playback: reject non-finite replay gain

Review follow-ups for the apply slice.

The parse guard in the base branch keeps non-finite gains and peaks out
of the library, but nothing here should depend on that: a value already
stored, or written by a future caller, must not be able to silence a
track or blast it at full scale.

- the gain maths ignores a non-finite peak and falls back to unity when
  the result is not finite.
- the processor falls back to unity for a non-finite gain, so
  `roundToInt` can never be handed a NaN.
- the preamp is sanitised on both read and write, since `coerceIn` passes
  NaN straight through.
- the renderers factory is pulled out of `PlaybackService` into
  `ReplayGainRenderersFactory` so a test can assert the sink really is
  built with the processor in its chain; the wiring was otherwise
  untested.
- a null active source clears the gain rather than leaving the previous
  track's applied.

app/src/main/kotlin/com/subtracks/data/prefs/UserPreferences.kt

@@ -163,13 +163,15 @@ class UserPreferences(
 
     private val replayGainModeKey = stringPreferencesKey("replay_gain_mode")
 
-    fun replayGainPreampDb(): Flow<Float> =
-        store.data.map { prefs -> (prefs[replayGainPreampKey] ?: 0f).coerceIn(MIN_REPLAY_GAIN_PREAMP_DB, MAX_REPLAY_GAIN_PREAMP_DB) }
+    fun replayGainPreampDb(): Flow<Float> = store.data.map { prefs -> prefs[replayGainPreampKey].sanitizedPreampDb() }
 
     suspend fun setReplayGainPreampDb(value: Float) {
-        store.edit { prefs -> prefs[replayGainPreampKey] = value.coerceIn(MIN_REPLAY_GAIN_PREAMP_DB, MAX_REPLAY_GAIN_PREAMP_DB) }
+        store.edit { prefs -> prefs[replayGainPreampKey] = value.sanitizedPreampDb() }
     }
 
+    private fun Float?.sanitizedPreampDb(): Float =
+        this?.takeIf { it.isFinite() }?.coerceIn(MIN_REPLAY_GAIN_PREAMP_DB, MAX_REPLAY_GAIN_PREAMP_DB) ?: 0f
+
     private val replayGainPreampKey = floatPreferencesKey("replay_gain_preamp_db")
 
     fun offlineMode(): Flow<Boolean> = store.data.map { prefs -> prefs[offlineModeKey] ?: false }

app/src/main/kotlin/com/subtracks/playback/PlaybackService.kt

@@ -20,10 +20,7 @@ import androidx.media3.datasource.cache.CacheDataSource
 import androidx.media3.datasource.cache.LeastRecentlyUsedCacheEvictor
 import androidx.media3.datasource.cache.SimpleCache
 import androidx.media3.datasource.okhttp.OkHttpDataSource
-import androidx.media3.exoplayer.DefaultRenderersFactory
 import androidx.media3.exoplayer.ExoPlayer
-import androidx.media3.exoplayer.audio.AudioSink
-import androidx.media3.exoplayer.audio.DefaultAudioSink
 import androidx.media3.exoplayer.source.DefaultMediaSourceFactory
 import androidx.media3.session.CommandButton
 import androidx.media3.session.MediaSession
@@ -113,24 +110,10 @@ class PlaybackService : MediaSessionService() {
                 .combine(preferences.replayGainPreampDb()) { mode, preamp -> ReplayGainSettings(mode, preamp) }
                 .collect(replayGain::setSettings)
         }
-        val renderersFactory =
-            object : DefaultRenderersFactory(this) {
-                override fun buildAudioSink(
-                    context: Context,
-                    enableFloatOutput: Boolean,
-                    enableAudioOutputPlaybackParameters: Boolean,
-                ): AudioSink =
-                    DefaultAudioSink
-                        .Builder(context)
-                        .setEnableFloatOutput(enableFloatOutput)
-                        .setEnableAudioOutputPlaybackParameters(enableAudioOutputPlaybackParameters)
-                        .setAudioProcessors(arrayOf(gainProcessor))
-                        .build()
-            }
         val exoPlayer =
             ExoPlayer
                 .Builder(this)
-                .setRenderersFactory(renderersFactory)
+                .setRenderersFactory(ReplayGainRenderersFactory(this, gainProcessor))
                 .setAudioAttributes(
                     AudioAttributes
                         .Builder()
@@ -204,7 +187,11 @@ class PlaybackService : MediaSessionService() {
         session?.setMediaButtonPreferences(listOf(starButton(false)))
         songJob =
             scope.launch {
-                val sourceId = sourceRepository?.activeSourceIdOnce() ?: return@launch
+                val sourceId = sourceRepository?.activeSourceIdOnce()
+                if (sourceId == null) {
+                    replayGain.setTrack(null)
+                    return@launch
+                }
                 library.song(sourceId, songId).collect { song ->
                     if (currentSongId != songId) return@collect
                     starred = song?.starred != null

