tell the user when a sync fails

The failure was only recorded as SyncStatus.Failed, which nothing shows, so a
bad username or an unreachable server folded silently. It goes through the same
toast the rest of the data layer uses, with the server's own message.

app/src/main/kotlin/com/subtracks/data/sync/SyncManager.kt

@@ -29,6 +29,7 @@ class SyncManager(
     private val db: SubtracksDatabase,
     private val sourceRepository: SourceRepository,
     private val queueRepository: QueueRepository,
+    private val showMessage: (String) -> Unit = {},
 ) {
     private val scope = CoroutineScope(SupervisorJob() + Dispatchers.Default)
     private val requests = Channel<Unit>(Channel.CONFLATED)
@@ -56,7 +57,9 @@ class SyncManager(
                 throw cancellation
             } catch (failure: Exception) {
                 Log.w(TAG, "Sync failed", failure)
-                SyncStatus.Failed(failure.message ?: "Sync failed")
+                val message = failure.message ?: "unknown error"
+                showMessage("Sync failed: $message")
+                SyncStatus.Failed(message)
             }
         queueRepository.invalidateLibraryCache()
         _status.value = result

app/src/main/kotlin/com/subtracks/di/AppModule.kt

@@ -57,7 +57,7 @@ fun appModule(
     single<ServerActionSink> { NetworkServerActionSink(get()) }
     single { LibraryRepository(get(), get(), get(), toast) }
     single<ArtworkSeedStore> { ArtworkSeedRepository(get()) }
-    single { SyncManager(get(), get(), get()) }
+    single { SyncManager(get(), get(), get(), showMessage = toast) }
     single { QueueRepository(get()) }
     single { ArtworkStore(downloadsRoot(context)) }
     single<ArtworkFetcher> { OkHttpArtworkFetcher(get()) }

app/src/test/kotlin/com/subtracks/data/sync/SyncManagerTest.kt

@@ -21,6 +21,7 @@ import org.junit.Before
 import org.junit.Test
 import org.junit.runner.RunWith
 import java.io.File
+import java.util.concurrent.CopyOnWriteArrayList
 
 @RunWith(AndroidJUnit4::class)
 class SyncManagerTest {
@@ -28,6 +29,7 @@ class SyncManagerTest {
     private lateinit var sourceRepository: SourceRepository
     private lateinit var queueRepository: QueueRepository
     private lateinit var manager: SyncManager
+    private val messages = CopyOnWriteArrayList<String>()
 
     @Before
     fun setUp() {
@@ -39,7 +41,7 @@ class SyncManagerTest {
                 .build()
         sourceRepository = SourceRepository(db, OkHttpClient(), fakeUserPreferences(), ArtworkStore(File(context.cacheDir, "art")))
         queueRepository = QueueRepository(db)
-        manager = SyncManager(db, sourceRepository, queueRepository)
+        manager = SyncManager(db, sourceRepository, queueRepository, showMessage = { messages += it })
     }
 
     @After
@@ -48,6 +50,15 @@ class SyncManagerTest {
         db.close()
     }
 
+    @Test
+    fun aFailedSyncTellsTheUser() =
+        runBlocking {
+            manager.requestSync()
+            withTimeout(10_000) { manager.status.first { it is SyncStatus.Failed } }
+
+            assertEquals(listOf("Sync failed: No server configured"), messages)
+        }
+
     @Test
     fun aFailedSyncStillInvalidatesTheLibraryCache() =
         runBlocking {