feat(LibCarla, Carla): add Zenoh ROS2 middleware

[habby1012]

Description

Adds Zenoh as a third ROS 2 middleware backend on top of the existing abstraction. Users select it at runtime via --rmw=zenoh, alongside --rmw=fastdds and --rmw=cyclonedds.

New Zenoh middleware — LibCarla/source/carla/ros2/middleware/zenoh/:

• ZenohSharedSession — opens a process-wide Zenoh session on first use and declares the carla_bridge node so rmw_zenoh peers see it. Defaults to connecting at tcp/localhost:7447, and it can be overridden via ZENOH_SESSION_CONFIG_URI or ZENOH_CONFIG_OVERRIDE.
• ZenohWireFormat — small helpers that build the keyexpr, liveliness token string, and per-message attachment in the format rmw_zenoh expects.
• ZenohPublisherMiddleware / ZenohSubscriberMiddleware — implement the existing IPublisherMiddleware / ISubscriberMiddleware interfaces. CDR serialization is shared with the FastDDS / CycloneDDS paths.

Wire it into the factory:

• Middleware.h, MiddlewareFactory.h — add a Zenoh enum value and a factory branch that constructs ZenohPublisherMiddleware / ZenohSubscriberMiddleware when CARLA_ROS2_MIDDLEWARE_ZENOH is defined.
• LibCarla/cmake/CMakeLists.txt, cmake/ros2/CMakeLists.txt, cmake/zenoh/CMakeLists.txt (new) — compile zenoh sources into carla_ros2 and install zenoh headers + libzenohc.a to the LibCarla dependencies dir.

Build & runtime selection:

• Util/BuildTools/Setup.sh — download prebuilt zenoh-c 1.8.0 and strip --strip-debug the static library.
• Carla.Build.cs — define CARLA_ROS2_MIDDLEWARE_ZENOH and link libzenohc.a into the UE4 plugin.
• CarlaSettings.h — accept zenoh as a value for --rmw=.

Tests — test_ros2_middleware.cpp: mirror the existing fastdds / cyclonedds cases for zenoh (to/from string, availability check, factory get/set, factory create-when-unavailable).

Where has this been tested?

• Platform(s): Linux (Ubuntu 24.04)
• Python version(s): 3.12
• Unreal Engine version(s): UE4 4.26

Tests run:

• make LibCarla ARGS="--ros2": passed.
• make check.LibCarla ARGS="--ros2": 406 / 406 pass across all four configs (server release 137, client release 66, server debug 137, client debug 66).
• make launch ARGS="--ros2 --rmw=fastdds" then python -m nose2 -v smoke.test_ros2: 5 / 5 pass in 39.1 s.
• make launch ARGS="--ros2 --rmw=cyclonedds" then python -m nose2 -v smoke.test_ros2: 5 / 5 pass in 38.4 s.
• make launch ARGS="--ros2 --rmw=zenoh" then python -m nose2 -v smoke.test_ros2: 5 / 5 pass in 38.6 s.

Possible Drawbacks

• Adds a new build dependency (zenoh-c 1.8.0, ~5 MB after strip). Fetched as prebuilt archive on first make setup --ros2.
• No change to --rmw=fastdds / --rmw=cyclonedds paths; existing behavior preserved.

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This change is 

[JArmandoAnaya]

I added one very small suggestion.

[LuisPovedaCano]

The PR adds --rmw=zenohz support to CARLA but the companion PythonAPI/examples/ros2/Dockerfile only handles rmw_fastrtps_cpp and rmw_cyclonedds_cpp.
It should be extended to install ros-${ROS_DISTRO}-rmw-zenoh-cpp when RMW_IMPLEMENTATION=rmw_zenoh_cpp is passed

I've tested this end-to-end on the feat/zenoh-ros2-middleware branch on Ubuntu 24.04.
The implementation is clean.
Great addition.

[JArmandoAnaya]

@LuisPovedaCano It would be great to add this update to the project's Docker script so we can test Zenoh features in the future.

[habby1012]

Thanks for the reviews! Added Zenoh support to the ROS2 example as suggested by @JArmandoAnaya and @LuisPovedaCano:

• DockerFile: installs ros-${ROS_DISTRO}-rmw-zenoh-cpp when --rmw=zenoh is selected.
• run_rviz.sh: accepts --rmw=zenoh.

