fix(system_tray): dispatch tray updates asynchronously to avoid blocking callers (#4199)

[moimart]

Summary

update_tray_*() functions in src/system_tray.cpp call tray_update(), which on Linux blocks the caller inside pthread_cond_wait until the tray library's GTK main-loop callback runs. That callback calls libayatana-appindicator and libnotify synchronously. If the active desktop notification daemon is unresponsive — a common failure mode on Wayland compositors (swaync/dunst/Quickshell) during desktop transitions, lock screens, or daemon restarts — the callback blocks indefinitely and every caller of update_tray_* blocks with it.

One of those callers is stream::session::join(), which arms a 10-second NVENC-deadlock watchdog. When the tray update doesn't return in time, the watchdog fires debug_trap() and the entire sunshine process dies with SIGTRAP. Reported as #4199.

Before: `session::join() → update_tray_pausing() → tray_update() → pthread_cond_wait → tray_update_internal → notify_notification_show → GDBus blocked on a hung notification daemon → 10 s → debug_trap() → process dies.**

After: session::join() → update_tray_pausing() → spawn detached worker → return. session teardown completes. The worker stays blocked until the daemon recovers; if the process exits first, the thread is cleaned up by the OS.

Change

• New helper run_tray_async(fn) that spawns a detached std::thread, takes tray_async_mutex, and runs fn. Catches exceptions so a runaway tray/notify library call can't kill the worker. No-ops cleanly if tray_initialized is false at post time or at worker start time (race window).
• tray_async_mutex serializes mutation of the shared tray struct across workers.
• Persistent file-scope std::string buffers (playing_msg_buf, pausing_msg_buf, stopped_msg_buf) replace the static std::string msg = std::format(...) pattern. The old pattern only evaluated the format expression the first time each function ran, permanently capturing the first app_name; this also fixes that latent bug.
• Each update_tray_* body moves verbatim into a lambda passed to run_tray_async.

What stays the same

• Public API (src/system_tray.h). No call-site changes in src/stream.cpp, src/main.cpp, src/nvhttp.cpp, src/process.cpp.
• Tray event loop (tray_thread_worker + process_tray_events using tray_loop(1)) is untouched — fix(tray): use the blocking event loop to avoid wasting power #4457's power optimization is preserved.
• The session::join() NVENC-deadlock watchdog is untouched — it exists for a separate, legitimate NVIDIA-driver issue, and remains the intended failsafe. The fix just prevents a non-critical tray subsystem from tripping it.
• Windows and macOS tray paths work unchanged. tray_update() on those platforms isn't known to block, but routing through the async worker costs essentially nothing and eliminates any possibility of blocking the caller.

Trade-offs / caveats

• Each update spawns a transient thread (~one per streaming state change — a few per session). If the notification daemon is hung and never recovers, the workers stay blocked on tray_async_mutex + tray_update's internal cond var. Memory impact is bounded by stream state transitions per session.
• If the notification daemon is permanently wedged, tray state stops updating but sunshine otherwise runs normally. Before this patch, sunshine would SIGTRAP instead.
• std::thread construction can throw on resource exhaustion; this is caught and logged (warning) and the update is dropped. Dropping a tray icon update is preferable to surfacing the error on session teardown.

Test plan

• Reproduced SIGTRAP crash in stream::session::join due to deadlock with system tray/notification thread on Wayland #4199 locally on CachyOS / Hyprland (Wayland, Quickshell-based notification daemon) with the upstream v2025.924.154138 tag: cluster of SIGTRAP crashes on Moonlight disconnect, coredumpctl showed raise → pthread_once in the tray handler path.
• Built sunshine with this patch applied (v2025.924 tag port; this PR is forward-ported to master); ran 10+ connect/disconnect cycles from Moonlight — no SIGTRAP, session teardown completes cleanly each time.
• Verified tray icon and notifications still work when the notification daemon is healthy.
• CI: I have only tested on Linux. Windows and macOS reviewers: please confirm tray behavior is unchanged (the only user-visible change is that updates run on a short-lived worker thread).

Fixes #4199

[sonarqubecloud]

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[ReenigneArcher]

Did you test the current branch to see if it's fixed? Because there were significant changes to the tray library. Other's reported it's now working for them.

Second, tray is being migrated in #4907

[moimart]

Second, tray is being migrated in #4907

Ohh. Didn't realize about that one.. then indeed ignore this. Thanks!