util: introduce compile time API version checking

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.

Reviewed-by: Pavel Hrdina <phrdina@redhat.com>
Reviewed-by: Daniel Henrique Barboza <danielhb413@gmail.com>
Signed-off-by: Daniel P. Berrangé <berrange@redhat.com>

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

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.h we can
+ * 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