Initial download service startup fixes and range request fix

android/app/build.gradle

@@ -60,6 +60,9 @@ android {
       proguardFiles getDefaultProguardFile('proguard-android.txt'), 'proguard-rules.pro'
     }
   }
+  testOptions {
+    unitTests.returnDefaultValues = true
+  }
 }
 
 repositories {
@@ -81,6 +84,7 @@ configurations.configureEach {
 }
 
 dependencies {
+  testImplementation "junit:junit:$junit_version"
   implementation "androidx.core:core-splashscreen:$coreSplashScreenVersion"
   implementation fileTree(include: ['*.jar'], dir: 'libs')
   implementation "androidx.appcompat:appcompat:$androidxAppCompatVersion"

android/app/src/main/java/com/audiobookshelf/app/managers/DownloadItemManager.kt

@@ -5,6 +5,7 @@ import android.net.Uri
 import android.os.StatFs
 import android.util.Log
 import androidx.documentfile.provider.DocumentFile
+import com.anggrayudi.storage.file.fullName
 import com.audiobookshelf.app.device.DeviceManager
 import com.audiobookshelf.app.device.FolderScanner
 import com.audiobookshelf.app.models.DownloadItem
@@ -22,7 +23,6 @@ import kotlinx.coroutines.SupervisorJob
 import kotlinx.coroutines.cancel
 import kotlinx.coroutines.delay
 import kotlinx.coroutines.launch
-import okhttp3.Call
 
 /** Manages the process-owned queue for app-managed downloads. */
 class DownloadItemManager(
@@ -32,10 +32,11 @@ class DownloadItemManager(
 ) {
   private val tag = "DownloadItemManager"
   private val scope = CoroutineScope(SupervisorJob() + Dispatchers.IO)
-  private val activeCalls = ConcurrentHashMap<String, Call>()
+  private val activeCalls = ConcurrentHashMap<String, InternalDownloadManager.DownloadHandle>()
   private val safFolderLocks = ConcurrentHashMap<String, Any>()
   private val reservations = mutableMapOf<String, Long>()
   private val lastPersistTime = mutableMapOf<String, Long>()
+  private val finalizingItems = mutableSetOf<String>()
   private var watcherRunning = false
   private val jacksonMapper =
           jacksonObjectMapper()
@@ -58,10 +59,6 @@ class DownloadItemManager(
     fun onComplete(failed: Boolean)
   }
 
-  init {
-    IncompleteDownloadCleanup.cleanupExpired(context)
-  }
-
   @Synchronized
   fun setEventEmitter(eventEmitter: DownloadEventEmitter) {
     clientEventEmitter = eventEmitter
@@ -73,6 +70,15 @@ class DownloadItemManager(
   fun restoreQueue() {
     if (downloadItemQueue.isNotEmpty()) return
     DeviceManager.dbManager.getDownloadItems().forEach { item ->
+      item.downloadItemParts.filter { it.moved }.forEach { part ->
+        if (!finalizedFileExists(part)) {
+          Log.w(tag, "Finalized file is missing; resetting ${part.filename}")
+          part.moved = false
+          part.completed = false
+          part.completedDestinationUri = null
+          part.downloadId = null
+        }
+      }
       if (item.isDownloadFinished) {
         downloadItemQueue.add(item)
         checkDownloadItemFinished(item)
@@ -80,19 +86,26 @@ class DownloadItemManager(
       }
       item.downloadItemParts.forEach { part ->
         if (part.moved) return@forEach
-        if (item.terminalFailureAt != null && part.failed) return@forEach
+        if (item.terminalFailureAt != null) {
+          part.downloadId = null
+          part.isMoving = false
+          part.failed = true
+          part.waitingForSpace = false
+          part.bytesDownloaded = File(part.destinationPath).takeIf(File::exists)?.length() ?: 0L
+          return@forEach
+        }
         part.downloadId = null
         part.isMoving = false
         part.failed = false
-        part.completed = false
         part.waitingForSpace = false
-        part.bytesDownloaded = File(part.destinationPath).takeIf(File::exists)?.length() ?: 0L
+        val stagingLength = File(part.destinationPath).takeIf(File::exists)?.length() ?: 0L
+        part.bytesDownloaded = stagingLength
+        if (part.completed && stagingLength <= 0L) part.completed = false
       }
       downloadItemQueue.add(item)
       if (item.terminalFailureAt != null) IncompleteDownloadCleanup.schedule(context, item)
       clientEventEmitter.onDownloadItem(item)
     }
-    checkUpdateDownloadQueue()
     notifyQueueChanged()
   }
 
