ParseFromString returns bytes parsed (#5166)

* ParseFromString returns bytes parsed

ParseFromString is documented as returning the number of bytes parsed,
and the C++ implementation does this, so the Python implementation
should too.

See #5165.

* Fix docstring

* Add a test for ParseFromString length behavior

python/google/protobuf/internal/message_test.py

@@ -139,9 +139,10 @@ class MessageTest(BaseTestCase):
   def testGoldenPackedMessage(self, message_module):
     golden_data = test_util.GoldenFileData('golden_packed_fields_message')
     golden_message = message_module.TestPackedTypes()
-    golden_message.ParseFromString(golden_data)
+    parsed_bytes = golden_message.ParseFromString(golden_data)
     all_set = message_module.TestPackedTypes()
     test_util.SetAllPackedFields(all_set)
+    self.assertEqual(parsed_bytes, len(golden_data))
     self.assertEqual(all_set, golden_message)
     self.assertEqual(golden_data, all_set.SerializeToString())
     golden_copy = copy.deepcopy(golden_message)

python/google/protobuf/message.py

@@ -178,11 +178,10 @@ class Message(object):
   def ParseFromString(self, serialized):
     """Parse serialized protocol buffer data into this message.
 
-    Like MergeFromString(), except we clear the object first and
+    Like MergeFromString(), except we clear the object first.
-    do not return the value that MergeFromString returns.
     """
     self.Clear()
-    self.MergeFromString(serialized)
+    return self.MergeFromString(serialized)
 
   def SerializeToString(self, **kwargs):
     """Serializes the protocol message to a binary string.