wire the sync fetch concurrency to a Settings option

The bound was only a constructor default with no production consumer. Add a
"Sync concurrency" preference (Settings -> Network, default 4) and pass it
through SourceRepository to SubsonicSource, so sync parallelism is tunable
without a rebuild. Both the app-scoped source and the on-demand
`activeMusicSource()` used by sync read the preference.

- UserPreferences.syncConcurrency()/setSyncConcurrency() (clamped >= 1)
- SourceRepository combines the preference into toMusicSource
- Settings row + dialog; SettingsViewModel state and setter
- Preferences test for default/round-trip/clamp

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

@@ -43,6 +43,8 @@ data class StreamQuality(
     val transcodes: Boolean get() = maxBitrate > 0 || !format.isNullOrEmpty()
 }
 
+const val DEFAULT_SYNC_CONCURRENCY = 4
+
 private val Context.preferences: DataStore<Preferences> by preferencesDataStore("user_prefs")
 
 data class ArtworkSeedValue(
@@ -77,6 +79,14 @@ class UserPreferences(
 
     private fun streamQualityKey(mode: NetworkMode) = stringPreferencesKey("stream_quality_${mode.key}")
 
+    fun syncConcurrency(): Flow<Int> = store.data.map { prefs -> prefs[syncConcurrencyKey] ?: DEFAULT_SYNC_CONCURRENCY }
+
+    suspend fun setSyncConcurrency(value: Int) {
+        store.edit { prefs -> prefs[syncConcurrencyKey] = value.coerceAtLeast(1) }
+    }
+
+    private val syncConcurrencyKey = intPreferencesKey("sync_concurrency")
+
     suspend fun lastSeed(): ArtworkSeedValue? {
         val prefs = store.data.first()
         val cacheKey = prefs[lastSeedKey] ?: return null

app/src/main/kotlin/com/subtracks/data/repo/SourceRepository.kt

@@ -22,6 +22,7 @@ import kotlinx.coroutines.flow.Flow
 import kotlinx.coroutines.flow.MutableStateFlow
 import kotlinx.coroutines.flow.StateFlow
 import kotlinx.coroutines.flow.combine
+import kotlinx.coroutines.flow.first
 import kotlinx.coroutines.flow.flowOf
 import kotlinx.coroutines.launch
 import kotlinx.coroutines.withContext
@@ -55,16 +56,18 @@ class SourceRepository(
                 networkMode,
                 prefs.streamQuality(NetworkMode.Wifi),
                 prefs.streamQuality(NetworkMode.Mobile),
-            ) { config, mode, wifi, mobile -> config to (if (mode == NetworkMode.Wifi) wifi else mobile) }
-                .collect { (config, quality) ->
-                    active = config?.toMusicSource(quality)
-                    _quality.value = quality
-                    val sourceChanged = config?.id != activeSourceId
-                    activeSourceId = config?.id
-                    if (sourceChanged && config != null && config.useTokenAuth) {
-                        scope.launch { runCatching { config.toClient().check("ping") } }
-                    }
+                prefs.syncConcurrency(),
+            ) { config, mode, wifi, mobile, concurrency ->
+                Triple(config, if (mode == NetworkMode.Wifi) wifi else mobile, concurrency)
+            }.collect { (config, quality, concurrency) ->
+                active = config?.toMusicSource(quality, concurrency)
+                _quality.value = quality
+                val sourceChanged = config?.id != activeSourceId
+                activeSourceId = config?.id
+                if (sourceChanged && config != null && config.useTokenAuth) {
+                    scope.launch { runCatching { config.toClient().check("ping") } }
                 }
+            }
         }
     }
 
@@ -172,7 +175,7 @@ class SourceRepository(
 
     suspend fun activeMusicSource(): MusicSource? {
         val config = db.sourcesDao().activeSubsonicConfigOnce() ?: return null
-        return config.toMusicSource(quality.value)
+        return config.toMusicSource(quality.value, prefs.syncConcurrency().first())
     }
 
     private fun SubsonicConfig.toClient(): SubsonicClient =
@@ -184,12 +187,16 @@ class SourceRepository(
             onTokenAuthUnsupported = { disableTokenAuth(id) },
         )
 
