failed to deserialize ROS message: Fast CDR exception deserializing message of type std_srvs::srv::dds_::Empty_Response_.

[theksg]

Hi,

I am creating a ROS Service in Unity Simulator of type std_srvs/Empty.

I am able to call the service, service is doing the intended work.

But when I return the empty response, I get the following error:

The following exception was never retrieved: failed to deserialize ROS message: Fast CDR exception deserializing message of type stdsrvs::srv::dds::EmptyResponse., at /home/user/ros2_humble/src/ros2/rmw_fastrtps/rmw_fastrtps_cpp/src/type_support_common.cpp:118

can anyone help me with what could be going wrong?

[clalancette]

Usually this happens when you have two nodes on the same service and with the same type, but the actual "type" has been changed. Then the deserialization will attempt to deserialize the wrong thing. Ensure that there are no other nodes in your network that are on the same service name and try again.

[sloretz]

@theksg would you be willing to provide a minimal reproducible example?

[JingyuQian]

I met the same problem when trying this demo at https://github.com/foxglove/mcap/blob/main/python/examples/ros2/py_mcap_demo/py_mcap_demo/reader.py. I'm reading a recorded .mcap file. Not sure what this error means since I'm new in ros2. @clalancette would you be able to clarify a bit more? thank you.

To be specific, this demo code runs well on string messages but crashed on bytes messages.

[fujitatomoya]

@JingyuQian

https://github.com/foxglove/mcap/blob/main/python/examples/ros2/py_mcap_demo/py_mcap_demo/reader.py.

this is maintained by https://github.com/foxglove but ros2 organization. i suggest you to create the issue to https://github.com/foxglove/mcap instead.

[clalancette]

Given that there was no response to the original, I'm going to close this one out. As @fujitatomoya says, if you are having trouble with mcap please open an issue on that repository.

[mjforan]

I have this problem with one custom message type, while other similar custom messages are fine. Creating a subscription causes CPU to hit 100%. Occasionally a message will make it through successfully, but mostly I get "A message was lost!!!" warnings. This happens with both ros2 topic echo and C++ subscribers. I am playing data from an MCAP bag, but Cyclone DDS does not exhibit this issue.

[MiguelCompany]

@mjforan If the bag was captured with service data coming from Cyclone DDS, you should be aware that the RMWs for Cyclone DDS and Fast DDS do not interoperate for services

[mjforan]

@MiguelCompany My bag is in the FastRTPS format- it is actually a ROS1 bag which has been converted. This error comes from a regular topic (not service).

[mjforan]

Apparently my team's custom message type changed between ROS 1 and 2. That likely caused the deserialization problem. It would be nice to get an exception instead of locking up the CPU.

[JingyuQian]

@mjforan Yes. After extensive debugging, I can confirm it is the mismatch between recorded mcap and messages.

For my case, I can use https://mcap.dev/docs/python/mcap-ros2-apidoc/mcap_ros2.reader#:~:text=mcap_ros2.reader.read_ros2_messages to read each mcap message. I found that each message contains the schema being used and can be compared with schemas I'm deserializing with.

[Pattybhjd]

change the DDS to Cyclone for humble