@@ -100,39 +113,66 @@ class DownloadItemManager(
   fun addDownloadItem(downloadItem: DownloadItem) {
     val existingItem = downloadItemQueue.find { it.id == downloadItem.id }
     if (existingItem != null) {
-      if (existingItem.terminalFailureAt != null) {
-        retryDownloadItem(existingItem)
-        checkUpdateDownloadQueue()
-        notifyQueueChanged()
-      }
       return
     }
     persist(downloadItem, force = true)
     downloadItemQueue.add(downloadItem)
     clientEventEmitter.onDownloadItem(downloadItem)
+    notifyQueueChanged()
+  }
+
+  @Synchronized
+  fun retryDownloadItem(downloadItemId: String): Boolean {
+    val item = downloadItemQueue.find { it.id == downloadItemId } ?: return false
+    if (item.downloadItemParts.any { it in currentDownloadItemParts }) return false
+    if (item.isDownloadFinished) return false
+    synchronized(IncompleteDownloadCleanup) {
+      item.terminalFailureAt = null
+      item.stagingCleanupAt = null
+      IncompleteDownloadCleanup.cancel(context, item.id)
+      item.downloadItemParts.filter { !it.moved }.forEach { part ->
+        part.failed = false
+        part.isMoving = false
+        part.downloadId = null
+        part.retryCount = 0
+        part.waitingForSpace = false
+        val stagingLength = File(part.destinationPath).takeIf(File::exists)?.length() ?: 0L
+        part.bytesDownloaded = stagingLength
+        part.completed = part.completed && stagingLength > 0L
+      }
+      persist(item, force = true)
+    }
+    clientEventEmitter.onDownloadItem(item)
+    notifyQueueChanged()
+    return true
+  }
+
+  @Synchronized
+  fun resumeWork() {
     checkUpdateDownloadQueue()
     notifyQueueChanged()
   }
 
-  private fun retryDownloadItem(item: DownloadItem) {
-    item.terminalFailureAt = null
-    IncompleteDownloadCleanup.cancel(context, item.id)
-    item.downloadItemParts.filter { it.failed }.forEach { part ->
-      part.failed = false
-      part.completed = false
-      part.isMoving = false
-      part.downloadId = null
-      part.retryCount = 0
-    }
-    persist(item, force = true)
-  }
-
   @Synchronized
   fun cancelAll() {
-    activeCalls.values.forEach(Call::cancel)
+    activeCalls.values.forEach(InternalDownloadManager.DownloadHandle::cancel)
     activeCalls.clear()
     downloadItemQueue.forEach { item ->
-      item.downloadItemParts.forEach { part -> File(part.destinationPath).delete() }
+      item.downloadItemParts.forEach { part ->
+        File(part.destinationPath).delete()
+        if (part.moved && part.isInternalStorage) {
+          File(part.finalDestinationPath).delete()
+        } else if (part.moved) {
+          part.completedDestinationUri?.let { uri ->
+            try {
+              DocumentFile.fromSingleUri(context, Uri.parse(uri))?.delete()
+            } catch (e: Exception) {
+              Log.w(tag, "Could not delete cancelled SAF file ${part.filename}", e)
+            }
+          }
+        }
+      }
+      IncompleteDownloadCleanup.cancel(context, item.id)
       DeviceManager.dbManager.removeDownloadItem(item.id)
     }
     currentDownloadItemParts.clear()
@@ -145,14 +185,26 @@ class DownloadItemManager(
   fun hasWork(): Boolean =
           downloadItemQueue.any { item ->
             item.downloadItemParts.any { part ->
-              (!part.completed && !part.failed) || part.isMoving
+              (!part.moved && !part.failed) || part.isMoving
             }
           }
 
   @Synchronized
   private fun checkUpdateDownloadQueue() {
     downloadItemQueue.toList().forEach { item ->
-      val slots = MAX_SIMULTANEOUS_DOWNLOADS - currentDownloadItemParts.size
+      var slots = MAX_SIMULTANEOUS_DOWNLOADS - currentDownloadItemParts.size
+      if (slots <= 0) return@forEach
+      item.downloadItemParts
+              .filter { part ->
+                part.completed && !part.moved && !part.failed && !part.isMoving &&
+                        part !in currentDownloadItemParts && File(part.destinationPath).exists()
+              }
+              .take(slots)
+              .forEach { part ->
+                currentDownloadItemParts.add(part)
+                part.downloadId = APP_MANAGED_DOWNLOAD_ID
+              }
+      slots = MAX_SIMULTANEOUS_DOWNLOADS - currentDownloadItemParts.size
       if (slots <= 0) return@forEach
       item.getNextDownloadItemParts(slots).forEach { part ->
         val existingFile = findSharedStorageFile(part)
@@ -189,7 +241,7 @@ class DownloadItemManager(
             else
                     DeviceManager.getServerConnectionConfig(item.serverConnectionConfigId)?.token
                             ?: DeviceManager.token
-    activeCalls[part.id] =
+    val handle =
             InternalDownloadManager(
                             stagingFile,
                             part.fileSize,
@@ -216,8 +268,10 @@ class DownloadItemManager(
                               }
                             },
                             { hasAvailableSpace(part) }
-                    )
-                    .download(serverUrl(item, part), token)
+                    ).download(serverUrl(item, part), token)
+    if (part in currentDownloadItemParts && !part.completed && !part.failed) {
+      activeCalls[part.id] = handle
+    }
   }
 
