util: add guard abstraction

Add an abstraction over the generation system called GenerationGuard.

This allows cheap cooperative threading to be implemented consistently
in many places.

app/src/main/java/org/oxycblt/auxio/detail/DetailViewModel.kt

@@ -39,6 +39,7 @@ import org.oxycblt.auxio.settings.Settings
 import org.oxycblt.auxio.ui.Sort
 import org.oxycblt.auxio.ui.recycler.Header
 import org.oxycblt.auxio.ui.recycler.Item
+import org.oxycblt.auxio.util.GenerationGuard
 import org.oxycblt.auxio.util.application
 import org.oxycblt.auxio.util.logD
 import org.oxycblt.auxio.util.logW
@@ -112,6 +113,8 @@ class DetailViewModel(application: Application) :
             currentGenre.value?.let(::refreshGenreData)
         }
 
+    private val songGuard = GenerationGuard()
+
     fun setSongId(id: Long) {
         if (_currentSong.value?.run { song.id } == id) return
         val library = unlikelyToBeNull(musicStore.library)
@@ -158,16 +161,12 @@ class DetailViewModel(application: Application) :
     private fun generateDetailSong(song: Song) {
         _currentSong.value = DetailSong(song, null)
         viewModelScope.launch(Dispatchers.IO) {
+            val generation = songGuard.newHandle()
             val info = generateDetailSongInfo(song)
-
+            songGuard.yield(generation)
-            // Theoretically, the song could have been changed again while we were
-            // extracting song information, so make sure that we can update the song
-            // in the first place.
-            if (_currentSong.value?.run { this.song.id } == song.id) {
             _currentSong.value = DetailSong(song, info)
         }
     }
-    }
 
     private fun generateDetailSongInfo(song: Song): SongInfo {
         val extractor = MediaExtractor()

app/src/main/java/org/oxycblt/auxio/image/BaseFetcher.kt

@@ -53,7 +53,7 @@ import org.oxycblt.auxio.util.logW
  * The base implementation for all image fetchers in Auxio.
  * @author OxygenCobalt
  *
- * TODO: Artist images
+ * TODO: File-system derived images [cover.jpg, Artist Images]
  */
 abstract class BaseFetcher : Fetcher {
     /**

app/src/main/java/org/oxycblt/auxio/image/BitmapProvider.kt

@@ -25,6 +25,7 @@ import coil.request.Disposable
 import coil.request.ImageRequest
 import coil.size.Size
 import org.oxycblt.auxio.music.Song
+import org.oxycblt.auxio.util.GenerationGuard
 
 /**
  * A utility to provide bitmaps in a manner less prone to race conditions.
@@ -39,7 +40,7 @@ import org.oxycblt.auxio.music.Song
  */
 class BitmapProvider(private val context: Context) {
     private var currentRequest: Request? = null
-    private var currentGeneration = 0L
+    private var guard = GenerationGuard()
 
     /** If this provider is currently attempting to load something. */
     val isBusy: Boolean
@@ -51,9 +52,7 @@ class BitmapProvider(private val context: Context) {
      */
     @Synchronized
     fun load(song: Song, target: Target) {
-        // Increment the generation value so that previous requests are invalidated.
+        val generation = guard.newHandle()
-        // This is a second safeguard to mitigate instruction-by-instruction race conditions.
-        val generation = ++currentGeneration
 
         currentRequest?.run { disposable.dispose() }
         currentRequest = null
@@ -65,12 +64,12 @@ class BitmapProvider(private val context: Context) {
                     .size(Size.ORIGINAL)
                     .target(
                         onSuccess = {
-                            if (generation == currentGeneration) {
+                            if (guard.check(generation)) {
                                 target.onCompleted(it.toBitmap())
                             }
                         },
                         onError = {
-                            if (generation == currentGeneration) {
+                            if (guard.check(generation)) {
                                 target.onCompleted(null)
                             }
                         })

app/src/main/java/org/oxycblt/auxio/music/system/Indexer.kt

@@ -23,7 +23,6 @@ import android.content.pm.PackageManager
 import android.database.Cursor
 import android.os.Build
 import androidx.core.content.ContextCompat
-import kotlin.coroutines.cancellation.CancellationException
 import kotlinx.coroutines.Dispatchers
 import kotlinx.coroutines.withContext
 import org.oxycblt.auxio.BuildConfig
@@ -34,6 +33,7 @@ import org.oxycblt.auxio.music.MusicStore
 import org.oxycblt.auxio.music.Song
 import org.oxycblt.auxio.settings.Settings
 import org.oxycblt.auxio.ui.Sort
+import org.oxycblt.auxio.util.GenerationGuard
 import org.oxycblt.auxio.util.logD
 import org.oxycblt.auxio.util.logE
 import org.oxycblt.auxio.util.logW
@@ -64,7 +64,7 @@ class Indexer {
     private var lastResponse: Response? = null
     private var indexingState: Indexing? = null
 
-    private var currentGeneration = 0L
+    private var guard = GenerationGuard()
     private var controller: Controller? = null
     private var callback: Callback? = null
 