-    private fun SubsonicConfig.toMusicSource(quality: StreamQuality): SubsonicMusicSource =
+    private fun SubsonicConfig.toMusicSource(
+        quality: StreamQuality,
+        syncConcurrency: Int,
+    ): SubsonicMusicSource =
         SubsonicMusicSource(
             id = id,
             client = toClient(),
             maxBitrate = quality.maxBitrate,
             streamFormat = quality.format,
+            maxConcurrentFetches = syncConcurrency,
         )
 
     private fun disableTokenAuth(sourceId: Long) {

app/src/main/kotlin/com/subtracks/data/source/subsonic/SubsonicSource.kt

@@ -1,6 +1,5 @@
 package com.subtracks.data.source.subsonic
 
-import androidx.annotation.VisibleForTesting
 import com.subtracks.data.model.Album
 import com.subtracks.data.model.Artist
 import com.subtracks.data.model.Playlist
@@ -35,7 +34,6 @@ class SubsonicSource(
     private val maxBitrate: Int = 0,
     private val streamFormat: String? = null,
     private val maxPages: Int = MAX_PAGES,
-    @VisibleForTesting(otherwise = VisibleForTesting.PRIVATE)
     private val maxConcurrentFetches: Int = MAX_CONCURRENT_FETCHES,
 ) : MusicSource {
     private var emptyQuerySearchSupported: Boolean? = null

app/src/main/kotlin/com/subtracks/ui/settings/SettingsScreen.kt

@@ -54,7 +54,7 @@ import org.koin.compose.viewmodel.koinViewModel
 private val bitrateOptions = listOf(0, 24, 32, 64, 96, 128, 192, 256, 320)
 private val streamFormats = listOf(null, "mp3", "opus", "ogg", "webm", "aac", "flac")
 
-private enum class SettingsDialog { WifiQuality, MobileQuality }
+private enum class SettingsDialog { WifiQuality, MobileQuality, SyncConcurrency }
 
 @Composable
 fun SettingsRoute(
@@ -66,15 +66,18 @@ fun SettingsRoute(
     val activeSourceId by viewModel.activeSourceId.collectAsStateWithLifecycle()
     val wifiQuality by viewModel.wifiQuality.collectAsStateWithLifecycle()
     val mobileQuality by viewModel.mobileQuality.collectAsStateWithLifecycle()
+    val syncConcurrency by viewModel.syncConcurrency.collectAsStateWithLifecycle()
     SettingsScreen(
         sources = sources,
         activeSourceId = activeSourceId,
         wifiQuality = wifiQuality,
         mobileQuality = mobileQuality,
+        syncConcurrency = syncConcurrency,
         onSelectSource = viewModel::selectSource,
         onDeleteSource = viewModel::deleteSource,
         onWifiQualityChange = viewModel::setWifiQuality,
         onMobileQualityChange = viewModel::setMobileQuality,
+        onSyncConcurrencyChange = viewModel::setSyncConcurrency,
         onAddServer = onAddServer,
         onBack = onBack,
     )
@@ -87,10 +90,12 @@ fun SettingsScreen(
     activeSourceId: Long?,
     wifiQuality: StreamQuality,
     mobileQuality: StreamQuality,
+    syncConcurrency: Int,
     onSelectSource: (Long) -> Unit,
     onDeleteSource: (Long) -> Unit,
     onWifiQualityChange: (StreamQuality) -> Unit,
     onMobileQualityChange: (StreamQuality) -> Unit,
+    onSyncConcurrencyChange: (Int) -> Unit,
     onAddServer: () -> Unit,
     onBack: () -> Unit,
     modifier: Modifier = Modifier,
@@ -167,6 +172,14 @@ fun SettingsScreen(
                     colors = ListItemDefaults.colors(containerColor = Color.Transparent),
                 )
             }
+            item {
+                ListItem(
+                    headlineContent = { Text("Sync concurrency") },
+                    supportingContent = { Text(concurrencyLabel(syncConcurrency)) },
+                    modifier = Modifier.clickable { dialog = SettingsDialog.SyncConcurrency },
+                    colors = ListItemDefaults.colors(containerColor = Color.Transparent),
+                )
+            }
             item { Spacer(Modifier.height(fill)) }
         }
     }
@@ -190,6 +203,14 @@ fun SettingsScreen(
             )
         }
 
+        SettingsDialog.SyncConcurrency -> {
+            ConcurrencyDialog(
+                selected = syncConcurrency,
+                onSelect = onSyncConcurrencyChange,
+                onDismiss = { dialog = null },
+            )
+        }
+
         null -> {
             Unit
         }
@@ -200,6 +221,42 @@ private fun qualityLabel(quality: StreamQuality): String = "${bitrateLabel(quali
 
 private fun bitrateLabel(kbps: Int): String = if (kbps == 0) "Unlimited" else "${kbps}kbps"
 
