Fix sd-bus thread-safety crash in Linux tray (#405)

sd-bus objects are not thread-safe, but the event-loop thread
(sd_bus_process) and the JVM threads driving the sni_tray_set_*/
sni_tray_item_* mutators (sd_bus_emit_*) accessed tray->bus with no
synchronization. Under frequent updates an emit races with an in-flight
sd_bus_process and trips 'Assertion !bus->current_slot ... Aborting'.

- Add a bus_lock mutex serializing all sd_bus_* access and the tray
  state it touches: held across setup, each sd_bus_process batch and
  teardown, and acquired by every public mutator.
- Defer user callbacks (click / menu) through a queue flushed by the
  event loop after releasing bus_lock, so upcalls into Kotlin can
  re-enter the mutators without deadlocking against the caller's lock.
- Add a standalone C concurrency regression test that aborts on the
  old code and passes on the fixed code.

Author: kdroidFilter

src/native/linux/run_concurrency_test.sh

@@ -0,0 +1,35 @@
+#!/bin/bash
+# Build and run the issue #405 concurrency regression test.
+# Runs the stress harness several times under a private D-Bus session.
+# Exit 0 only if every run completes without crashing.
+
+set -u
+SCRIPT_DIR="$(cd "$(dirname "$0")" && pwd)"
+SDBUS_CFLAGS=$(pkg-config --cflags libsystemd)
+SDBUS_LIBS=$(pkg-config --libs libsystemd)
+BIN="$SCRIPT_DIR/test_sni_concurrency"
+
+echo "Compiling concurrency test..."
+gcc -O2 -g -fPIC -Wall -Wextra -Wno-unused-parameter \
+    -I "$SCRIPT_DIR" $SDBUS_CFLAGS \
+    "$SCRIPT_DIR/sni.c" "$SCRIPT_DIR/test_sni_concurrency.c" \
+    $SDBUS_LIBS -lpthread -lm -o "$BIN" || { echo "compile failed"; exit 2; }
+
+RUNS="${1:-5}"
+rc=0
+for i in $(seq 1 "$RUNS"); do
+    echo "--- run $i/$RUNS ---"
+    dbus-run-session -- "$BIN"
+    status=$?
+    if [ $status -ne 0 ]; then
+        echo "FAIL: run $i exited with status $status (signal $((status-128)) if >128)"
+        rc=1
+        break
+    fi
+done
+
+rm -f "$BIN"
+if [ $rc -eq 0 ]; then
+    echo "PASS: all $RUNS runs completed without crash"
+fi
+exit $rc