overrides: fix callback setter overrides (bis)

The previous commit broke those by trying to pass weak refs
through pygobject, but we should probably have tested the elements
beyond instantiation: weakref.WeakMethod returns a callable, but
that callable when called only returns the ephemeral bound method,
which is the object we want to call, but pygobject has no support
for that.

Instead, fix the memory leaks we were going after by decoupling the
lifecycle of the callback and that of the pad, by passing functors
to pygobject.

gi/overrides/Gst.py

@@ -140,42 +140,39 @@ class Caps(Gst.Caps):
 Caps = override(Caps)
 __all__.append('Caps')
 
-class Pad(Gst.Pad):
-    def __init__(self, *args, **kwargs):
-        self._real_chain_func = None
-        self._real_event_func = None
-        self._real_query_func = None
-        super(Gst.Pad, self).__init__(*args, **kwargs)
+class PadFunc:
+    def __init__(self, func):
+        self.func = func
 
-    def _chain_override(self, pad, parent, buf):
-        return self._real_chain_func(pad, buf)
+    def __call__(self, pad, parent, obj):
+        if isinstance(self.func, weakref.WeakMethod):
+            func = self.func()
+        else:
+            func = self.func
 
-    def _event_override(self, pad, parent, event):
-        return self._real_event_func(pad, event)
-
-    def _query_override(self, pad, parent, query):
         try:
-            res = self._real_query_func(pad, query)
+            res = func(pad, obj)
         except TypeError:
             try:
-                res = self._real_query_func(pad, parent, query)
+                res = func(pad, parent, obj)
             except TypeError:
-                raise TypeError("Invalid query method %s, 2 or 3 arguments required"
-                                % self._real_query_func)
+                raise TypeError("Invalid method %s, 2 or 3 arguments required"
+                                % func)
 
         return res
 
+class Pad(Gst.Pad):
+    def __init__(self, *args, **kwargs):
+        super(Gst.Pad, self).__init__(*args, **kwargs)
+
     def set_chain_function(self, func):
-        self._real_chain_func = func
-        self.set_chain_function_full(weakref.WeakMethod(self._chain_override), None)
+        self.set_chain_function_full(PadFunc(func), None)
 
     def set_event_function(self, func):
-        self._real_event_func = func
-        self.set_event_function_full(weakref.WeakMethod(func), None)
+        self.set_event_function_full(PadFunc(func), None)
 
     def set_query_function(self, func):
-        self._real_query_func = func
-        self.set_query_function_full(weakref.WeakMethod(self._query_override), None)
+        self.set_query_function_full(PadFunc(func), None)
 
     def query_caps(self, filter=None):
         return Gst.Pad.query_caps(self, filter)