   @Synchronized
@@ -277,6 +331,7 @@ class DownloadItemManager(
       part.completed = false
       part.downloadId = null
       item.terminalFailureAt = item.terminalFailureAt ?: System.currentTimeMillis()
+      item.stagingCleanupAt = null
       persist(item, force = true)
       IncompleteDownloadCleanup.schedule(context, item)
       notifyQueueChanged()
@@ -341,13 +396,13 @@ class DownloadItemManager(
         }
         if (temporary.length() != staging.length())
                 throw IllegalStateException("SAF copy size mismatch")
-        val existing = folder.findFile(part.filename)
+        val existing = findDocumentByFilename(folder, part)
         if (existing != null && !existing.delete())
                 throw IllegalStateException("Could not replace existing file")
         if (!temporary.renameTo(part.filename))
                 throw IllegalStateException("Could not finalize SAF temporary file")
         val destination =
-                folder.findFile(part.filename)
+                findDocumentByFilename(folder, part)
                         ?: throw IllegalStateException("Could not reopen finalized SAF file")
         if (destination.length() != staging.length())
                 throw IllegalStateException("SAF final size mismatch")
@@ -380,8 +435,10 @@ class DownloadItemManager(
     checkDownloadItemFinished(item)
   }
 
+  @Synchronized
   private fun checkDownloadItemFinished(item: DownloadItem) {
-    if (!item.isDownloadFinished) return
+    if (!item.isDownloadFinished || !finalizingItems.add(item.id)) return
+    IncompleteDownloadCleanup.cancel(context, item.id)
     scope.launch {
       folderScanner.scanDownloadItem(item) { scanResult ->
         val event =
@@ -397,6 +454,7 @@ class DownloadItemManager(
                 }
         clientEventEmitter.onDownloadItemComplete(event)
         synchronized(this@DownloadItemManager) {
+          finalizingItems.remove(item.id)
           downloadItemQueue.remove(item)
           DeviceManager.dbManager.removeDownloadItem(item.id)
           notifyQueueChanged()
@@ -457,7 +515,7 @@ class DownloadItemManager(
   }
 
   fun destroy() {
-    activeCalls.values.forEach(Call::cancel)
+    activeCalls.values.forEach(InternalDownloadManager.DownloadHandle::cancel)
     activeCalls.clear()
     scope.cancel()
   }
@@ -479,13 +537,44 @@ class DownloadItemManager(
       if (segment == "." || segment == "..") return null
       folder = folder.findFile(segment) ?: return null
     }
-    val file = folder.findFile(part.filename) ?: return null
+    val file = findDocumentByFilename(folder, part) ?: return null
     if (!file.isFile) return null
     if (part.fileSize > 0L && file.length() != part.fileSize) return null
     if (part.fileSize <= 0L && file.length() <= 0L) return null
     return file
   }
 
+  private fun finalizedFileExists(part: DownloadItemPart): Boolean {
+    if (part.isInternalStorage) {
+      val file = File(part.finalDestinationPath)
+      return file.isFile &&
+              if (part.fileSize > 0L) file.length() == part.fileSize else file.length() > 0L
+    }
+    part.completedDestinationUri?.let { uri ->
+      try {
+        val file = DocumentFile.fromSingleUri(context, Uri.parse(uri))
+        if (file?.isFile == true &&
+                        (part.fileSize <= 0L || file.length() == part.fileSize)) return true
+      } catch (e: Exception) {
+        Log.w(tag, "Could not validate SAF file ${part.filename}", e)
+      }
+    }
+    return findSharedStorageFile(part) != null
+  }
+
+  private fun findDocumentByFilename(folder: DocumentFile, part: DownloadItemPart): DocumentFile? {
+    folder.findFile(part.filename)?.let { return it }
+    val expectedBaseName = part.filename.substringBeforeLast('.')
+    return folder.listFiles().firstOrNull { document ->
+      document.name == part.filename ||
+              document.fullName == part.filename ||
+              (part.audioTrack != null && document.isFile &&
+                      (document.name ?: "").substringBeforeLast('.') == expectedBaseName) ||
+              (part.audioTrack != null && document.isFile &&
+                      document.fullName.substringBeforeLast('.') == expectedBaseName)
+    }
+  }
+
   private fun mimeTypeFor(part: DownloadItemPart): String =
           part.audioTrack?.mimeType
                   ?: when (part.ebookFile?.ebookFormat?.lowercase()) {

android/app/src/main/java/com/audiobookshelf/app/managers/DownloadResumePolicy.kt

