Factory and multiple messages in a proto file

[chapulina]

While debugging gazebosim/gz-gui#69, I noticed that the Factory::New function fails for ignition.msgs.SerializedStepMap. This test currently fails:

diff --git a/src/Factory_TEST.cc b/src/Factory_TEST.cc
index c5d6516..28d7a52 100644
--- a/src/Factory_TEST.cc
+++ b/src/Factory_TEST.cc
@@ -78,3 +78,12 @@ TEST(FactoryTest, NewDynamicFactory)
   msg = msgs::Factory::New("example.msgs.StringMsg");
   EXPECT_TRUE(msg.get() != nullptr);
 }
+
+/////////////////////////////////////////////////
+TEST(FactoryTest, NewSerializedStepMap)
+{
+  auto msg = msgs::Factory::New<msgs::SerializedStepMap>(
+      "ignition.msgs.SerializedStepMap");
+
+  ASSERT_NE(nullptr, msg.get());
+}

I noticed that the serialized_map.proto file doesn't have the option java_outer_classname line like other messages do. But when I added it, I noticed that that proto file has multiple messages defined, so we can add only one classname. I checked that adding

option java_outer_classname = "SerializedStepMapProtos";

fixes the test, but only for the 1st message defined in the proto file - whichever one it is.

If we ignore the factory issue, the message works otherwise. We've been publishing and using a few messages from serialized_map.proto on ign-gazebo without issues.

I'm wondering if the only solution to this is splitting the messages into multiple files, or if there's another more appropriate solution to this. I think moving the messages to other files wouldn't break API / ABI as long as the existing message includes all others.