fix(server): race condition and cache corruption in playback routes

Two related bugs in ServerPlaybackRoutes:

1. _getSourcedTrack looked up the requested media in
   audioPlayer.playlist.medias by URI, but MPV issues its HTTP request
   before media_kit's Dart-side state has synced with the native
   player, so the lookup threw "Bad state: No element" and the proxy
   returned 500 ("Failed to open" in MPV). Now resolves the track
   directly from playlist.tracks by trackId via firstWhereOrNull,
   independent of the player's media list.

2. MPV opens multiple concurrent range requests for the same track
   (buffering/seek/reconnect), each opening its own writeOnlyAppend
   sink to the same .part cache file; the final rename would then fail
   with a file-in-use error (or corrupt the cache from concurrent
   appends). Added a _cachingInProgress guard so only one request
   caches a given track at a time, cache only full requests
   (contentRange.start == 0), write with truncate instead of append,
   and clean up the in-progress marker/partial file in finally.

Fixes #3089

lib/provider/server/routes/playback.dart

@@ -2,6 +2,7 @@ import 'dart:async';
 import 'dart:io';
 import 'dart:math';
 
+import 'package:collection/collection.dart';
 import 'package:dio/dio.dart' hide Response;
 import 'package:dio/dio.dart' as dio_lib;
 import 'package:flutter/foundation.dart';
@@ -42,6 +43,13 @@ class ServerPlaybackRoutes {
   AudioPlayerState get playlist => ref.read(audioPlayerProvider);
   final Dio dio;
 
+  /// Track IDs that are currently being written to the music cache.
+  /// MediaKit/MPV opens several concurrent range requests for the same track
+  /// (buffering, seeking, reconnects); without this guard every request would
+  /// open its own sink to the same `.part` file and the final `rename` would
+  /// fail with a file-in-use error on Windows.
+  final Set<String> _cachingInProgress = {};
+
   ServerPlaybackRoutes(this.ref) : dio = Dio();
 
   Future<String> _getTrackCacheFilePath(SourcedTrack track) async {
@@ -53,25 +61,19 @@ class ServerPlaybackRoutes {
     );
   }
 
-  Future<SourcedTrack?> _getSourcedTrack(
+  Future<SourcedTrack?> _getSourcedTrack(String trackId) async {
-    Request request,
+    final track = playlist.tracks
-    String trackId,
+        .firstWhereOrNull((element) => element.id == trackId);
-  ) async {
+
-    final track =
+    if (track == null) return null;
-        playlist.tracks.firstWhere((element) => element.id == trackId);
 
     final activeSourcedTrack =
         await ref.read(activeTrackSourcesProvider.future);
 
-    final media = audioPlayer.playlist.medias
-        .firstWhere((e) => e.uri == request.requestedUri.toString());
-    final spotubeMedia =
-        media is SpotubeMedia ? media : SpotubeMedia.media(media);
     final sourcedTrack = activeSourcedTrack?.track.id == track.id
         ? activeSourcedTrack?.source
         : await ref.read(
-            sourcedTrackProvider(spotubeMedia.track as SpotubeFullTrackObject)
+            sourcedTrackProvider(track as SpotubeFullTrackObject).future,
-                .future,
           );
 
     return sourcedTrack;
@@ -103,11 +105,16 @@ class ServerPlaybackRoutes {
       );
     }
 
-    String url = track.url ??
+    final swappedUrl0 = track.url ??
         await ref
             .read(sourcedTrackProvider(track.query).notifier)
             .swapWithNextSibling()
-            .then((track) => track.url!);
+            .then((t) => t.url);
+    if (swappedUrl0 == null) {
+      AppLogger.log.e("No playable URL for ${track.query.name}");
+      throw Exception("No playable URL for ${track.query.name}");
+    }
+    String url = swappedUrl0;
 
     final options = Options(
       headers: {
@@ -156,11 +163,16 @@ class ServerPlaybackRoutes {
       );
     }
 
