remove rmw_localhost_only_t.

[CihatAltiparmak]

Hello, maybe it's also in your plan, but i wanted to open PR about deprecated usage of rmw_localhost_only_t just in case you have overlooked this PR ( ros2/rcl#1169 ) in rcl. I have discovered this one while running ros-industrial ci in my repo and some errors happen like below in the ROS_REPO=testing case.

       Finished release [optimized] target(s) in 7m 07s
  ---
  Finished <<< zenoh_c_vendor [7min 14s]
  Starting >>> rmw_zenoh_cpp
  --- stderr: rmw_zenoh_cpp
  /root/upstream_ws/src/rmw_zenoh/rmw_zenoh_cpp/src/rmw_init_options.cpp: In function ‘rmw_ret_t rmw_init_options_init(rmw_init_options_t*, rcutils_allocator_t)’:
  /root/upstream_ws/src/rmw_zenoh/rmw_zenoh_cpp/src/rmw_init_options.cpp:50:17: error: ‘rmw_init_options_t’ {aka ‘struct rmw_init_options_s’} has no member named ‘localhost_only’
     50 |   init_options->localhost_only = RMW_LOCALHOST_ONLY_DEFAULT;
        |                 ^~~~~~~~~~~~~~
  /root/upstream_ws/src/rmw_zenoh/rmw_zenoh_cpp/src/rmw_init_options.cpp:50:34: error: ‘RMW_LOCALHOST_ONLY_DEFAULT’ was not declared in this scope
     50 |   init_options->localhost_only = RMW_LOCALHOST_ONLY_DEFAULT;
        |                                  ^~~~~~~~~~~~~~~~~~~~~~~~~~
  /root/upstream_ws/src/rmw_zenoh/rmw_zenoh_cpp/src/rmw_init_options.cpp: In function ‘rmw_ret_t rmw_init_options_copy(const rmw_init_options_t*, rmw_init_options_t*)’:
  /root/upstream_ws/src/rmw_zenoh/rmw_zenoh_cpp/src/rmw_init_options.cpp:92:7: error: ‘rmw_init_options_t’ {aka ‘struct rmw_init_options_s’} has no member named ‘localhost_only’
     92 |   tmp.localhost_only = src->localhost_only;
        |       ^~~~~~~~~~~~~~
  /root/upstream_ws/src/rmw_zenoh/rmw_zenoh_cpp/src/rmw_init_options.cpp:92:29: error: ‘const rmw_init_options_t’ {aka ‘const struct rmw_init_options_s’} has no member named ‘localhost_only’
     92 |   tmp.localhost_only = src->localhost_only;
        |                             ^~~~~~~~~~~~~~
  gmake[2]: *** [CMakeFiles/rmw_zenoh_cpp.dir/build.make:384: CMakeFiles/rmw_zenoh_cpp.dir/src/rmw_init_options.cpp.o] Error 1
  gmake[2]: *** Waiting for unfinished jobs....
  gmake[1]: *** [CMakeFiles/Makefile2:139: CMakeFiles/rmw_zenoh_cpp.dir/all] Error 2
  gmake: *** [Makefile:146: all] Error 2
  ---
  Failed   <<< rmw_zenoh_cpp [14.0s, exited with code 2]
  
  Summary: 14 packages finished [7min 29s]
    1 package failed: rmw_zenoh_cpp
    2 packages had stderr output: rmw_zenoh_cpp zenoh_c_vendor
'build_upstream_workspace' returned with code '2' after 7 min 29 sec

[Yadunund]

Thanks for the PR. One question I have is whether we retain support for Iron + Jazzy users who may continue to specify ROS_LOCALHOST_ONLY. If so, we would need to create iron and jazzy branches in rmw_zenoh before we merge this in. @clalancette what do you think?

[Timple]

Just an interested bystander here: I am following this repository out of curiosity.

For such a small delta, I would advise some guard around the statement instead of branching off. Otherwise a huge backport effort will probably be undertaken. With tooling like mergify bots etc. Making a lot of PR creation noise. Making repositories often un-watchable.

Of course this should not be your main concern but also take into account the maintenance effort this brings in. I think a (rather ugly) guard in the code is less of an effort.

[MichaelOrlov]

@Yadunund Friendly ping here to follow up.

[clalancette]

@Yadunund , @ahcorde , and I discussed this change.

In the long run (end of this year), we are going to have to branch off, due to releasing into distributions. But until we get there, it is convenient to use the same branch for everything. Thus, for now, I think should strive in this PR to maintain compatibility with Iron, Jazzy, and Rolling.

The simple way to do that would be to just remove all references to the localhost_only structure member. That works because we actually don't support the localhost_only parameter here in rmw_zenoh_cpp anyway. However, that is only half the story. If we just remove all references, then on Iron and Jazzy, localhost_only might be in an undefined state.

Thus, in addition to removing those references, we should also make sure that the structure is initialized. For rmw_init_options_init, we can accomplish that by calling memset on the structure before doing any work. For rmw_init_options_copy, we can accomplish that by calling memcpy from the src to the tmp structure before doing any additional work there.

[CihatAltiparmak]

Friendly feedback @clalancette @Yadunund @ahcorde , this problem has started in main repo as well for rolling.

Github action log : https://github.com/CihatAltiparmak/moveit_middleware_benchmark/actions/runs/10504255092/job/29099303697#step:3:1285