@@ -0,0 +1,23 @@
+package com.audiobookshelf.app.managers
+
+internal object DownloadResumePolicy {
+  enum class InitialAction { COMPLETE, RESTART, FULL_DOWNLOAD, RANGE_DOWNLOAD }
+
+  fun initialAction(existingBytes: Long, expectedSize: Long): InitialAction =
+          when {
+            expectedSize > 0L && existingBytes == expectedSize -> InitialAction.COMPLETE
+            expectedSize > 0L && existingBytes > expectedSize -> InitialAction.RESTART
+            existingBytes > 0L -> InitialAction.RANGE_DOWNLOAD
+            else -> InitialAction.FULL_DOWNLOAD
+          }
+
+  fun unsatisfiedRangeSize(contentRange: String?): Long? {
+    if (contentRange == null) return null
+    return UNSATISFIED_CONTENT_RANGE.matchEntire(contentRange)
+            ?.groupValues
+            ?.get(1)
+            ?.toLongOrNull()
+  }
+
+  private val UNSATISFIED_CONTENT_RANGE = Regex("bytes \\*/(\\d+)")
+}

android/app/src/main/java/com/audiobookshelf/app/managers/IncompleteDownloadCleanup.kt

@@ -1,9 +1,7 @@
 package com.audiobookshelf.app.managers
 
 import android.content.Context
-import android.net.Uri
 import android.util.Log
-import androidx.documentfile.provider.DocumentFile
 import androidx.work.ExistingWorkPolicy
 import androidx.work.OneTimeWorkRequestBuilder
 import androidx.work.WorkManager
@@ -14,7 +12,7 @@ import com.audiobookshelf.app.models.DownloadItem
 import java.io.File
 import java.util.concurrent.TimeUnit
 
-/** Removes terminally failed downloads after their retention window elapses. */
+/** Removes only staging data for terminally failed downloads after their retention window. */
 object IncompleteDownloadCleanup {
   private const val tag = "IncompleteDownloadCleanup"
   private const val RETENTION_MS = 24L * 60L * 60L * 1000L
@@ -22,6 +20,7 @@ object IncompleteDownloadCleanup {
 
   fun schedule(context: Context, item: DownloadItem) {
     val failedAt = item.terminalFailureAt ?: return
+    if (item.stagingCleanupAt != null) return
     val delay = (failedAt + RETENTION_MS - System.currentTimeMillis()).coerceAtLeast(0L)
     val request = OneTimeWorkRequestBuilder<IncompleteDownloadCleanupWorker>()
             .setInitialDelay(delay, TimeUnit.MILLISECONDS)
@@ -34,7 +33,8 @@ object IncompleteDownloadCleanup {
     WorkManager.getInstance(context).cancelUniqueWork(WORK_PREFIX + itemId)
   }
 
-  /** Removes failures retained longer than 24 hours when scheduled work did not run. */
+  /** Cleans staging data retained longer than 24 hours when scheduled work did not run. */
+  @Synchronized
   fun cleanupExpired(context: Context) {
     val now = System.currentTimeMillis()
     DeviceManager.dbManager.getDownloadItems()
@@ -46,6 +46,7 @@ object IncompleteDownloadCleanup {
 
   private fun isEligible(item: DownloadItem, now: Long): Boolean {
     val failedAt = item.terminalFailureAt ?: return false
+    if (item.stagingCleanupAt != null) return false
     if (now - failedAt < RETENTION_MS) return false
     return item.downloadItemParts.all { part ->
       part.moved || (part.failed && !part.isMoving)
@@ -55,21 +56,15 @@ object IncompleteDownloadCleanup {
   private fun deleteItem(context: Context, item: DownloadItem) {
     item.downloadItemParts.forEach { part ->
       deleteAppOwnedFile(context, File(part.destinationPath))
-      if (part.isInternalStorage && part.moved) {
-        deleteAppOwnedFile(context, File(part.finalDestinationPath))
-      } else if (!part.isInternalStorage && part.moved) {
-        part.completedDestinationUri?.let { uriString ->
-          try {
-            DocumentFile.fromSingleUri(context, Uri.parse(uriString))?.delete()
-          } catch (e: Exception) {
-            Log.w(tag, "Could not delete expired SAF document for ${part.filename}", e)
-          }
-        }
+      if (!part.moved) {
+        part.bytesDownloaded = 0L
+        part.completed = false
       }
     }
-    DeviceManager.dbManager.removeDownloadItem(item.id)
+    item.stagingCleanupAt = System.currentTimeMillis()
+    DeviceManager.dbManager.saveDownloadItem(item)
     cancel(context, item.id)
-    Log.i(tag, "Deleted terminally failed download item ${item.id}")
+    Log.i(tag, "Deleted staging files for terminally failed download item ${item.id}")
   }
 
   private fun deleteAppOwnedFile(context: Context, file: File) {

android/app/src/main/java/com/audiobookshelf/app/managers/InternalDownloadManager.kt

