fix(isaac-sim): pegasus drone retains PX4 state across Stop/Play

[andrewjong]

What features did you add and/or bugs did you address?

Bumps the PegasusSimulator submodule to fe8b5a10, which fixes a bug where pressing Stop and then Play in Isaac Sim caused the drone's props to spin and veer off toward its previous PX4 setpoint instead of resetting cleanly.

No AirStack-side code changes — just a submodule pointer update and a version bump.

How did you implement it?

All fixes live in the submodule commit. Summary of what changed there:

1. PX4 was not actually killed on Stop. Popen.kill() only targeted the parent and never wait()ed, so helper threads/children survived and kept talking to the simulator on the next Play. PX4 is now launched with start_new_session=True and the whole process group is killed via SIGTERM -> wait -> SIGKILL -> wait in kill_px4().

2. Leaked physics callbacks reapplied stale rotor speeds. After Stop, callbacks could still read the backend's last commanded rotor speeds and apply them as thrust. _rotor_data is now zeroed on backend.stop(), and backend.update() early-returns when not running / no connection, so any stale callback applies zero thrust.

3. Tearing down and rebuilding the multirotor caused [simulator_mavlink] poll timeout on the second Play. Isaac's add_physics_callback does not reliably overwrite an existing key, so the new multirotor's sensor callbacks failed to register and PX4 never received HIL_SENSOR. The multirotor and its physics callbacks are now kept alive across Stop/Play cycles; the per-vehicle sim_start_stop callback drives backend.start()/stop() (which relaunches a clean PX4 process each Play).

See the submodule commit message for the full rationale.

How do you run and use it?

# From the repo root, ensure the submodule is at the new pointer
git submodule update --init --recursive

# Bring up Isaac Sim with Pegasus (single drone is fine to reproduce)
airstack up isaac-sim

Then in the Isaac Sim UI:

1. Press Play — drone spawns and PX4 connects.
2. Press Stop.
3. Press Play again.

Before this fix: props spin immediately and the drone drifts toward the previous setpoint; PX4 logs may show [simulator_mavlink] poll timeout.

After this fix: drone sits idle with zero thrust, PX4 reconnects cleanly, and behavior is identical to the first Play.

Testing with PyTest

No new pytests added — this is a submodule pointer bump for a UI-driven Stop/Play interaction that the current tests/ harness does not exercise (the takeoff/hover/land test runs a single Play cycle).

Existing relevant suites should continue to pass:

airstack test -m liveliness --sim isaacsim --num-robots 1 --stress-iterations 1 -v
airstack test -m takeoff_hover_land --sim isaacsim --num-robots 1 -v

Expected: same pass/fail status as develop.

Documentation

• Was mkdocs.yml updated? n — submodule-only change, no new user-facing surface.
• Newcomer reproduction steps are in the "How do you run and use it?" section above.
• No new visual media; the fix is observable via the manual repro above.

Versioning

Bumped .env VERSION from 0.19.0-alpha.1 to 0.19.0-alpha.2 (bug fix on the current alpha line per the template).

[Copilot]

Pull request overview

Updates the Isaac Sim Pegasus integration to prevent PX4 and rotor-state from persisting across Isaac Sim Stop/Play cycles by bumping the PegasusSimulator submodule, and increments the AirStack release version to reflect the fix.

Changes:

• Bump simulation/isaac-sim/extensions/PegasusSimulator submodule to include Stop/Play reset fixes (PX4 process-group teardown, rotor speed state reset, and callback lifecycle adjustments).
• Bump .env VERSION from 0.19.0-alpha.1 to 0.19.0-alpha.2.

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated no comments.

File	Description
.env	Increments the project version to 0.19.0-alpha.2 to reflect the Isaac Sim Stop/Play bug fix release.
simulation/isaac-sim/extensions/PegasusSimulator (submodule)	Advances the PegasusSimulator integration to a commit that resets PX4/rotor/callback state cleanly across Stop/Play.

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