Fix PyUnknownFields memory leak

Properly release internal data structure on deallocation.
Fix #7301

python/google/protobuf/internal/unknown_fields_test.py

@@ -39,6 +39,7 @@
   import unittest2 as unittest  #PY26
 except ImportError:
   import unittest
+import tracemalloc
 from google.protobuf import map_unittest_pb2
 from google.protobuf import unittest_mset_pb2
 from google.protobuf import unittest_pb2
@@ -312,6 +313,22 @@ def testClear(self):
     self.assertIn('UnknownFields does not exist.',
                   str(context.exception))
 
+  def testUnknownFieldsNoMemoryLeak(self):
+    # Call to UnknownFields must not leak memory
+    nb_leaks = 1234
+    def leaking_function():
+      for _ in range(nb_leaks):
+        self.empty_message.UnknownFields()
+    tracemalloc.start()
+    snapshot1 = tracemalloc.take_snapshot()
+    leaking_function()
+    snapshot2 = tracemalloc.take_snapshot()
+    top_stats = snapshot2.compare_to(snapshot1, 'lineno')
+    tracemalloc.stop()
+    # There's no easy way to look for a precise leak source.
+    # Rely on a "marker" count value while checking allocated memory.
+    self.assertEqual([], [x for x in top_stats if x.count_diff == nb_leaks])
+
   def testSubUnknownFields(self):
     message = unittest_pb2.TestAllTypes()
     message.optionalgroup.a = 123

python/google/protobuf/pyext/unknown_fields.cc

@@ -142,6 +142,7 @@ static void Dealloc(PyObject* pself) {
   }
   Py_CLEAR(self->parent);
   self->~PyUnknownFields();
+  Py_TYPE(pself)->tp_free(pself);
 }
 
 static PySequenceMethods SqMethods = {