@@ -4,6 +4,8 @@ import android.util.Log
 import java.io.File
 import java.io.FileOutputStream
 import java.io.IOException
+import java.util.concurrent.atomic.AtomicBoolean
+import java.util.concurrent.atomic.AtomicReference
 import java.util.concurrent.TimeUnit
 import okhttp3.Call
 import okhttp3.Callback
@@ -19,16 +21,62 @@ class InternalDownloadManager(
         private val hasAvailableSpace: () -> Boolean
 ) {
   private val tag = "InternalDownloadManager"
+
+  interface DownloadHandle {
+    fun cancel()
+  }
+
+  private class ActiveDownloadHandle : DownloadHandle {
+    private val cancelled = AtomicBoolean(false)
+    private val activeCall = AtomicReference<Call?>()
+
+    fun setCall(call: Call) {
+      activeCall.set(call)
+      if (cancelled.get()) call.cancel()
+    }
+
+    override fun cancel() {
+      cancelled.set(true)
+      activeCall.get()?.cancel()
+    }
+  }
   /**
    * Starts or resumes a download.
    *
    * @param url download URL
    * @param token access token sent in the Authorization header
-   * @return active call, used to cancel a stalled transfer
+   * @return logical handle used to cancel the active request, including a restarted request
    */
-  fun download(url: String, token: String): Call {
+  fun download(url: String, token: String): DownloadHandle {
     destinationFile.parentFile?.mkdirs()
-    val existingBytes = destinationFile.takeIf { it.exists() }?.length() ?: 0L
+    val handle = ActiveDownloadHandle()
+    startRequest(url, token, handle, allowRestart = true)
+    return handle
+  }
+
+  private fun startRequest(
+          url: String,
+          token: String,
+          handle: ActiveDownloadHandle,
+          allowRestart: Boolean
+  ) {
+    var existingBytes = destinationFile.takeIf { it.exists() }?.length() ?: 0L
+    when (DownloadResumePolicy.initialAction(existingBytes, expectedSize)) {
+      DownloadResumePolicy.InitialAction.COMPLETE -> {
+        progressCallback.onProgress(existingBytes, 100L)
+        progressCallback.onComplete(false)
+        return
+      }
+      DownloadResumePolicy.InitialAction.RESTART -> {
+        if (!destinationFile.delete()) {
+          Log.e(tag, "Could not delete oversized staging file ${destinationFile.name}")
+          progressCallback.onComplete(true)
+          return
+        }
+        existingBytes = 0L
+      }
+      else -> Unit
+    }
     val request =
             Request.Builder()
                     .url(url)
@@ -37,6 +85,7 @@ class InternalDownloadManager(
                     .apply { if (existingBytes > 0L) header("Range", "bytes=$existingBytes-") }
                     .build()
     val call = client.newCall(request)
+    handle.setCall(call)
     call.enqueue(
             object : Callback {
               override fun onFailure(call: Call, e: IOException) {
@@ -47,10 +96,20 @@ class InternalDownloadManager(
               override fun onResponse(call: Call, response: Response) {
                 response.use {
                   try {
-                    if (response.code == 416 && expectedSize > 0L && existingBytes == expectedSize
-                    ) {
-                      progressCallback.onProgress(existingBytes, 100L)
-                      progressCallback.onComplete(false)
+                    if (response.code == 416) {
+                      val serverSize =
+                              DownloadResumePolicy.unsatisfiedRangeSize(
+                                      response.header("Content-Range"))
+                      if (serverSize != null && serverSize > 0L && existingBytes == serverSize) {
+                        progressCallback.onProgress(existingBytes, 100L)
+                        progressCallback.onComplete(false)
+                      } else if (allowRestart && destinationFile.delete()) {
+                        Log.w(tag, "Restarting stale range from byte zero")
+                        startRequest(url, token, handle, allowRestart = false)
+                      } else {
+                        Log.e(tag, "Could not recover invalid range at offset $existingBytes")
+                        progressCallback.onComplete(true)
+                      }
                       return
                     }
                     val append =
@@ -112,7 +171,6 @@ class InternalDownloadManager(
               }
             }
     )
-    return call
   }
 
   private fun hasExpectedRange(response: Response, offset: Long): Boolean {

android/app/src/main/java/com/audiobookshelf/app/models/DownloadItem.kt

@@ -20,7 +20,8 @@ data class DownloadItem(
         val itemSubfolder: String,
         val media: MediaType,
         val downloadItemParts: MutableList<DownloadItemPart>,
-        @JsonIgnore var terminalFailureAt: Long? = null
+        @JsonIgnore var terminalFailureAt: Long? = null,
+        @JsonIgnore var stagingCleanupAt: Long? = null
 ) {
   @get:JsonIgnore
   val isInternalStorage
@@ -28,7 +29,9 @@ data class DownloadItem(
 
