car audio: run the audio threads at audio priority so a busy phone doesn't make it skip

gearslip/src/main/java/app/seb3thehacker/gearslip/GearslipRunner.kt

@@ -45,7 +45,10 @@ class GearslipRunner(
     private lateinit var identityTask: FutureTask<PhoneTls>
 
     /** Sends every message, in order, so no other thread ever blocks on the USB link. See [send]. */
-    private val writer = java.util.concurrent.Executors.newSingleThreadExecutor { Thread(it, "gearslip-writer") }
+    private val writer = java.util.concurrent.Executors.newSingleThreadExecutor {
+        // Sound goes out through here too, so it gets audio's priority: late writes are skips.
+        Thread({ android.os.Process.setThreadPriority(android.os.Process.THREAD_PRIORITY_AUDIO); it.run() }, "gearslip-writer")
+    }
 
     /** Waits for the certificate only when TLS is first needed; see [run]. */
     private val tls: PhoneTls by lazy { identityTask.get() }
@@ -88,6 +91,8 @@ class GearslipRunner(
     @Volatile private var lastKeyframeSentAt = 0L
 
     fun run() {
+        // This thread reads the car's acks; audio waits on them, so it can't be left at the back.
+        android.os.Process.setThreadPriority(android.os.Process.THREAD_PRIORITY_AUDIO)
         SessionReport.begin()
         log.i("--- session starting ---")
         SessionStatus.connecting("Connected to the head unit")

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

@@ -16,6 +16,7 @@ import android.media.AudioRecord
 import android.media.projection.MediaProjection
 import android.media.projection.MediaProjectionManager
 import android.os.IBinder
+import android.os.Process
 import app.seb3thehacker.gearslip.GearslipLog
 import kotlin.math.abs
 
@@ -147,6 +148,9 @@ class AudioCaptureService : Service() {
 
         reading = true
         thread = Thread({
+            // A busy phone (a map app drawing, the video encoder) would otherwise starve this
+            // loop, and the car hears every late read as a skip.
+            Process.setThreadPriority(Process.THREAD_PRIORITY_URGENT_AUDIO)
             val buffer = ByteArray(CHUNK_BYTES)
             var peak = 0
             var heardAt = System.currentTimeMillis()