Fix ContainerBase::DeepCopy to not modify the source object.

In the worst case, the source object is a constant (eg a global default instance) and modifying them has undefined behavior.

PiperOrigin-RevId: 673402981

python/google/protobuf/internal/reflection_cpp_test.py

@@ -0,0 +1,35 @@
+# -*- coding: utf-8 -*-
+# Protocol Buffers - Google's data interchange format
+# Copyright 2008 Google Inc.  All rights reserved.
+#
+# Use of this source code is governed by a BSD-style
+# license that can be found in the LICENSE file or at
+# https://developers.google.com/open-source/licenses/bsd
+
+"""Unittest for reflection.py, with C++ protos linked in."""
+
+import copy
+import unittest
+
+from google.protobuf.internal import testing_refleaks
+from google.protobuf.internal import _parameterized
+from google.protobuf import unittest_pb2
+from google.protobuf import unittest_proto3_arena_pb2
+
+
+@_parameterized.named_parameters(
+    ('_proto2', unittest_pb2), ('_proto3', unittest_proto3_arena_pb2)
+)
+@testing_refleaks.TestCase
+class ReflectionTest(unittest.TestCase):
+
+  def testEmptyCompositeContainerDeepCopy(self, message_module):
+    proto1 = message_module.NestedTestAllTypes(
+        payload=message_module.TestAllTypes(optional_string='foo')
+    )
+    nested2 = copy.deepcopy(proto1.child.repeated_child)
+    self.assertEqual(0, len(nested2))
+
+
+if __name__ == '__main__':
+  unittest.main()

python/google/protobuf/pyext/message.cc

@@ -2651,11 +2651,17 @@ PyObject* ContainerBase::DeepCopy() {
       cmessage::NewEmptyMessage(this->parent->GetMessageClass());
   new_parent->message = this->parent->message->New(nullptr);
 
-  // Copy the map field into the new message.
-  this->parent->message->GetReflection()->SwapFields(
-      this->parent->message, new_parent->message,
-      {this->parent_field_descriptor});
-  this->parent->message->MergeFrom(*new_parent->message);
+  // There is no API to copy a single field. The closest operation we have is
+  // SwapFields.
+  // So, we copy the source into a disposable message and then swap the one
+  // field we care about from it.
+  // If the performance of this operation matters we can do a copy of the single
+  // field, but that would require huge switches for each type+cardinality to
+  // call the right read/write field functions.
+  std::unique_ptr<Message> tmp(this->parent->message->New(nullptr));
+  tmp->MergeFrom(*this->parent->message);
+  tmp->GetReflection()->SwapFields(tmp.get(), new_parent->message,
+                                   {this->parent_field_descriptor});
 
   PyObject* result =
       cmessage::GetFieldValue(new_parent, this->parent_field_descriptor);