map ROS_LOCALHOST_ONLY to UXRCE_DDS_PTCFG

[varadVaidya]

In SITL startup, mirror ROS_LOCALHOST_ONLY into UXRCE_DDS_PTCFG so PX4 and ROS middleware locality policy stay aligned with environment-based bringup.
Also document ROS_LOCALHOST_ONLY as an env override for UXRCE_DDS_PTCFG.

Solved Problem

When running PX4 SITL with ROS 2, ROS_LOCALHOST_ONLY and PX4 UXRCE_DDS_PTCFG can become misaligned unless set manually.
That mismatch causes DDS discovery/data flow failure from ROS 2 (e.g. missing /fmu/out/* topics).

Fixes #N/A

Solution

• In ROMFS/px4fmu_common/init.d-posix/rcS, map ROS_LOCALHOST_ONLY to UXRCE_DDS_PTCFG during SITL startup:
  • if ROS_LOCALHOST_ONLY is set and equals "1": UXRCE_DDS_PTCFG=1
  • if ROS_LOCALHOST_ONLY is set and is anything else: UXRCE_DDS_PTCFG=0
  • if ROS_LOCALHOST_ONLY is unset: leave parameter default behavior unchanged
• Update docs/en/middleware/uxrce_dds.md to document ROS_LOCALHOST_ONLY override and update example command.

Changelog Entry

For release notes:

Bugfix: SITL startup now maps ROS_LOCALHOST_ONLY to UXRCE_DDS_PTCFG.
Documentation: Added ROS_LOCALHOST_ONLY env override in uxrce_dds docs and updated example command.

Alternatives

• Keep manual param set UXRCE_DDS_PTCFG in scripts.

Test coverage

Tested with SITL (gz_x500, headless), MicroXRCEAgent UDP 8888, ROS_DOMAIN_ID=42.

Host ROS_LOCALHOST_ONLY	PX4 ROS_LOCALHOST_ONLY -> UXRCE_DDS_PTCFG	Result
0	0	PASS: /fmu/out/* visible and echo succeeds
1	1	PASS: /fmu/out/* visible and echo succeeds
1	0	FAIL (expected): ROS side only sees /parameter_events and /rosout
0	1	FAIL (expected): ROS side only sees /parameter_events and /rosout

Echo command used:

ros2 topic echo --once --no-daemon /fmu/out/vehicle_status_v1
--qos-reliability best_effort --qos-durability transient_local

Context

This keeps env-based SITL bringup minimal and deterministic: setting ROS_LOCALHOST_ONLY directly drives PX4 uXRCE locality mode.

[varadVaidya]

If this is accepted on main, I can open follow-up backport PRs for older maintained release branches