fix(playback): 修复切歌永久假死，并让元数据抓取不再抢播放带宽

问题（后台播一段时间后无法缓冲，点下一首也没反应，只能重启）：
1. _isChangingTrack 是没有超时的锁：_playCurrent() 置 true 后，
   只有走完 finally 才会清。mpv 的 open() 在弱网下可能挂很久，
   锁就一直不释放，之后 next()/previous() 全被静默拒绝。
2. 元数据用 dio.download 整文件下载，和 mpv 流式播放抢带宽，
   20Mbps FRP 下播放流只剩一两百 K/s，缓冲长期为 0。
3. 没有订阅 mpv 的 error/buffering，失败无感知、无法自愈。

改动：
- playback_service: open 20s / 控制命令 5s 超时；失败时 stop 回空闲并上抛异常
- audio_service:
  * 去掉「正在切歌就拒绝」的逻辑，改为 latest-wins（epoch 校验，
    旧请求自行放弃，只有最新请求解锁）
  * 播放失败不再吞掉：记录 lastPlaybackError 并 notifyListeners
  * 订阅 stream.error / stream.buffering
  * 缓冲超过 30s 自动重连当前歌曲并 seek 回原位置（2 分钟内最多一次）
- file_provider: 元数据改为 Range 只取头部 2MB，流式写入并在切歌时取消上一请求
- home_page: 去掉重复的 PlaybackService().play()（同一首歌会被 open 两次），
  登录校验提前

验证：dart analyze 无 error；gradlew assembleDebug BUILD SUCCESSFUL。
真机验证待设备重新连接后进行。

lib/metadata/file_provider.dart

@@ -1,6 +1,6 @@
 // lib/metadata/file_provider.dart
 import 'dart:io';
-import 'package:flutter/foundation.dart'; // ⭐ 添加
+import 'package:flutter/foundation.dart';
 import 'package:path_provider/path_provider.dart';
 import 'package:dio/dio.dart';
 import '../services/webdav_service.dart';
@@ -11,46 +11,107 @@ abstract class FileProvider {
 }
 
 class WebDAVFileProvider implements FileProvider {
+  /// 只为读标签取文件头部，绝不整文件下载。
+  /// 之前用 dio.download 会把整首歌拉到临时目录，和 mpv 的流式播放抢带宽，
+  /// 20Mbps 的 FRP 链路上尤其明显（表现为流量很高但缓冲一直为 0）。
+  static const int _maxBytes = 2 * 1024 * 1024;
+
+  /// 切歌时取消上一次未完成的下载，避免多个下载叠加占满带宽。
+  static CancelToken? _activeToken;
+
   final Dio _dio = Dio();
 
   @override
   Future<File?> getFile(String url) async {
     try {
-      // 1. 获取缓存目录
       final cacheDir = await getTemporaryDirectory();
-      final cachePath = '${cacheDir.path}/metadata_${songKeyForUrl(url)}.tmp';
+      final cachePath = '${cacheDir.path}/metadata_${songKeyForUrl(url)}.part';
       final cacheFile = File(cachePath);
 
-      // 2. 如果缓存存在且未过期，直接返回
+      // 1. 命中本地缓存（头部文件同样按 24h 过期）
       if (await cacheFile.exists()) {
         final stat = await cacheFile.stat();
-        // 缓存 1 小时内有效（86400 秒）
-        if (DateTime.now().difference(stat.modified).inSeconds < 86400) {
+        if (stat.size > 0 &&
+            DateTime.now().difference(stat.modified).inSeconds < 86400) {
           return cacheFile;
         }
       }
 
-      // 3. 获取认证头
+      // 2. 认证头
       final headers = await WebDAVService.instance.getAuthHeaders();
       if (headers.isEmpty) return null;
 
-      // 4. 只下载前 1MB
-      await _dio.download(
+      // 3. 取消上一次未完成的下载
+      _activeToken?.cancel('superseded');
+      final token = CancelToken();
+      _activeToken = token;
+
+      // 4. 只请求前 2MB
+      final response = await _dio.get<ResponseBody>(
         url,
-        cachePath,
+        cancelToken: token,
         options: Options(
-          headers: headers,
-          receiveTimeout: const Duration(seconds: 10),
+          headers: {
+            ...headers,
+            'Range': 'bytes=0-${_maxBytes - 1}',
+          },
+          responseType: ResponseType.stream,
+          receiveTimeout: const Duration(seconds: 15),
           sendTimeout: const Duration(seconds: 10),
+          followRedirects: true,
         ),
       );
 
