Configure logging to not write to file system

[sloretz]

Currently the default logger rcl_logging_spdlog always tries to write to the file system, and errors if it can't. This prevents it from being used on systems with read-only file systems. It should be configurable to allow outputting to the console only.

Related discussion #86 (comment)
Summary: New environment variable RCL_LOGGING_OUTPUT_SINK that defaults to console,file if unset.

[ros-discourse]

This issue has been mentioned on ROS Discourse. There might be relevant details there:

https://discourse.ros.org/t/ros2-logging-to-docker-daemon/26563/2

[clalancette]

Currently the default logger rcl_logging_spdlog always tries to write to the file system, and errors if it can't. This prevents it from being used on systems with read-only file systems. It should be configurable to allow outputting to the console only.

So after reading through the logging code the last couple of weeks, I actually think we don't need a new environment variable at all.

The way that logging works is that as log messages come through the rcutils layer, a callback into rcl gets called. That callback formats the log message, and then figures out where to send it:

• To the console via rcutils
• To disk via rcl_logging_spdlog
• To the network via /rosout

In turn, it is possible to enable or disable each of those at node initialization time. For instance, to have messages only go out to the console (and not written to disk or to the network):

ros2 run demo_nodes_cpp talker --ros-args --disable-external-lib-logs --disable-rosout-logs

And this can even be set on a per-composable node basis. So I think we have this functionality already, we just don't have it well documented.

@sloretz What do you think?