remove the padding member from structs which should be empty

[dirk-thomas]

IDL and some programming languages like C require a struct to have at least one member. Python though doesn't have that restriction and can define a class without any slots.

Therefore this patch removes the member structure_needs_at_least_one_member only used to "pad" empty structures from the Python data type. As a consequence printing the message won't show the member as well as the user won't see a getter and setter for the member anymore.

In the conversion from Python to C the member variable present in C is just set to 0 in order to not be uninitialized. In the other direction the value in the C struct is just ignored and not transferred to the Python object.

Fixes ros2/ros2cli#297. Requires ros2/rosidl#389.

[sloretz]

I think this spot at line 379 in __eq__ might need to be skipped

rosidl_python/rosidl_generator_py/resource/_msg.py.em

Lines 372 to 382 in 000cf6b

	def __eq__(self, other):
	if not isinstance(other, self.__class__):
	return False
	@[for member in message.structure.members]@
	@[ if isinstance(member.type, Array) and isinstance(member.type.value_type, BasicType) and member.type.value_type.typename in SPECIAL_NESTED_BASIC_TYPES]@
	if all(self.@(member.name) != other.@(member.name)):
	@[ else]@
	if self.@(member.name) != other.@(member.name):
	@[ end if]@
	return False
	@[end for]@

[dirk-thomas]

I think this spot at line 379 in __eq__ might need to be skipped

Very good point. Fixed in d97cc46. I am curious if the running CI builds will complain about it 🤔

Update: and the tests did catch this 😥 http://build.ros2.org/view/Eci/job/Eci__overlay_ubuntu_bionic_amd64/23/testReport/junit/rosidl_runtime_py.test.rosidl_runtime_py/test_set_message/test_set_message_fields_none/

[dirk-thomas]

CI build including the second commit: (same test failures as with master)