+      final body = response.data;
+      if (body == null) return null;
+
+      // 5. 写入临时文件（最多 _maxBytes）
+      final sink = cacheFile.openWrite();
+      var written = 0;
+      try {
+        await for (final chunk in body.stream) {
+          final remain = _maxBytes - written;
+          if (remain <= 0) break;
+          final data =
+              chunk.lengthInBytes > remain ? chunk.sublist(0, remain) : chunk;
+          sink.add(data);
+          written += data.lengthInBytes;
+          if (written >= _maxBytes) break;
+        }
+      } finally {
+        await sink.flush();
+        await sink.close();
+      }
+
+      if (written == 0) {
+        try {
+          await cacheFile.delete();
+        } catch (_) {}
+        return null;
+      }
+
+      debugPrint(
+          '📦 [WebDAVFileProvider] 取头部 ${(written / 1024).toStringAsFixed(0)}KB'
+          '（上限 ${_maxBytes ~/ 1024}KB，不再整文件下载）');
       return cacheFile;
+    } on DioException catch (e) {
+      if (CancelToken.isCancel(e)) {
+        debugPrint('↩️ [WebDAVFileProvider] 下载已取消（切歌触发）');
+        return null;
+      }
+      debugPrint('⚠️ [WebDAVFileProvider] download failed: $e');
+      return null;
     } catch (e) {
       debugPrint('⚠️ [WebDAVFileProvider] download failed: $e');
       return null;
     }
   }
+
+  /// 主动取消进行中的元数据下载（切歌时可由上层调用）。
+  static void cancelActive() {
+    _activeToken?.cancel('superseded');
+    _activeToken = null;
+  }
 }
 
 class LocalFileProvider implements FileProvider {

lib/pages/home_page.dart

@@ -2,7 +2,6 @@ import 'package:flutter/material.dart';
 import 'package:provider/provider.dart';
 import '../services/audio_service.dart';
 import '../services/webdav_service.dart';
-import '../services/playback_service.dart';
 import '../widgets/magnetic_scroll_physics.dart';
 import 'webdav_setup_page.dart';
 import 'webdav_file_list_page.dart';
@@ -246,16 +245,10 @@ class _HomePageState extends BaseState<HomePage> {
 
   void _playSong(SongItem song) async {
     try {
-      final url = WebDAVService.instance.getFileUrl(song.path);
-
-      final audioService = context.read<AudioService>();
-      audioService.playSong(Song(
-        id: song.path,
-        title: song.displayTitle,
-        artist: song.displaySubtitle,
-        url: url,
-      ));
-
+      // 先校验登录，再开始播放。
+      // 以前这里是：playSong() 不 await，紧跟着又 PlaybackService().play() 一次，
+      // 同一个 Player 会被 open 两次，media_kit 串行化时第二次会顶掉第一次，
+      // 表现就是"媒体加载了但没有声音"。
       final headers = await WebDAVService.instance.getAuthHeaders();
 
       if (headers.isEmpty) {
@@ -270,7 +263,16 @@ class _HomePageState extends BaseState<HomePage> {
         return;
       }
 
-      await PlaybackService().play(url, headers: headers);
+      if (!mounted) return;
+
+      final url = WebDAVService.instance.getFileUrl(song.path);
+      final audioService = context.read<AudioService>();
+      await audioService.playSong(Song(
+        id: song.path,
+        title: song.displayTitle,
+        artist: song.displaySubtitle,
+        url: url,
+      ));
     } catch (e) {
       if (mounted) {
         ScaffoldMessenger.of(context).showSnackBar(

lib/services/audio_service.dart

@@ -81,6 +81,18 @@ class AudioService extends ChangeNotifier {
   bool _hasStartedCurrentPlayback = false;
   bool _isUserSeeking = false;
 
+  /// 切歌请求序号：只有最新一次请求有权解锁、写状态。
+  int _switchEpoch = 0;
+
+  /// 缓冲卡死看门狗
+  static const Duration _stallThreshold = Duration(seconds: 30);
+  Timer? _stallTimer;
+  DateTime? _lastStallRecoveryAt;
+
+  /// 最近一次播放失败的原因，供 UI 提示；成功播放后自动清空。
+  String? _lastPlaybackError;
+  String? get lastPlaybackError => _lastPlaybackError;
+
   int _playbackGeneration = 0;
 
   // ---- 待恢复的播放进度 ----
@@ -253,16 +265,14 @@ class AudioService extends ChangeNotifier {
   // ════════════════════════════════════════════════════════════
 
   Future<void> _playCurrent() async {
-    if (_isChangingTrack) {
-      debugPrint('⚠️ [AudioService] track switch already running');
-      return;
-    }
-
     if (_currentIndex < 0 || _currentIndex >= _queue.length) {
       stopPlay();
       return;
     }
 
+    // 「最新请求胜出」：不再因为 _isChangingTrack 为 true 就直接丢弃请求。
+    // 旧请求会通过 epoch 校验自行放弃后续步骤，避免和新的切歌互相覆盖。
+    final epoch = ++_switchEpoch;
     _isChangingTrack = true;
     _handlingCompletion = false;
     _hasStartedCurrentPlayback = false;
@@ -283,9 +293,29 @@ class AudioService extends ChangeNotifier {
         return;
       }
 