   @get:JsonIgnore
   val isDownloadFinished
-    get() = !downloadItemParts.any { !it.completed || it.isMoving || it.failed }
+    get() = downloadItemParts.isNotEmpty() && downloadItemParts.all {
+      it.completed && it.moved && !it.isMoving && !it.failed
+    }
 
   @JsonIgnore
   fun getNextDownloadItemParts(limit: Int): MutableList<DownloadItemPart> {

android/app/src/main/java/com/audiobookshelf/app/plugins/AbsDownloader.kt

@@ -87,54 +87,57 @@ class AbsDownloader : Plugin() {
     Log.d(tag, "Download library item $libraryItemId to folder $localFolderId / episode: $episodeId")
 
     val downloadId = if (episodeId.isEmpty()) libraryItemId else "$libraryItemId-$episodeId"
-    if (downloadItemManager.downloadItemQueue.find { it.id == downloadId } != null) {
-      Log.d(tag, "Download already started for this media entity $downloadId")
-      return call.resolve(JSObject("{\"error\":\"Download already started for this media entity\"}"))
-    }
-
-    apiHandler.getLibraryItemWithProgress(libraryItemId, episodeId) { libraryItem ->
-      if (libraryItem == null) {
-        call.resolve(JSObject("{\"error\":\"Server request failed\"}"))
-      } else {
-        Log.d(tag, "Got library item from server ${libraryItem.id}")
-
-        if (localFolderId == "") {
-          localFolderId = "internal-${libraryItem.mediaType}"
+    DownloadServiceHost.retryExisting(mainActivity, downloadId) { result ->
+      when (result) {
+        DownloadServiceHost.ExistingDownloadResult.RETRIED -> call.resolve()
+        DownloadServiceHost.ExistingDownloadResult.ACTIVE -> {
+          Log.d(tag, "Download already started for this media entity $downloadId")
+          call.resolve(JSObject("{\"error\":\"Download already started for this media entity\"}"))
         }
-        var localFolder = DeviceManager.dbManager.getLocalFolder(localFolderId)
-
-        if (localFolder == null && localFolderId.startsWith("internal-")) {
-          Log.d(tag, "Creating new App Storage internal LocalFolder $localFolderId")
-          localFolder = LocalFolder(localFolderId, "Internal App Storage", "", "", "", "internal", libraryItem.mediaType)
-          DeviceManager.dbManager.saveLocalFolder(localFolder)
-        }
-
-        if (localFolder != null) {
-          if (episodeId.isNotEmpty() && libraryItem.mediaType != "podcast") {
-            Log.e(tag, "Library item is not a podcast but episode was requested")
-            call.resolve(JSObject("{\"error\":\"Invalid library item not a podcast\"}"))
-          } else if (episodeId.isNotEmpty()) {
-            val podcast = libraryItem.media as Podcast
-            val episode = podcast.episodes?.find { podcastEpisode ->
-              podcastEpisode.id == episodeId
-            }
-            if (episode == null) {
-              call.resolve(JSObject("{\"error\":\"Invalid podcast episode not found\"}"))
+        DownloadServiceHost.ExistingDownloadResult.SERVICE_START_FAILED ->
+                call.resolve(JSObject("{\"error\":\"Unable to start the Android download service\"}"))
+        DownloadServiceHost.ExistingDownloadResult.NOT_FOUND -> {
+          apiHandler.getLibraryItemWithProgress(libraryItemId, episodeId) { libraryItem ->
+            if (libraryItem == null) {
+              call.resolve(JSObject("{\"error\":\"Server request failed\"}"))
             } else {
-              startLibraryItemDownload(libraryItem, localFolder, episode)
-              call.resolve()
+              Log.d(tag, "Got library item from server ${libraryItem.id}")
+
+              if (localFolderId == "") localFolderId = "internal-${libraryItem.mediaType}"
+              var localFolder = DeviceManager.dbManager.getLocalFolder(localFolderId)
+              if (localFolder == null && localFolderId.startsWith("internal-")) {
+                Log.d(tag, "Creating new App Storage internal LocalFolder $localFolderId")
+                localFolder = LocalFolder(localFolderId, "Internal App Storage", "", "", "", "internal", libraryItem.mediaType)
+                DeviceManager.dbManager.saveLocalFolder(localFolder)
+              }
+
+              if (localFolder == null) {
+                call.resolve(JSObject("{\"error\":\"Local Folder Not Found\"}"))
+              } else if (episodeId.isNotEmpty() && libraryItem.mediaType != "podcast") {
+                call.resolve(JSObject("{\"error\":\"Invalid library item not a podcast\"}"))
+              } else if (episodeId.isNotEmpty()) {
+                val podcast = libraryItem.media as Podcast
+                val episode = podcast.episodes?.find { it.id == episodeId }
+                if (episode == null) {
+                  call.resolve(JSObject("{\"error\":\"Invalid podcast episode not found\"}"))
+                } else {
+                  startLibraryItemDownload(libraryItem, localFolder, episode) { error -> resolveDownloadCall(call, error) }
+                }
+              } else {
+                startLibraryItemDownload(libraryItem, localFolder, null) { error -> resolveDownloadCall(call, error) }
+              }
             }
-          } else {
-            startLibraryItemDownload(libraryItem, localFolder, null)
-            call.resolve()
           }
-        } else {
-          call.resolve(JSObject("{\"error\":\"Local Folder Not Found\"}"))
         }
       }
     }
   }
 
