playback: cache streamed audio to stop redundant refetches

A streamed MP3 with no Xing/Info/VBRI header gets an estimated duration
from Mp3Extractor. At end of stream the extractor rewrites it, which
refreshes the timeline; MediaPeriodQueue.updateQueuedPeriods then sees
the playing period's duration change, drops every period after it and
re-enqueues the next one, so the next track is opened from byte 0 again.
After a seek near the end that repeats a few times.

Route streamed bytes through a bounded SimpleCache (DefaultDataSource ->
CacheDataSource -> KnownLengthDataSource -> OkHttp). A re-opened next
period is served from the bytes the prefetch already committed, so the
redundant network fetches stop without changing playback length or
gapless padding. DefaultDataSource still handles file:/content:, so
downloads bypass the cache.

The cache is a process-scoped 64 MiB LRU in cacheDir/stream-cache; a
construction failure falls back to no cache so playback still starts.

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

@@ -12,8 +12,13 @@ import androidx.media3.common.MediaItem
 import androidx.media3.common.Player
 import androidx.media3.common.util.ExperimentalApi
 import androidx.media3.common.util.UnstableApi
+import androidx.media3.database.StandaloneDatabaseProvider
 import androidx.media3.datasource.DataSource
 import androidx.media3.datasource.DefaultDataSource
+import androidx.media3.datasource.cache.Cache
+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.ExoPlayer
 import androidx.media3.exoplayer.source.DefaultMediaSourceFactory
@@ -41,6 +46,7 @@ import kotlinx.coroutines.cancel
 import kotlinx.coroutines.launch
 import okhttp3.OkHttpClient
 import org.koin.core.context.GlobalContext
+import java.io.File
 
 @OptIn(UnstableApi::class)
 class PlaybackService : MediaSessionService() {
@@ -91,6 +97,7 @@ class PlaybackService : MediaSessionService() {
         val sources = GlobalContext.get().get<SourceRepository>()
         sourceRepository = sources
         library = GlobalContext.get().get<LibraryRepository>()
+        val cache = StreamCache.open(this)
         val exoPlayer =
             ExoPlayer
                 .Builder(this)
@@ -106,7 +113,11 @@ class PlaybackService : MediaSessionService() {
                     DefaultMediaSourceFactory(this)
                         .setEnableClippingInMediaPeriod(true)
                         .setDataSourceFactory(
-                            mediaDataSourceFactory(this, OkHttpDataSource.Factory(streamingClient()).setUserAgent(UserAgent.value)),
+                            mediaDataSourceFactory(
+                                this,
+                                cache,
+                                OkHttpDataSource.Factory(streamingClient()).setUserAgent(UserAgent.value),
+                            ),
                         ),
                 ).build()
         exoPlayer.addListener(
@@ -216,8 +227,46 @@ class PlaybackService : MediaSessionService() {
     }
 }
 
+// Process-scoped so it is never released while a loader thread is still committing to it, and so
+// a recreated service does not try to lock the directory a second time.
+@OptIn(UnstableApi::class)
+private object StreamCache {
+    @Volatile
+    private var instance: SimpleCache? = null
+
+    fun open(context: Context): SimpleCache? =
+        instance ?: synchronized(this) {
+            instance ?: runCatching {
+                SimpleCache(
+                    File(context.cacheDir, STREAM_CACHE_DIR),
+                    LeastRecentlyUsedCacheEvictor(STREAM_CACHE_BYTES),
+                    StandaloneDatabaseProvider(context),
+                )
+            }.onFailure { Log.w("playback", "stream cache unavailable: ${it.message}") }
+                .getOrNull()
+                ?.also { instance = it }
+        }
+
+    private const val STREAM_CACHE_DIR = "stream-cache"
+    private const val STREAM_CACHE_BYTES = 64L * 1024 * 1024
+}
+
 @OptIn(UnstableApi::class)
 internal fun mediaDataSourceFactory(
     context: Context,
+    cache: Cache?,
     upstream: DataSource.Factory,
-): DataSource.Factory = DefaultDataSource.Factory(context, KnownLengthDataSourceFactory(upstream))
+): DataSource.Factory {
+    val known = KnownLengthDataSourceFactory(upstream)
+    val source: DataSource.Factory =
+        if (cache == null) {
+            known
+        } else {
+            CacheDataSource
+                .Factory()
+                .setCache(cache)
+                .setUpstreamDataSourceFactory(known)
+                .setFlags(CacheDataSource.FLAG_IGNORE_CACHE_ON_ERROR)
+        }
+    return DefaultDataSource.Factory(context, source)
+}

app/src/test/kotlin/com/subtracks/playback/MediaDataSourceTest.kt

@@ -2,7 +2,12 @@ package com.subtracks.playback
 
 import android.content.Context
 import android.net.Uri
+import androidx.media3.common.C
+import androidx.media3.database.StandaloneDatabaseProvider
+import androidx.media3.datasource.DataSource
 import androidx.media3.datasource.DataSpec
+import androidx.media3.datasource.cache.NoOpCacheEvictor
+import androidx.media3.datasource.cache.SimpleCache
 import androidx.media3.datasource.okhttp.OkHttpDataSource
 import androidx.test.core.app.ApplicationProvider
 import androidx.test.ext.junit.runners.AndroidJUnit4
@@ -13,13 +18,16 @@ import okhttp3.mockwebserver.MockWebServer
 import org.junit.Assert.assertEquals
 import org.junit.Test
 import org.junit.runner.RunWith
+import java.io.ByteArrayOutputStream
 import java.io.File
+import java.nio.charset.StandardCharsets
 
 @RunWith(AndroidJUnit4::class)
 class MediaDataSourceTest {
     private val context = ApplicationProvider.getApplicationContext<Context>()
 
-    private fun dataSource() = mediaDataSourceFactory(context, OkHttpDataSource.Factory(OkHttpClient())).createDataSource()
+    private fun dataSource(cache: SimpleCache? = null) =
+        mediaDataSourceFactory(context, cache, OkHttpDataSource.Factory(OkHttpClient())).createDataSource()
 
     @Test
     fun aDownloadedFileIsReadThroughTheStreamingChain() {
@@ -50,4 +58,44 @@ class MediaDataSourceTest {
 
         assertEquals(1_000_000L, length)
     }
+
+    @Test
+    fun reopeningAStreamIsServedFromTheCache() {
+        val server = MockWebServer()
+        server.enqueue(MockResponse().setBody("0123456789"))
+        server.start()
+        val url = "http://${server.hostName}:${server.port}/rest/stream"
+        val cache =
+            SimpleCache(
+                File(context.cacheDir, "stream-cache-${System.nanoTime()}"),
+                NoOpCacheEvictor(),
+                StandaloneDatabaseProvider(context),
+            )
+
+        val first = readAll(dataSource(cache), url)
+        val second = readAll(dataSource(cache), url)
+        val requests = server.requestCount
+        server.shutdown()
+        cache.release()
+
+        assertEquals("0123456789", String(first, StandardCharsets.UTF_8))
+        assertEquals(first.toList(), second.toList())
+        assertEquals(1, requests)
+    }
+
+    private fun readAll(
+        dataSource: DataSource,
+        url: String,
+    ): ByteArray {
+        dataSource.open(DataSpec(Uri.parse(url)))
+        val out = ByteArrayOutputStream()
+        val buffer = ByteArray(1024)
+        while (true) {
+            val read = dataSource.read(buffer, 0, buffer.size)
+            if (read == C.RESULT_END_OF_INPUT) break
+            out.write(buffer, 0, read)
+        }
+        dataSource.close()
+        return out.toByteArray()
+    }
 }