address review: test the source wiring, one default, clamp the stored value

- Add a `SourceRepositoryTest` that sets the preference to 1 and proves the
  active source fetches sequentially; removing the `maxConcurrentFetches`
  wiring now fails it.
- `playlistSongsRespectsASmallerFetchBound` now holds the first `bound`
  fetches and asserts no `(bound + 1)`th starts, instead of timing an
  overlap; it fails when the semaphore ignores the bound.
- One `DEFAULT_FETCH_CONCURRENCY` (in `data.source`) backs both the source
  default and the preference default, so they cannot drift.
- Clamp the stored preference in `syncConcurrency()` too, so a bad stored
  value can't make `SubsonicSource` throw on construction.

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

@@ -8,6 +8,7 @@ import androidx.datastore.preferences.core.intPreferencesKey
 import androidx.datastore.preferences.core.stringPreferencesKey
 import androidx.datastore.preferences.preferencesDataStore
 import com.subtracks.data.net.NetworkMode
+import com.subtracks.data.source.DEFAULT_FETCH_CONCURRENCY
 import kotlinx.coroutines.flow.Flow
 import kotlinx.coroutines.flow.first
 import kotlinx.coroutines.flow.map
@@ -43,8 +44,6 @@ 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(
@@ -79,7 +78,7 @@ 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 }
+    fun syncConcurrency(): Flow<Int> = store.data.map { prefs -> (prefs[syncConcurrencyKey] ?: DEFAULT_FETCH_CONCURRENCY).coerceAtLeast(1) }
 
     suspend fun setSyncConcurrency(value: Int) {
         store.edit { prefs -> prefs[syncConcurrencyKey] = value.coerceAtLeast(1) }

app/src/main/kotlin/com/subtracks/data/source/MusicSource.kt

@@ -7,6 +7,8 @@ import com.subtracks.data.model.PlaylistSong
 import com.subtracks.data.model.Song
 import kotlinx.coroutines.flow.Flow
 
+const val DEFAULT_FETCH_CONCURRENCY = 4
+
 interface MusicSource {
     val id: Long
 

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

@@ -5,6 +5,7 @@ import com.subtracks.data.model.Artist
 import com.subtracks.data.model.Playlist
 import com.subtracks.data.model.PlaylistSong
 import com.subtracks.data.model.Song
+import com.subtracks.data.source.DEFAULT_FETCH_CONCURRENCY
 import com.subtracks.data.source.MusicSource
 import com.subtracks.data.source.StarType
 import kotlinx.coroutines.CancellationException
@@ -34,7 +35,7 @@ class SubsonicSource(
     private val maxBitrate: Int = 0,
     private val streamFormat: String? = null,
     private val maxPages: Int = MAX_PAGES,
-    private val maxConcurrentFetches: Int = MAX_CONCURRENT_FETCHES,
+    private val maxConcurrentFetches: Int = DEFAULT_FETCH_CONCURRENCY,
 ) : MusicSource {
     private var emptyQuerySearchSupported: Boolean? = null
 
@@ -306,8 +307,6 @@ class SubsonicSource(
     }
 }
 
-internal const val MAX_CONCURRENT_FETCHES = 4
-
 private class PageCapExceeded(
     message: String,
 ) : SubsonicException(-1, message)

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

@@ -4,11 +4,11 @@ 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
 import com.subtracks.data.repo.SourceRepository
+import com.subtracks.data.source.DEFAULT_FETCH_CONCURRENCY
 import kotlinx.coroutines.flow.SharingStarted
 import kotlinx.coroutines.flow.StateFlow
 import kotlinx.coroutines.flow.stateIn
@@ -38,7 +38,7 @@ class SettingsViewModel(
     val syncConcurrency: StateFlow<Int> =
         userPreferences
             .syncConcurrency()
-            .stateIn(viewModelScope, SharingStarted.WhileSubscribed(5_000), DEFAULT_SYNC_CONCURRENCY)
+            .stateIn(viewModelScope, SharingStarted.WhileSubscribed(5_000), DEFAULT_FETCH_CONCURRENCY)
 
     fun selectSource(id: Long) {
         viewModelScope.launch { sourceRepository.selectSource(id) }