+  private fun resolveDownloadCall(call: PluginCall, error: String?) {
+    if (error == null) call.resolve()
+    else call.resolve(JSObject().put("error", error))
+  }
+
   // Item filenames could be the same if they are in sub-folders, this will make them unique
   private fun getFilenameFromRelPath(relPath: String): String {
     var cleanedRelPath = relPath.replace("\\", "_").replace("/", "_")
@@ -155,7 +158,12 @@ class AbsDownloader : Plugin() {
     return newTitle
   }
 
-  private fun startLibraryItemDownload(libraryItem: LibraryItem, localFolder: LocalFolder, episode:PodcastEpisode?) {
+  private fun startLibraryItemDownload(
+          libraryItem: LibraryItem,
+          localFolder: LocalFolder,
+          episode: PodcastEpisode?,
+          callback: (String?) -> Unit
+  ) {
     val isInternal = localFolder.id.startsWith("internal-")
 
     val finalInternalFolderPath = "${mainActivity.filesDir}/downloads/${libraryItem.id}"
@@ -224,8 +232,8 @@ class AbsDownloader : Plugin() {
           downloadItem.downloadItemParts.add(downloadItemPart)
         }
 
-        DownloadServiceHost.enqueue(mainActivity, downloadItem)
-      }
+        DownloadServiceHost.enqueue(mainActivity, downloadItem, callback)
+      } else callback("No downloadable files found")
     } else {
       // Podcast episode download
       val podcastTitle = cleanStringForFileSystem(libraryItem.media.metadata.title)
@@ -262,7 +270,7 @@ class AbsDownloader : Plugin() {
         downloadItem.downloadItemParts.add(downloadItemPart)
       }
 
-        DownloadServiceHost.enqueue(mainActivity, downloadItem)
+        DownloadServiceHost.enqueue(mainActivity, downloadItem, callback)
     }
   }
 }

android/app/src/main/java/com/audiobookshelf/app/services/DownloadService.kt

@@ -28,7 +28,7 @@ class DownloadService : Service() {
       ACTION_CANCEL -> DownloadServiceHost.cancelAll(this)
       else -> {
         startForegroundWithType(DownloadServiceHost.notificationStrings(this).preparing)
-        DownloadServiceHost.ensure(this)
+        DownloadServiceHost.startWork(this)
       }
     }
     return START_STICKY

android/app/src/main/java/com/audiobookshelf/app/services/DownloadServiceHost.kt

@@ -1,16 +1,25 @@
 package com.audiobookshelf.app.services
 
 import android.content.Context
+import android.util.Log
 import androidx.core.content.ContextCompat
 import com.audiobookshelf.app.device.FolderScanner
 import com.audiobookshelf.app.managers.DbManager
 import com.audiobookshelf.app.managers.DownloadItemManager
+import com.audiobookshelf.app.managers.IncompleteDownloadCleanup
 import com.audiobookshelf.app.models.DownloadItem
 import com.getcapacitor.JSObject
 import java.util.Collections
+import kotlinx.coroutines.CoroutineScope
+import kotlinx.coroutines.Dispatchers
+import kotlinx.coroutines.Job
+import kotlinx.coroutines.SupervisorJob
+import kotlinx.coroutines.launch
 
 /** Shared process owner used by the foreground service and the Capacitor bridge. */
 object DownloadServiceHost {
+  enum class ExistingDownloadResult { NOT_FOUND, ACTIVE, RETRIED, SERVICE_START_FAILED }
+
   data class NotificationStrings(
           val preparing: String,
           val downloadingFile: String,
@@ -21,9 +30,11 @@ object DownloadServiceHost {
 
   private var manager: DownloadItemManager? = null
   private var bridgeEmitter: DownloadItemManager.DownloadEventEmitter = NoopEmitter
-  private var service: DownloadService? = null
+  @Volatile private var service: DownloadService? = null
   @Volatile private var bridgeReady = false
   private val deferredCompletions = Collections.synchronizedList(mutableListOf<JSObject>())
+  private val scope = CoroutineScope(SupervisorJob() + Dispatchers.IO)
+  private var restoreJob: Job? = null
 
