car audio: send Start once, even when the car answers the focus request while it's starting

Starting the stream at once and the car's focus answer could both get through, so the car got
Start twice and two AAC encoders ran. A 2014 aftermarket head unit dropped the connection on the
first audio. Directions now also log why a turn went unspoken.

gearslip/src/main/java/app/seb3thehacker/gearslip/audio/AudioLink.kt

@@ -233,7 +233,19 @@ class AudioLink(
         startStream()
     }
 
+    /**
+     * Guards starting and stopping. [start] starts the stream at once for head units that never
+     * answer the focus request, and the car's answer arrives on another thread and starts it
+     * too: without this both got through, so the car got Start twice and two AAC encoders, and a
+     * 2014 head unit dropped the connection on the first audio.
+     */
+    private val startLock = Any()
+
     private fun startStream() {
+        synchronized(startLock) { startStreamLocked() }
+    }
+
+    private fun startStreamLocked() {
         if (!ready || started) return
         sendOnChannel(
             MSG_START,
@@ -252,6 +264,10 @@ class AudioLink(
     }
 
     override fun stop() {
+        synchronized(startLock) { stopLocked() }
+    }
+
+    private fun stopLocked() {
         if (!started) return
         started = false
         synchronized(ackLock) { ackLock.notifyAll() }

gearslip/src/main/java/app/seb3thehacker/gearslip/car/DirectionsVoice.kt

@@ -87,16 +87,26 @@ object DirectionsVoice {
         running = false
     }
 
+    /** Why the last turn wasn't spoken, so the log says it once rather than every tick. */
+    private var silentBecause: String? = null
+
+    private fun quiet(reason: String) {
+        if (silentBecause == reason) return
+        silentBecause = reason
+        GearslipLog.i("guidance: not speaking - $reason")
+    }
+
     private fun tick() {
-        if (!CarSettings.speakDirections.value) return
         val turn = NavTurn.current()
         if (turn == null) {
             turnKey = null
             return
         }
-        if (turn.rerouting) return
+        if (!CarSettings.speakDirections.value) return quiet("Spoken directions is off in the car's Settings")
+        if (turn.rerouting) return quiet("the map app is rerouting")
         val step = turn.step
-        val instruction = instruction(step) ?: return
+        val instruction = instruction(step) ?: return quiet("the map app gave this turn no words")
+        silentBecause = null
         val meters = turn.distance?.let(NavTurn::toMeters)
         val close = (turn.seconds != null && turn.seconds <= NEAR_SECONDS) || (meters != null && meters <= NEAR_METERS)