@@ -133,7 +133,7 @@ class Indexer {
     suspend fun index(context: Context) {
         requireBackgroundThread()
 
-        val generation = synchronized(this) { ++currentGeneration }
+        val generation = guard.newHandle()
 
         val notGranted =
             ContextCompat.checkSelfPermission(context, Manifest.permission.READ_EXTERNAL_STORAGE) ==
@@ -184,13 +184,13 @@ class Indexer {
     @Synchronized
     fun cancelLast() {
         logD("Cancelling last job")
-        currentGeneration++
+        val generation = guard.newHandle()
-        emitIndexing(null, currentGeneration)
+        emitIndexing(null, generation)
     }
 
     @Synchronized
     private fun emitIndexing(indexing: Indexing?, generation: Long) {
-        checkGeneration(generation)
+        guard.yield(generation)
 
         if (indexing == indexingState) {
             // Ignore redundant states used when the backends just want to check for
@@ -210,32 +210,23 @@ class Indexer {
     }
 
     private suspend fun emitCompletion(response: Response, generation: Long) {
-        synchronized(this) {
+        guard.yield(generation)
-            checkGeneration(generation)
-
-            // Do not check for redundancy here, as we actually need to notify a switch
-            // from Indexing -> Complete and not Indexing -> Indexing or Complete -> Complete.
-
-            lastResponse = response
-            indexingState = null
-        }
-
-        val state = State.Complete(response)
 
         // Swap to the Main thread so that downstream callbacks don't crash from being on
         // a background thread. Does not occur in emitIndexing due to efficiency reasons.
         withContext(Dispatchers.Main) {
+            synchronized(this) {
+                // Do not check for redundancy here, as we actually need to notify a switch
+                // from Indexing -> Complete and not Indexing -> Indexing or Complete -> Complete.
+                lastResponse = response
+                indexingState = null
+
+                val state = State.Complete(response)
+
                 controller?.onIndexerStateChanged(state)
                 callback?.onIndexerStateChanged(state)
             }
         }
-
-    private fun checkGeneration(generation: Long) {
-        if (currentGeneration != generation) {
-            // Not the running task anymore, cancel this co-routine. This allows a yield-like
-            // behavior to be implemented in a far cheaper manner for each backend.
-            throw CancellationException()
-        }
     }
 
     /**

app/src/main/java/org/oxycblt/auxio/music/system/MediaStoreBackend.kt

@@ -471,7 +471,6 @@ class Api21MediaStoreBackend : MediaStoreBackend() {
 open class BaseApi29MediaStoreBackend : MediaStoreBackend() {
     private var volumeIndex = -1
     private var relativePathIndex = -1
-    private var dateTakenIndex = -1
 
     override val projection: Array<String>
         get() =
@@ -499,7 +498,6 @@ open class BaseApi29MediaStoreBackend : MediaStoreBackend() {
             volumeIndex = cursor.getColumnIndexOrThrow(MediaStore.Audio.AudioColumns.VOLUME_NAME)
             relativePathIndex =
                 cursor.getColumnIndexOrThrow(MediaStore.Audio.AudioColumns.RELATIVE_PATH)
-            dateTakenIndex = cursor.getColumnIndexOrThrow(MediaStore.Audio.AudioColumns.DATE_TAKEN)
         }
 
         val volumeName = cursor.getString(volumeIndex)
@@ -512,7 +510,6 @@ open class BaseApi29MediaStoreBackend : MediaStoreBackend() {
             audio.dir = Directory(volume, relativePath.removeSuffix(File.separator))
         }
 
-
         return audio
     }
 }

app/src/main/java/org/oxycblt/auxio/util/PrimitiveUtil.kt

@@ -22,6 +22,7 @@ import android.text.format.DateUtils
 import androidx.core.math.MathUtils
 import java.lang.reflect.Field
 import java.lang.reflect.Method
+import java.util.concurrent.CancellationException
 import kotlin.reflect.KClass
 import org.oxycblt.auxio.BuildConfig
 
@@ -77,3 +78,34 @@ fun lazyReflectedField(clazz: KClass<*>, field: String) = lazy {
 fun lazyReflectedMethod(clazz: KClass<*>, method: String) = lazy {
     clazz.java.getDeclaredMethod(method).also { it.isAccessible = true }
 }
+
+/**
+ * A generation-based abstraction that allows cheap cooperative multi-threading in shared object
+ * contexts. Every new task should call [newHandle], while every running task should call [check] or
+ * [yield] depending on the context.
+ *
+ * @author OxygenCobalt
+ */
+class GenerationGuard {
+    private var currentHandle = 0L
+
+    /**
+     * Returns a new handle to the calling task while invalidating the generations of the previous
+     * task.
+     */
+    @Synchronized fun newHandle() = ++currentHandle
+
+    /** Check if the given [handle] is still the one represented by this class. */
+    @Synchronized fun check(handle: Long) = handle == currentHandle
+
+    /**
+     * Alternative to [kotlinx.coroutines.yield], that achieves the same behavior but in a much
+     * cheaper manner.
+     */
+    @Synchronized
+    fun yield(handle: Long) {
+        if (!check(handle)) {
+            throw CancellationException()
+        }
+    }
+}