   @Synchronized
   fun ensure(context: Context): DownloadItemManager {
@@ -31,7 +42,11 @@ object DownloadServiceHost {
       val appContext = context.applicationContext
       DbManager.initialize(appContext)
       manager = DownloadItemManager(FolderScanner(appContext), appContext, ForwardingEmitter)
-      manager!!.restoreQueue()
+      restoreJob = scope.launch {
+        IncompleteDownloadCleanup.cleanupExpired(appContext)
+        manager!!.restoreQueue()
+        onRestoreComplete(appContext)
+      }
     }
     return manager!!
   }
@@ -41,14 +56,12 @@ object DownloadServiceHost {
   fun attachBridge(context: Context, emitter: DownloadItemManager.DownloadEventEmitter) {
     bridgeReady = false
     bridgeEmitter = emitter
-    val queue = ensure(context)
-    queue.setEventEmitter(ForwardingEmitter)
+    ensure(context).setEventEmitter(ForwardingEmitter)
     bridgeReady = true
     val completions = synchronized(deferredCompletions) {
       deferredCompletions.toList().also { deferredCompletions.clear() }
     }
     completions.forEach(bridgeEmitter::onDownloadItemComplete)
-    if (queue.hasWork()) startService(context)
   }
 
   @Synchronized
@@ -57,14 +70,44 @@ object DownloadServiceHost {
     bridgeEmitter = NoopEmitter
   }
 
-  @Synchronized
-  fun enqueue(context: Context, item: DownloadItem) {
-    ensure(context).addDownloadItem(item)
-    startService(context)
+  fun enqueue(context: Context, item: DownloadItem, callback: (String?) -> Unit) {
+    val queue = ensure(context)
+    scope.launch {
+      restoreJob?.join()
+      queue.addDownloadItem(item)
+      if (startService(context)) callback(null)
+      else callback("Unable to start the Android download service")
+    }
   }
 
-  @Synchronized
-  fun cancelAll(context: Context) { ensure(context).cancelAll() }
+  fun retryExisting(
+          context: Context,
+          downloadItemId: String,
+          callback: (ExistingDownloadResult) -> Unit
+  ) {
+    val queue = ensure(context)
+    scope.launch {
+      restoreJob?.join()
+      val existing = queue.downloadItemQueue.find { it.id == downloadItemId }
+      if (existing == null) {
+        callback(ExistingDownloadResult.NOT_FOUND)
+      } else if (!queue.retryDownloadItem(downloadItemId)) {
+        callback(ExistingDownloadResult.ACTIVE)
+      } else if (startService(context)) {
+        callback(ExistingDownloadResult.RETRIED)
+      } else {
+        callback(ExistingDownloadResult.SERVICE_START_FAILED)
+      }
+    }
+  }
+
+  fun cancelAll(context: Context) {
+    val queue = ensure(context)
+    scope.launch {
+      restoreJob?.join()
+      queue.cancelAll()
+    }
+  }
 
   fun setNotificationStrings(
           context: Context,
@@ -96,10 +139,9 @@ object DownloadServiceHost {
             preferences.getString(KEY_CANCEL, DEFAULT_CANCEL) ?: DEFAULT_CANCEL)
   }
 
-  @Synchronized
   fun attachService(downloadService: DownloadService) {
-    service = downloadService
-    service?.onQueueChanged(ensure(downloadService).hasWork())
+    synchronized(this) { service = downloadService }
+    startWork(downloadService)
   }
 
   @Synchronized
@@ -107,8 +149,31 @@ object DownloadServiceHost {
     if (service === downloadService) service = null
   }
 
-  private fun startService(context: Context) {
-    ContextCompat.startForegroundService(context, DownloadService.intent(context))
+  fun startWork(context: Context) {
+    val queue = ensure(context)
+    scope.launch {
+      restoreJob?.join()
+      queue.resumeWork()
+    }
+  }
+
+  private fun onRestoreComplete(context: Context) {
+    val attachedService = synchronized(this) { service }
+    if (attachedService != null) {
+      manager?.resumeWork()
+    } else if (bridgeReady && manager?.hasWork() == true) {
+      startService(context)
+    }
+  }
+
+  private fun startService(context: Context): Boolean {
+    return try {
+      ContextCompat.startForegroundService(context, DownloadService.intent(context))
+      true
+    } catch (e: RuntimeException) {
+      Log.e(TAG, "Could not start download foreground service", e)
+      false
+    }
   }
 
   private object ForwardingEmitter : DownloadItemManager.DownloadEventEmitter {
@@ -144,4 +209,5 @@ object DownloadServiceHost {
   private const val DEFAULT_WAITING_FOR_STORAGE = "Waiting for available storage"
   private const val DEFAULT_DOWNLOADS = "Downloads"
   private const val DEFAULT_CANCEL = "Cancel"
+  private const val TAG = "DownloadServiceHost"
 }