Message data not inferred for subscribe operation

[qiuzhong]

According to rosbridge v2 protocol section 3.4.4 subscription

If left off, type will be inferred.

Let's take a look at the subscribe operation:

{ "op": "subscribe",
  (optional) "id": <string>,
  "topic": <string>,
  (optional) "type": <string>,
  (optional) "throttle_rate": <int>,
  (optional) "queue_length": <int>,
  (optional) "fragment_size": <int>,
  (optional) "compression": <string>
}

type field is optional, however, sending a subscribe operation without type field to the bridge server will get an error response.

Request: {op: 'subscribe', id: 'subscribe_id1', topic: 'example_topic'}
Response: {"op":"set_level","id":"subscribe_id1","level":"error"}

This error happened in _exractMessageType(type) before creating the rclnodejs subscription.

[minggangw]

I am afraid that we can not keep compatible with the rosbridge v2 protocol in ROS 2. The reason is below:

In ROS 1 era, we can get the message type by calling rostopic, see this example. But the ros2cli in ROS 2 doesn't have this interface, and we have to carry the exact message type in the request from browser. The result is what you have gotten if the request didn't have the type part.

[minggangw]

Close this issue as we can not keep compatible with the rosbridge protocol. And we have added the compatibility section into README to point out the differences (#60).