[Qemu-devel] [PATCH v3 3/3] glib: enforce the minimum required version and warn about old APIs

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

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. This workaround is illustrated with the
implementation of the g_strv_contains method to satisfy the test suite.

Signed-off-by: Daniel P. Berrangé <berrange@redhat.com>
---
 include/glib-compat.h | 68 +++++++++++++++++++++++++++++++++++++++++++
 tests/test-qga.c      |  2 --
 2 files changed, 68 insertions(+), 2 deletions(-)

diff --git a/include/glib-compat.h b/include/glib-compat.h
index 3b340ab33c..fdf95a255d 100644
--- a/include/glib-compat.h
+++ b/include/glib-compat.h
@@ -16,8 +16,74 @@
 #ifndef QEMU_GLIB_COMPAT_H
 #define QEMU_GLIB_COMPAT_H
 
+/* 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_40
+
+/* 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_40
+
+#pragma GCC diagnostic push
+#pragma GCC diagnostic ignored "-Wdeprecated-declarations"
+
 #include <glib.h>
 
+/*
+ * Note that because of the GLIB_VERSION_MAX_ALLOWED constant above, allowing
+ * use of functions from newer GLib via this compat header 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 static inline function with the same signature that does
+ * what we need, but with a "_qemu" suffix e.g.
+ *
+ * static inline void g_foo_qemu(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 we can do
+ *
+ *   #define g_foo(a) g_foo_qemu(a)
+ *
+ * So now the code elsewhere in QEMU, which *does* have the
+ * -Wdeprecated-declarations warning active, can call g_foo(...) as normal,
+ * without generating warnings.
+ */
+
+static inline gboolean g_strv_contains_qemu(const gchar *const *strv,
+                                            const gchar *str)
+{
+#if GLIB_CHECK_VERSION(2, 44, 0)
+    return g_strv_contains(strv, str);
+#else
+    g_return_val_if_fail(strv != NULL, FALSE);
+    g_return_val_if_fail(str != NULL, FALSE);
+
+    for (; *strv != NULL; strv++) {
+        if (g_str_equal(str, *strv)) {
+            return TRUE;
+        }
+    }
+
+    return FALSE;
+#endif
+}
+#define g_strv_contains(a, b) g_strv_contains_qemu(a, b)
+
+
 #if defined(_WIN32) && !GLIB_CHECK_VERSION(2, 50, 0)
 /*
  * g_poll has a problem on Windows when using
@@ -45,4 +111,6 @@ gint g_poll_fixed(GPollFD *fds, guint nfds, gint timeout);
     } while (0)
 #endif
 
+#pragma GCC diagnostic pop
+
 #endif
diff --git a/tests/test-qga.c b/tests/test-qga.c
index 18e63cb533..30c9643257 100644
--- a/tests/test-qga.c
+++ b/tests/test-qga.c
@@ -744,12 +744,10 @@ static void test_qga_config(gconstpointer data)
 
     strv = g_key_file_get_string_list(kf, "general", "blacklist", &n, &error);
     g_assert_cmpint(n, ==, 2);
-#if GLIB_CHECK_VERSION(2, 44, 0)
     g_assert_true(g_strv_contains((const char * const *)strv,
                                   "guest-ping"));
     g_assert_true(g_strv_contains((const char * const *)strv,
                                   "guest-get-time"));
-#endif
     g_assert_no_error(error);
     g_strfreev(strv);
 
-- 
2.17.0