[habby1012]

Thanks again @JArmandoAnaya @LuisPovedaCano !
Added the Zenoh support for the ROS2 example (Dockerfile + run_rviz.sh --rmw=zenoh). Feel free to check whenever you have time 🙂

[JArmandoAnaya]

Hello @habby1012, Thank you for the commit. I'll test this locally as soon as possible. I will let you know my review.

[habby1012]

@JArmandoAnaya, Sure, thanks!

[LuisPovedaCano]

I wasn't able to run the example. The topics are published but not subscribed.

docker run --rm --net=host \
  --env="RMW_IMPLEMENTATION=rmw_zenoh_cpp" \
  carla-rviz-humble-zenoh \
  ros2 topic info /clock
Type: rosgraph_msgs/msg/Clock
Publisher count: 1
Subscription count: 0

Enabling the logs with debug it appeared this:
zenoh_ext::advanced_subscriber: Create AdvancedSubscriber{key_expr: 0/clock/rosgraph_msgs::msg::dds_::Clock_/TypeHashNotSupported}

so changing in ZenohWireFormat.h

inline std::string zenoh_make_topic_keyexpr(
    const std::string& domain,
    const std::string& topic_no_rt,
    const char*        type_name,
    const char*        type_hash)
{
  return domain + "/" + topic_no_rt + "/" + type_name + "/" + type_hash;
}

to:

inline std::string zenoh_make_topic_keyexpr(
    const std::string& domain,
    const std::string& topic_no_rt,
    const char*        type_name,
    const char* /* type_hash */)
{
  return domain + "/" + topic_no_rt + "/" + type_name + "/TypeHashNotSupported";
}

Has worked for me, now I can subscribe to the topics.
I tested running the RViz, I can see the cameras and all the topics.

Were you able to get it to work with RViz displaying the images without having to make this change?

The steps I followed to run the example are:

1. In one terminal run: docker run --rm --net=host carla-rviz-humble-zenoh ros2 run rmw_zenoh_cpp rmw_zenohd
2. Launch CARLA in another terminal with make launch ARGS="--ros2 --editor-flags='--ros2' --rmw=zenoh"
3. After start the simulation, in a third terminal go to PythonAPI/examples/ros2 and run python ros2_native.py --file stack.json
4. In a 4th terminal, go also to PythonAPI/examples/ros2 and run ./run_rviz.sh --distro=humble --rmw=zenoh

For DDS implementations, the step 1 was not required.
Should CARLA run the rmw_zenohd?
It is mandatory to run it? I tested without it and it does not work, but maybe it can be configured somehow.
All this should be documented

[habby1012]

Hi @LuisPovedaCano, sorry this one tripped you up on humble, and thanks a lot for digging into it. Even finding a workaround really helped me pin it down.

Humble doesn't subscribe because CARLA publishes each topic with its real RIHS01 type hash. On jazzy and rolling, rmw_zenoh builds the subscriber key expression with that same hash, so they line up. Humble's rmw_zenoh doesn't do type hashes, so it always uses TypeHashNotSupported, which never matches CARLA's RIHS01_.... That's the Subscription count: 0 you saw. (and why it worked for me, I was on jazzy).

Forcing TypeHashNotSupported fixes humble but would break jazzy/rolling, so instead I just wildcarded the hash part of the key expression (.../* instead of .../<hash>), which matches both. RViz shows everything on humble and jazzy now.

I also added one step to the example README, which is starting rmw_zenohd.

Thanks again, really appreciate the careful review! Pushed both changes whenever you get a chance to look.

[LuisPovedaCano]

Last commit fixes zenoh for both humble and jazzy.
I've been able to run simulations with both distros successfully.

[cgeller]

@habby1012 Really nice work, thanks a lot! Do you also plan to add Zenoh in the CARLA UE5 Version? I guess this would also he helpful for many other users.

[JArmandoAnaya]

Hello @cgeller 👋, yes, it is the idea. We are in the middle of a porting process to pull all existing ROS2 improvements from the UE4 branch into UE5. But since this process also requires closing a current gap between the two sides about some previous ROS2 features missing in UE5 before merging the ROS2 middleware decoupling that exists now in UE4. I hope we can finish that in the next few weeks.