rclcpp_components_register_node macro: --no-undefined linker option

[ottojo]

Hi! When converting nodes to components i found myself running into linker issues of undefined symbols at runtime, which would otherwise occur at compile/link time.

I suggest adding

target_link_options(${target} PRIVATE "-Wl,--no-undefined")

to the rclcpp_components_register_node macro, to ensure there are no undefined symbols when the generated executable is run or the component is loaded into a container later.

I'm unsure of the consequences, or if there are valid use cases for having undefined references in the component library, which is why i want to open this for discussion here first before making a PR. Please let me know if you know of any.
I think users of rclcpp_components_register_node would expect to be able to use the generated executable, which would require this anyways.

[mjcarroll]

It does seem like this would help a class of runtime errors that would be compile/link time errors if you were, for instance, building an executable.

if there are valid use cases for having undefined references in the component library

I think there are cases, but they would be the rare exception and not the rule. You can use weak symbols (https://en.wikipedia.org/wiki/Weak_symbol), but this isn't a commonly-used thing that I'm aware of.

Since it could have potential impact, I think that we could tick-tock it:

• Add an-opt in mechanism to the rclcpp_components_register_node and leave it off by default.
• Turn the opt-in mechanism on in all of the core places (just to see what happens)
• If there aren't any crazy user-facing impacts, turn it on by default at a later date.