python: Fix API break by not forcing usage of context manager

caps.get_structure(0).get_name() used to work, but the usage of
StructureWrapper broke it but forcing to wrap it in a "with" statement.

We already have a Structure subclass, simply make it reference its
parent Gst.Caps object.

Part-of: <https://gitlab.freedesktop.org/gstreamer/gstreamer/-/merge_requests/9148>

subprojects/gst-python/gi/overrides/Gst.py

@@ -142,11 +142,11 @@ __all__.append('NotWritableQuery')
 
 class Query(Gst.Query, MiniObject):
     def get_structure(self):
-        return StructureWrapper(_gi_gst.query_get_structure(self), self,
-                                False)
+        s = _gi_gst.query_get_structure(self)
+        return s._set_parent(self) if s is not None else None
 
     def writable_structure(self):
-        return StructureWrapper(_gi_gst.query_writable_structure(self), self, True)
+        return StructureWrapper(_gi_gst.query_writable_structure(self)._set_parent(self))
 
 
 Query = override(Query)
@@ -162,11 +162,11 @@ __all__.append('NotWritableEvent')
 
 class Event(Gst.Event, MiniObject):
     def get_structure(self):
-        return StructureWrapper(_gi_gst.event_get_structure(self), self,
-                                False)
+        s = _gi_gst.event_get_structure(self)
+        return s._set_parent(self) if s is not None else None
 
     def writable_structure(self):
-        return StructureWrapper(_gi_gst.event_writable_structure(self), self, True)
+        return StructureWrapper(_gi_gst.event_writable_structure(self)._set_parent(self))
 
 
 Event = override(Event)
@@ -182,11 +182,11 @@ __all__.append('NotWritableContext')
 
 class Context(Gst.Context, MiniObject):
     def get_structure(self):
-        return StructureWrapper(_gi_gst.context_get_structure(self), self,
-                                False)
+        s = _gi_gst.context_get_structure(self)
+        return s._set_parent(self) if s is not None else None
 
     def writable_structure(self):
-        return StructureWrapper(_gi_gst.context_writable_structure(self), self, True)
+        return StructureWrapper(_gi_gst.context_writable_structure(self)._set_parent(self))
 
 
 Context = override(Context)
@@ -251,11 +251,11 @@ class Caps(Gst.Caps, MiniObject):
         return self.get_size()
 
     def get_structure(self, index):
-        return StructureWrapper(_gi_gst.caps_get_structure(self, index), self,
-                                False)
+        s = _gi_gst.caps_get_structure(self, index)
+        return s._set_parent(self) if s is not None else None
 
     def writable_structure(self, index):
-        return StructureWrapper(_gi_gst.caps_writable_structure(self, index), self, True)
+        return StructureWrapper(_gi_gst.caps_writable_structure(self, index)._set_parent(self))
 
 
 Caps = override(Caps)
@@ -477,6 +477,16 @@ class Structure(Gst.Structure):
     def __str__(self):
         return self.to_string()
 
+    def _set_parent(self, parent):
+        self.__parent__ = parent
+        return self
+
+    def __enter__(self):
+        return self
+
+    def __exit__(self, _type, _value, _tb):
+        self._set_parent(None)
+
 
 Structure = override(Structure)
 __all__.append('Structure')
@@ -851,16 +861,16 @@ def pairwise(iterable):
 
 
 class StructureWrapper:
-    def __init__(self, structure, parent, writable):
+    """A Gst.Structure wrapper to force usage of a context manager.
+    """
+    def __init__(self, structure):
         self.__structure = structure
-        self.__parent__ = parent
 
     def __enter__(self):
         return self.__structure
 
     def __exit__(self, _type, _value, _tb):
-        self.__parent__ = False
-        return
+        self.__structure._set_parent(None)
 
 
 class MapInfo:

subprojects/gst-python/testsuite/test_gst.py

@@ -182,6 +182,25 @@ class TestCaps(TestCase):
         with caps.get_structure(0) as s:
             self.assertNotEqual(ptr, s.__ptr__())
 
+    def test_read_no_copy_no_cm(self):
+        Gst.init(None)
+        caps = Gst.Caps("audio/x-raw")
+
+        s = caps.get_structure(0)
+        ptr = s.__ptr__()
+
+        s2 = caps.get_structure(0)
+        self.assertEqual(ptr, s2.__ptr__())
+
+        c2 = caps.mini_object
+        self.assertEqual(c2.refcount, 2)
+        with self.assertRaises(Gst.NotWritableStructure):
+            caps.get_structure(0).set_value("rate", 44100)
+
+        caps.make_writable()
+        s = caps.get_structure(0)
+        self.assertNotEqual(ptr, s.__ptr__())
+
 
 class TestStructure(TestCase):
 
@@ -207,6 +226,12 @@ class TestEvent(TestCase):
             self.assertEqual(s["rate"], 44100)
             self.assertEqual(s["channels"], 2)
 
+    def test_no_structure(self):
+        Gst.init(None)
+        event = Gst.Event.new_flush_start()
+        s = event.get_structure()
+        self.assertIsNone(s)
+
 
 class TestQuery(TestCase):
     def test_writable(self):