app/src/main/kotlin/com/subtracks/playback/ReplayGain.kt

@@ -38,11 +38,11 @@ internal fun replayGainLinearGain(
         }
     val gainDb = requestedDb ?: return 1f
     val requested = gainDb + settings.preampDb
-    val applied = if (peak != null && peak > 0f) min(requested, maxSafeGainDb(peak)) else requested
-    return 10f.pow(applied / 20f)
+    val cap = peak?.takeIf { it.isFinite() && it > 0f }?.let(::maxSafeGainDb)
+    val applied = if (cap != null) min(requested, cap) else requested
+    return 10f.pow(applied / 20f).takeIf { it.isFinite() } ?: 1f
 }
 
-// A peak of 1.0 is full scale, so anything above it leaves no headroom and this goes negative. A
-// peak of zero or less carries no peak data (gonic sends 0 when only album gain is known), which
-// is why the caller ignores it rather than treating it as silence.
+// gonic sends a peak of 0 when it only has album gain, so a non-positive or non-finite peak means
+// "no peak data" and is ignored rather than treated as silence.
 private fun maxSafeGainDb(peak: Float): Float = -20f * log10(peak)

app/src/main/kotlin/com/subtracks/playback/ReplayGainProcessor.kt

@@ -21,7 +21,7 @@ internal class ReplayGainProcessor : BaseAudioProcessor() {
 
     override fun queueInput(inputBuffer: ByteBuffer) {
         val output = replaceOutputBuffer(inputBuffer.remaining())
-        val gain = linearGain
+        val gain = linearGain.takeIf { it.isFinite() } ?: 1f
         when (inputAudioFormat.encoding) {
             C.ENCODING_PCM_16BIT -> {
                 while (inputBuffer.remaining() >= 2) {

app/src/main/kotlin/com/subtracks/playback/ReplayGainRenderersFactory.kt

@@ -0,0 +1,29 @@
+package com.subtracks.playback
+
+import android.content.Context
+import androidx.annotation.OptIn
+import androidx.media3.common.util.UnstableApi
+import androidx.media3.exoplayer.DefaultRenderersFactory
+import androidx.media3.exoplayer.audio.AudioSink
+import androidx.media3.exoplayer.audio.DefaultAudioSink
+
+@OptIn(UnstableApi::class)
+internal class ReplayGainRenderersFactory(
+    context: Context,
+    private val processor: ReplayGainProcessor,
+) : DefaultRenderersFactory(context) {
+    // Mirrors the stock DefaultRenderersFactory.buildAudioSink, plus the processor. setAudioProcessors
+    // keeps the default chain and prepends ours, but DefaultAudioSink drops the whole chain when
+    // float output is enabled, so enabling that would silently disable replay gain.
+    public override fun buildAudioSink(
+        context: Context,
+        enableFloatOutput: Boolean,
+        enableAudioOutputPlaybackParameters: Boolean,
+    ): AudioSink =
+        DefaultAudioSink
+            .Builder(context)
+            .setEnableFloatOutput(enableFloatOutput)
+            .setEnableAudioOutputPlaybackParameters(enableAudioOutputPlaybackParameters)
+            .setAudioProcessors(arrayOf(processor))
+            .build()
+}