-    String url = track.url ??
+    final swappedUrl1 = track.url ??
         await ref
             .read(sourcedTrackProvider(track.query).notifier)
             .swapWithNextSibling()
-            .then((track) => track.url!);
+            .then((t) => t.url);
+    if (swappedUrl1 == null) {
+      AppLogger.log.e("No playable URL for ${track.query.name}");
+      throw Exception("No playable URL for ${track.query.name}");
+    }
+    String url = swappedUrl1;
 
     final options = Options(
       headers: {
@@ -214,9 +226,20 @@ class ServerPlaybackRoutes {
       "Headers: ${res.headers.map}",
     );
 
-    if (!userPreferences.cacheMusic) {
+    final contentRange = res.headers.value("content-range") != null
+        ? ContentRangeHeader.parse(res.headers.value("content-range") ?? "")
+        : ContentRangeHeader(0, 0, 0);
+
+    // Only cache full requests (range starting at 0) and only one writer per
+    // track at a time. Concurrent range requests just stream straight through.
+    final trackId = track.info.id;
+    final isFullRequest = contentRange.start == 0;
+    if (!userPreferences.cacheMusic ||
+        !isFullRequest ||
+        _cachingInProgress.contains(trackId)) {
       return res;
     }
+    _cachingInProgress.add(trackId);
 
     final resStream = res.data!.stream.asBroadcastStream();
 
@@ -227,23 +250,25 @@ class ServerPlaybackRoutes {
 
     // Write the stream to the file based on the range
     final partialCacheFileSink =
-        trackPartialCacheFile.openWrite(mode: FileMode.writeOnlyAppend);
+        trackPartialCacheFile.openWrite(mode: FileMode.writeOnly);
-    final contentRange = res.headers.value("content-range") != null
-        ? ContentRangeHeader.parse(res.headers.value("content-range") ?? "")
-        : ContentRangeHeader(0, 0, 0);
 
     resStream.listen(
       (data) {
         partialCacheFileSink.add(data);
       },
-      onError: (e, stack) {
+      onError: (e, stack) async {
-        partialCacheFileSink.close();
+        await partialCacheFileSink.close();
+        _cachingInProgress.remove(trackId);
       },
       onDone: () async {
+        try {
           await partialCacheFileSink.close();
 
           final fileLength = await trackPartialCacheFile.length();
-        if (fileLength != contentRange.total) return;
+          if (fileLength != contentRange.total) {
+            await trackPartialCacheFile.delete().catchError((_) => trackPartialCacheFile);
+            return;
+          }
 
           await trackPartialCacheFile.rename(trackCacheFile.path);
 
@@ -265,6 +290,11 @@ class ServerPlaybackRoutes {
           ).catchError((e, stackTrace) {
             AppLogger.reportError(e, stackTrace);
           });
+        } catch (e, stack) {
+          AppLogger.reportError(e, stack);
+        } finally {
+          _cachingInProgress.remove(trackId);
+        }
       },
       cancelOnError: true,
     );
@@ -277,7 +307,7 @@ class ServerPlaybackRoutes {
   /// @head('/stream/<trackId>')
   Future<Response> headStreamTrackId(Request request, String trackId) async {
     try {
-      final sourcedTrack = await _getSourcedTrack(request, trackId);
+      final sourcedTrack = await _getSourcedTrack(trackId);
 
       if (sourcedTrack == null) {
         return Response.notFound("Track not found in the current queue");
@@ -301,7 +331,7 @@ class ServerPlaybackRoutes {
   /// @get('/stream/<trackId>')
   Future<Response> getStreamTrackId(Request request, String trackId) async {
     try {
-      final sourcedTrack = await _getSourcedTrack(request, trackId);
+      final sourcedTrack = await _getSourcedTrack(trackId);
 
       if (sourcedTrack == null) {
         return Response.notFound("Track not found in the current queue");