble)
> + *     #else
> + *        g_something_equivalent_in_older_glib(wibble);
> + *     #endif
> + * }
> + *
> + * 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 in glibcompat.hu we can


s/glibcompat.hu/glibcompat.h



> + * add
> + *
> + *   #define g_foo(a) vir_g_foo(a)
> + *
> + * Thus all the code elsewhere in libvirt, which *does* have the
> + * -Wdeprecated-declarations warning active, can call g_foo(...) as
> + * normal, without generating warnings. The cost is an extra function
> + * call when using new glib, but this compat code will go away over
> + * time as we update the supported platforms target.
> + */
> +
> +#pragma GCC diagnostic push
> +#pragma GCC diagnostic ignored "-Wdeprecated-declarations"
> +
>   #undef g_canonicalize_filename
>   #undef g_fsync
>   #undef g_strdup_printf
>