[rmw] rmw_fastrtps_dds does not work well for C++ <-> Python pub/sub testing?

[EricCousineau-TRI]

In #285, I had to workaround testing expectations based on the effective RMW implementation being used:

drake-ros/drake_ros/drake_ros/test/core_test.py

Lines 122 to 169 in 5e8b52e

	def assert_equal_for_cyclone_dds(actual, expected, name):
	rmw_implementation = rclpy.utilities.get_rmw_implementation_identifier()
	is_cyclone_dds = rmw_implementation == "rmw_cyclonedds_cpp"
	if is_cyclone_dds:
	assert actual == expected
	else:
	# TODO(eric.cousineau): At least for rmw_fastrtps_cpp, the behavior of
	# publishing and receiving a message seems much less reliable than
	# cyclone. Demote errors to warnings in this case.
	if actual != expected:
	warnings.warn(
	f"WARNING! For RMW_IMPLEMENTATION={rmw_implementation}, "
	f"incorrect value for {name}: {actual} != {expected}"
	)
	def test_cpp_node(drake_ros_fixture):
	# Create a Python node.
	node_py = rclpy.create_node("node_py")
	sub_value_py = -1
	def on_sub(message):
	nonlocal sub_value_py
	sub_value_py = message.data
	node_py.create_subscription(Int32, "/cpp_pub", on_sub, 1)
	pub_py = node_py.create_publisher(Int32, "/cpp_sub", 1)
	# Create a C++ node.
	cpp_node_options = CppNodeOptions(use_global_arguments=False)
	assert not cpp_node_options.use_global_arguments
	node_cpp = CppNode("direct_node_cpp", node_options=cpp_node_options)
	assert node_cpp.get_name() == "direct_node_cpp"
	# Create a "fixture" that will publish and subscribe to above topics.
	fixture = CppPubAndSub(node=node_cpp)
	timeout_sec = 0.0
	# Show that we can publish from C++ to Python.
	fixture.Publish(value=10)
	rclpy.spin_once(node_py, timeout_sec=timeout_sec)
	assert_equal_for_cyclone_dds(sub_value_py, 10, "sub_value_py")
	# Show that we can publish from Python to C++.
	message = Int32()
	message.data = 100
	pub_py.publish(message)
	sub_value_cpp = fixture.SpinAndReturnLatest(timeout_sec=timeout_sec)
	assert_equal_for_cyclone_dds(sub_value_cpp, 100, "sub_value_cpp")

This makes me concerned about using or even supporting rmw_fastrtps_cpp if something this simple is flaky.

Some options local to drake-ros:
(a) Keep this pattern, just warn when rmw_cyclonedds_cpp is not in use whenever testing diverges.
(b) Only ever allow rmw_cyclonedds_cpp, at least for our testing in drake-ros (and usage in Anzu).

\cc @adityapande-1995 @calderpg-tri @IanTheEngineer

[EricCousineau-TRI]

Originally found this as a difference between Bazel build and CMake build.

Repro steps based on above commit:

sudo apt install ros-humble-rmw-cyclonedds-cpp ros-humble-rmw-fastrtps-cpp

cd drake_ros_ws
colcon build --packages-up-to drake_ros --symlink-install
source ./install/setup.bash
cd build/drake_ros/

# Good
export RMW_IMPLEMENTATION=rmw_cyclonedds_cpp
ctest -V -R core_test_py

# Bad
export RMW_IMPLEMENTATION=rmw_fastrtps_cpp
ctest -V -R core_test_py