roscpp service call doesn't return error message when service fails

[yli-cpr]

In rospy, service providers can raise rospy.ServiceException with error messages. However roscpp clients cannot get the error message. When you work with some roscpp nodes and rospy nodes, this can be a problem. If the error message is delivered through TCP (it should), so roscpp should be able to extract the message and return to caller.

[cwecht]

@yli-cpr could you elaborate on the concrete problem you are facing? Do you need an error message for debugging/logging purposes or do you want your nodes to be able to recover form different errors in different ways?

If you change the logger-level of ros.roscpp.roscpp_internal to Debug yo should get more information.

[yli-cpr]

@cwecht I just want to log the error message. There are ways to work around it, like including the error message in response. But I am just saying there is a gap between rospy and roscpp. Here is the code where rospy client reads the rospy.ServiceException error message: https://github.com/ros/ros_comm/blob/melodic-devel/clients/rospy/src/rospy/impl/tcpros_service.py#L365

And it is called when the OK byte is 0: https://github.com/ros/ros_comm/blob/melodic-devel/clients/rospy/src/rospy/impl/tcpros_service.py#L332

So it is supported by the protocol, but roscpp doesn't provide the service error.

[cwecht]

Well, it kind of does, the message is read here: https://github.com/ros/ros_comm/blob/29053c4832229efa7160fb944c05e3bc82e11540/clients/roscpp/src/libros/service_server_link.cpp#L246 and printed here: https://github.com/ros/ros_comm/blob/29053c4832229efa7160fb944c05e3bc82e11540/clients/roscpp/src/libros/service_server_link.cpp#L378

[yli-cpr]

Good to know. But in some cases, client wants to handle the error string by itself, for example: mapping to error messages in another language for globalization, or forward the error message to web client, so on. So it would be nice the call() method returns the string to caller.

[mkatliar]

So it would be nice the call() method returns the string to caller.

Or throw an exception to the caller, if the service also threw.