While building Navius (a GPS navigator for Ubuntu Touch) I ran into several GPS reliability issues in lomiri-location-service, particularly on devices that run Waydroid alongside UT.

I have collected the fixes in a branch called navius (currently at navius5). All patches are tested on a Xiaomi Redmi Note 9 Pro (HALIUM_10, arm64) running Ubuntu Touch Noble (24.04), but they address generic race conditions and watchdog issues that should benefit any device.

I would like to share them here and eventually propose the relevant ones upstream.

Bugs fixed

1. Waydroid GPS callback race → SIGSEGV navius1

On HALIUM_10, Waydroid and Ubuntu Touch both access the GPS HAL via host_hwbinder. Waydroid can call register_callbacks() at any time; that function does u_hardware_gps_delete() + u_hardware_gps_new(). If a GPS callback fires between those two calls it dereferences the freed handle — SIGSEGV, daemon crash.

Fix: std::shared_mutex callback_mutex in Impl. All seven GPS callbacks hold a shared lock (run concurrently with each other). register_callbacks() holds an exclusive lock only while deleting/installing the handle — callbacks complete first, then the swap happens safely.

2. EDEADLK at service start navius1

The initial mutex fix held the exclusive lock across the whole delete → new sequence. u_hardware_gps_new() fires on_set_capabilities synchronously on the calling thread, which tries to take a shared lock on the same mutex: EDEADLK, daemon aborts at start-up.

Fix: Three-phase lock:

1. Exclusive lock → delete old handle → release.
2. Create new handle without any lock (re-entrant callbacks are now safe).
3. Exclusive lock → install new handle → dispatch position mode → release.

3. start_positioning() blocking the daemon forever navius2

start_positioning() was called from a D-Bus handler thread. Its internal QEventLoop blocked that thread indefinitely. LLS started, satellites were tracked, but no position fix ever arrived.

Fix: start_positioning() dispatched to the main thread via Qt::QueuedConnection / QMetaObject::invokeMethod.

4. GPS stops after Waydroid closes and never restarts navius4

When Waydroid shuts down it calls u_hardware_gps_stop() on the shared HAL, halting GPS globally. LLS's handle remained valid so it never detected the stall — GPS frozen until LLS was manually restarted.

Fix — watchdog thread: a detached thread ticks every 5 s and reads last_gps_ms (updated by on_location_update and on_sv_status_update). If no GPS data for 10 s, it re-runs register_callbacks() and restarts the chipset automatically.

5. No fixes despite 37+ satellites in view navius4

dispatch_updated_modes_to_driver() (which sends the position mode to the chipset via u_hardware_gps_set_position_mode()) was called in register_callbacks() but not in the fast path inside start_positioning(). The chipset tracked satellites but never computed fixes.

Fix: call dispatch_updated_modes_to_driver() before u_hardware_gps_start() in the fast path.

Result after navius4: Waydroid and Ubuntu Touch GPS now work simultaneously. After Waydroid opens and steals the HAL, the watchdog reclaims callbacks within 10 s; accuracy converges to ~4 m while Waydroid keeps positioning.

Feature additions

GetVisibleSpaceVehicles D-Bus method navius2

QML clients that cannot subscribe to D-Bus property-change signals have no way to get a satellite snapshot without keeping a persistent session. Added a synchronous method and read-only property VisibleSpaceVehicles to the service interface:

dbus-send --system --dest=com.lomiri.location.Service \
          --print-reply \
          /com/lomiri/location/Service \
          com.lomiri.location.Service.Interface.GetVisibleSpaceVehicles

Compile-time debug traces (LLS_DEBUG) navius5

All GPS trace logging is gated behind LLS_DEBUG (default false) in include/…/lls_trace.h. Zero runtime cost when disabled; set to true and rebuild for full per-fix traces in journald.

Build & install

The branch ships a build-deb.sh script that produces a .deb inside an isolated Docker container (Ubuntu 24.04 + UBports repo). No host modifications needed. Requires Docker on an ARM64 host.

git clone https://github.com/woodyst/lomiri-location-service -b navius
cd lomiri-location-service
bash build-deb.sh            # output to ./debs/

# Deploy to device
scp debs/liblomiri-location-service3_*_arm64.deb phablet@<device>:/tmp/
ssh root@<device> "mount -o remount,rw / && \
    dpkg -i /tmp/liblomiri-location-service3_*_arm64.deb && \
    systemctl restart lomiri-location-service"

Upstream proposal

I intend to open two MRs on gitlab.com/ubports/…/lomiri-location-service:

Before opening the MRs I wanted to share the work here and get feedback, especially on whether there is already upstream work addressing the Waydroid race condition or the GetVisibleSpaceVehicles API.

Any review or testing on other devices is very welcome. Full patch documentation: doc/navius-patches.md.