Mcap Decoder is not Capable of Decoding "std_msgs/msg/Empty"

[shouhengyi]

Description
mcap decoder can't handle "std_msgs/msg/Empty" type and will throw exception.

• Version: mcap==1.1.0
• Platform: This was on ros-iron image with the mcap==1.1.0 and mcap-ros2-support==0.5.0 installed (both the latest releases)

Steps To Reproduce

1. Record a bag containing a "std_msgs/msg/Empty" message in MCAP format; either by running ros2 bag record -a -s mcap -o empty_message_bag and ros2 topic pub empty_msg std_msgs/msg/Empty, or by using Write Messages example in https://mcap.dev/docs/python/ros2_example#writing-messages.
2. Attempt to read all messages using the official Reading Messages example in https://mcap.dev/docs/python/ros2_noenv_example#reading-messages

Expected Behavior
mcap decoder can decode "std_msgs/msg/Empty" in the same way that the official rosbag2_py SequentialReader can do.

Actual Behavior
mcap decoder raises an exception when reading from a bag that contains a "std_msgs/msg/Empty" message.

Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "/root/ubuntu/robolog/ros2/ros2_bag_reader.py", line 274, in messages
    message = next(self._messages)
  File "/root/ubuntu/.venv/lib/python3.10/site-packages/mcap/reader.py", line 194, in iter_decoded_messages
    schema, channel, message, decoded_message(schema, channel, message)
  File "/root/ubuntu/.venv/lib/python3.10/site-packages/mcap/reader.py", line 182, in decoded_message
    decoder = factory.decoder_for(channel.message_encoding, schema)
  File "/root/ubuntu/.venv/lib/python3.10/site-packages/mcap_ros2/decoder.py", line 45, in decoder_for
    raise McapROS2DecodeError(f'schema parsing failed for "{schema.name}"')
mcap_ros2.decoder.McapROS2DecodeError: schema parsing failed for "std_msgs/msg/Empty"

[foxhubber]

Internal tracking ticket: FG-4583

[shouhengyi]

Ping on this. Thanks.

[shouhengyi]

Ping on this again. Thanks.

[achim-k]

This is fixed as of version 0.5.1