+private val syncConcurrencyOptions = listOf(1, 2, 4, 8, 16)
+
+private fun concurrencyLabel(value: Int): String = if (value <= 1) "1 (sequential)" else value.toString()
+
+@Composable
+private fun ConcurrencyDialog(
+    selected: Int,
+    onSelect: (Int) -> Unit,
+    onDismiss: () -> Unit,
+) {
+    var draft by remember(selected) { mutableStateOf(selected) }
+    AlertDialog(
+        onDismissRequest = onDismiss,
+        title = { Text("Sync concurrency") },
+        text = {
+            ChoiceGroup(
+                header = "Parallel server requests while syncing",
+                options = syncConcurrencyOptions.map { it to concurrencyLabel(it) },
+                selected = draft,
+                onSelect = { draft = it },
+            )
+        },
+        confirmButton = {
+            TextButton(
+                onClick = {
+                    onSelect(draft)
+                    onDismiss()
+                },
+            ) { Text("Done") }
+        },
+        dismissButton = {
+            TextButton(onClick = onDismiss) { Text("Cancel") }
+        },
+    )
+}
+
 @Composable
 private fun QualityDialog(
     title: String,

app/src/main/kotlin/com/subtracks/ui/settings/SettingsViewModel.kt

@@ -4,6 +4,7 @@ import androidx.lifecycle.ViewModel
 import androidx.lifecycle.viewModelScope
 import com.subtracks.data.model.Source
 import com.subtracks.data.net.NetworkMode
+import com.subtracks.data.prefs.DEFAULT_SYNC_CONCURRENCY
 import com.subtracks.data.prefs.StreamQuality
 import com.subtracks.data.prefs.UserPreferences
 import com.subtracks.data.repo.DownloadRepository
@@ -34,6 +35,11 @@ class SettingsViewModel(
             .streamQuality(NetworkMode.Mobile)
             .stateIn(viewModelScope, SharingStarted.WhileSubscribed(5_000), StreamQuality())
 
+    val syncConcurrency: StateFlow<Int> =
+        userPreferences
+            .syncConcurrency()
+            .stateIn(viewModelScope, SharingStarted.WhileSubscribed(5_000), DEFAULT_SYNC_CONCURRENCY)
+
     fun selectSource(id: Long) {
         viewModelScope.launch { sourceRepository.selectSource(id) }
     }
@@ -52,4 +58,8 @@ class SettingsViewModel(
     fun setMobileQuality(quality: StreamQuality) {
         viewModelScope.launch { userPreferences.setStreamQuality(NetworkMode.Mobile, quality) }
     }
+
+    fun setSyncConcurrency(value: Int) {
+        viewModelScope.launch { userPreferences.setSyncConcurrency(value) }
+    }
 }

app/src/test/kotlin/com/subtracks/data/prefs/UserPreferencesTest.kt

@@ -58,6 +58,24 @@ class UserPreferencesTest {
             file.delete()
         }
 
+    @Test
+    fun syncConcurrencyDefaultsAndClampsToAtLeastOne() =
+        runTest {
+            val file = File.createTempFile("user-prefs", ".preferences_pb").apply { delete() }
+            val store = PreferenceDataStoreFactory.create(scope = backgroundScope) { file }
+            val prefs = UserPreferences(store)
+
+            assertEquals(DEFAULT_SYNC_CONCURRENCY, prefs.syncConcurrency().first())
+
+            prefs.setSyncConcurrency(8)
+            assertEquals(8, prefs.syncConcurrency().first())
+
+            prefs.setSyncConcurrency(0)
+            assertEquals(1, prefs.syncConcurrency().first())
+
+            file.delete()
+        }
+
     @Test
     fun lastSeedRoundTripsWithAndWithoutASecondary() =
         runTest {

app/src/test/kotlin/com/subtracks/ui/AppScreenshotTest.kt

@@ -306,10 +306,12 @@ class AppScreenshotTest {
                     activeSourceId = 1,
                     wifiQuality = StreamQuality(),
                     mobileQuality = StreamQuality(192, "mp3"),
+                    syncConcurrency = 4,
                     onSelectSource = {},
                     onDeleteSource = {},
                     onWifiQualityChange = {},
                     onMobileQualityChange = {},
+                    onSyncConcurrencyChange = {},
                     onAddServer = {},
                     onBack = {},
                 )