Plumb rclpy.init context to get_default_launch_description

[pbaughman]

From #189

This lets you pass a launch context to get_default_launch_description into get_default_launch_description and plumbs it all the way down to the ROSSpecificLaunchStartup launch action.

I couldn't find where rclpy.shutdown is called, so I didn't hook that up anywhere.

I also had to change rclpy.get_global_executor to constructing my own SingleThreadedExecutor because the executor needs to be built with the same context as the node.

This Incomplete!

This is about 50% of what's needed for issue 189. The other half is to make ros2 launch actually use this plumbing. A question for @wjwwood - is it necessary to shut-down the context when we're done, or is it sufficient to just pass a context here?

[wjwwood]

A question for @wjwwood - is it necessary to shut-down the context when we're done, or is it sufficient to just pass a context here?

No need to call shutdown explicitly (it is called automatically by the pycapsule destructor), but if you create a context, it's a nice thing to do when you know it no longer should be used. I'd change this:

launch/ros2launch/ros2launch/api/api.py

Lines 140 to 141 in 632ef1f

	launch_service.include_launch_description(
	launch_ros.get_default_launch_description(prefix_output_with_name=False))

To be something like this:

context = rclpy.context.Context()
rclpy.init(argv=..., context=context)
launch_service.include_launch_description( 
  launch_ros.get_default_launch_description(prefix_output_with_name=False, context=context)) 
# ...
ret = launch_service.run()
rclpy.shutdown(context=context)
return ret

[wjwwood]

lgtm

[wjwwood]

CI:

• Linux
• Linux-aarch64
• macOS
• Windows

[wjwwood]

The test failures look unrelated/flaky:

• the test_cli one that is present on Linux and Windows: test_cli.test_params_yaml flaky build_farmer#166
• the gtest_waitable one that is present on macOS is listed here: Build farmer handoff 2019-03-01 build_farmer#168