Fix -Wincompatible-function-pointer-types warning on Clang

See:
libvips/libvips#3901 (comment)

Author: kleisauke

pyvips/gvalue.py

@@ -220,36 +220,17 @@ def set(self, value):
             memory = glib_lib.g_malloc(len(value))
             ffi.memmove(memory, value, len(value))
 
-            # this is horrible!
-            #
-            # * in API mode, we must have 8.6+ and use set_blob_free to
-            #   attach the metadata to avoid leaks
-            # * pre-8.6, we just pass a NULL free pointer and live with the
-            #   leak
-            #
-            # this is because in API mode you can't pass a builtin (what
-            # vips_lib.g_free() becomes) as a parameter to ffi.callback(), and
-            # vips_value_set_blob() needs a callback for arg 2
-            #
-            # additionally, you can't make a py def which calls g_free() and
-            # then use the py def in the callback, since libvips will trigger
-            # these functions during cleanup, and py will have shut down by
-            # then and you'll get a segv
-
-            if at_least_libvips(8, 6):
+            # In API mode, we use set_blob_free in a backwards compatible way.
+            # For pre-8.6 libvipses, in ABI mode, we declare the type of the
+            # free func in set_blob incorrectly so that we can pass g_free
+            # at runtime without triggering an exception.
+            if pyvips.API_mode or at_least_libvips(8, 6):
                 vips_lib.vips_value_set_blob_free(self.gvalue,
                                                   memory, len(value))
             else:
-                # we declare the type of the free func in set_blob incorrectly
-                # so that we can pass g_free at runtime without triggering an
-                # exception
-                if pyvips.API_mode:
-                    vips_lib.vips_value_set_blob(self.gvalue,
-                                                 ffi.NULL, memory, len(value))
-                else:
-                    vips_lib.vips_value_set_blob(self.gvalue,
-                                                 glib_lib.g_free,
-                                                 memory, len(value))
+                vips_lib.vips_value_set_blob(self.gvalue,
+                                             glib_lib.g_free,
+                                             memory, len(value))
         else:
             raise Error('unsupported gtype for set {0}, fundamental {1}'.
                         format(type_name(gtype), type_name(fundamental)))

pyvips/pyvips_build.py

@@ -9,15 +9,35 @@
 if pkgconfig.installed('vips', '< 8.2'):
     raise Exception('pkg-config "vips" is too old -- need libvips 8.2 or later')
 
+major, minor, micro = [int(s) for s in pkgconfig.modversion('vips').split('.')]
+
 ffibuilder = FFI()
 
+# vips_value_set_blob_free and vips_area_free_cb compat for libvips < 8.6
+compat = '''
+int
+vips_area_free_cb(void *mem, VipsArea *area)
+{
+    g_free(mem);
+
+    return 0;
+}
+
+void
+vips_value_set_blob_free(GValue* value, void* data, size_t length)
+{
+    vips_value_set_blob(value, (VipsCallbackFn) vips_area_free_cb,
+        data, length);
+}
+''' if major == 8 and minor < 6 else ''
+
 ffibuilder.set_source("_libvips",
-    r""" 
+    f"""
         #include <vips/vips.h>
+        {compat}
     """,
     **pkgconfig.parse('vips'))
 
-major, minor, micro = [int(s) for s in pkgconfig.modversion('vips').split('.')]
 
 features = {
     'major': major,

pyvips/vdecls.py

@@ -110,9 +110,6 @@ def cdefs(features):
         void vips_value_set_array_int (GValue* value,
             const int* array, int n );
         void vips_value_set_array_image (GValue *value, int n);
-        typedef void (*FreeFn)(void* a);
-        void vips_value_set_blob (GValue* value,
-            FreeFn free_fn, void* data, size_t length);
 
         int g_value_get_boolean (const GValue* value);
         int g_value_get_int (GValue* value);
@@ -405,6 +402,15 @@ def cdefs(features):
         int vips_cache_get_max_files();
     '''
 
+    # we must only define this in ABI mode ... in API mode we use
+    # vips_value_set_blob_free in a backwards compatible way
+    if not features['api']:
+        code += '''
+            typedef void (*FreeFn)(void* a);
+            void vips_value_set_blob (GValue* value,
+                FreeFn free_fn, void* data, size_t length);
+        '''
+
     if _at_least(features, 8, 5):
         code += '''
             char** vips_image_get_fields (VipsImage* image);