-      await _playWithHeaders(song.url!);
+      try {
+        await _playWithHeaders(song.url!);
+      } catch (e) {
+        // 播放失败（超时/401/网络中断）：必须走到 finally 解锁，
+        // 并把错误记下来，供上层提示或下一首重试。
+        _lastPlaybackError = '播放失败: $e';
+        debugPrint('❌ [AudioService] play failed: $e');
+        notifyListeners();
+        return;
+      }
+
+      if (epoch != _switchEpoch) {
+        debugPrint('↩️ [AudioService] 旧请求已被新的切歌取代，停止后续处理');
+        return;
+      }
+
       await _waitForPlaybackStarted();
 
+      if (epoch != _switchEpoch) {
+        debugPrint('↩️ [AudioService] 旧请求已被新的切歌取代，停止后续处理');
+        return;
+      }
+
       // ⭐ 如果有待恢复的进度，执行 seek
       if (_pendingSeekPosition != null &&
           _pendingSeekPosition!.inMilliseconds > 0) {
@@ -299,6 +329,7 @@ class AudioService extends ChangeNotifier {
       _syncPlayerStateDelayed();
 
       if (_hasStartedCurrentPlayback) {
+        _lastPlaybackError = null;
         _onSongChanged?.call(song);
         _loadMetadataForCurrentSong(generation);
       }
@@ -306,12 +337,51 @@ class AudioService extends ChangeNotifier {
       // 切歌完成后保存状态
       await savePlaybackState();
     } finally {
-      _isChangingTrack = false;
+      // 只有最新的一次切歌才有资格解锁，防止旧请求把新请求的锁提前放掉
+      if (epoch == _switchEpoch) {
+        _isChangingTrack = false;
+      }
     }
   }
 
-  Future<void> _waitForPlaybackStarted() async {
-    final player = PlaybackService().player;
+  /// 缓冲卡死自愈：重开当前歌曲并 seek 回原位置。
+  ///
+  /// 触发条件：mpv 的 buffering 持续为 true 超过 [_stallThreshold]。
+  /// 两次自动恢复之间至少间隔 2 分钟，避免源站长时间不可用时反复重连。
+  Future<void> _recoverFromStall() async {
+    if (_isChangingTrack) return;
+
+    final song = _currentSong;
+    if (song == null) return;
+    final url = song.url;
+    if (url == null || url.isEmpty) return;
+
+    final now = DateTime.now();
+    if (_lastStallRecoveryAt != null &&
+        now.difference(_lastStallRecoveryAt!) < const Duration(minutes: 2)) {
+      debugPrint('🩺 [AudioService] 仍在缓冲，但 2 分钟内已恢复过，交给用户手动切歌');
+      return;
+    }
+    _lastStallRecoveryAt = now;
+
+    final resumeAt = positionNotifier.value;
+    debugPrint(
+        '🩺 [AudioService] 缓冲超过 ${_stallThreshold.inSeconds}s，尝试重连：${song.title}');
+    try {
+      await _playWithHeaders(url);
+      if (resumeAt > Duration.zero) {
+        await PlaybackService().seek(resumeAt);
+        positionNotifier.value = resumeAt;
+      }
+      debugPrint('✅ [AudioService] 自动恢复完成');
+    } catch (e) {
+      debugPrint('❌ [AudioService] 自动恢复失败: $e');
+      _lastPlaybackError = '缓冲卡住，自动恢复失败';
+      notifyListeners();
+    }
+  }
+
+  Future<void> _waitForPlaybackStarted() async {    final player = PlaybackService().player;
 
     if (player.state.playing) {
       _hasStartedCurrentPlayback = true;
@@ -493,11 +563,7 @@ class AudioService extends ChangeNotifier {
       return;
     }
 
-    if (_isChangingTrack) {
-      debugPrint('⚠️ [next] REJECTED: track switch in progress');
-      return;
-    }
-
+    // 不再因为 _isChangingTrack 拒绝：新请求会取代进行中的切歌（latest wins）
     if (_playMode == PlayMode.shuffle) {
       if (_shuffledIndices.isEmpty) return;
       final nextIdx = (_shuffledIndex + 1) % _shuffledIndices.length;
@@ -515,12 +581,7 @@ class AudioService extends ChangeNotifier {
   Future<void> previous() async {
     if (_queue.isEmpty) return;
 
-    if (_isChangingTrack) {
-      debugPrint(
-          '⚠️ [AudioService] previous() ignored: track switch in progress');
-      return;
-    }
-
+    // 同 next()：最新请求胜出，不拒绝
     if (_playMode == PlayMode.shuffle) {
       if (_shuffledIndices.isEmpty) return;
       final prevIdx = (_shuffledIndex - 1) % _shuffledIndices.length;
@@ -664,6 +725,27 @@ class AudioService extends ChangeNotifier {
       }),
     );
 
+    // mpv 的错误（401 / 连接失败 / 解码失败）以前没有任何订阅，失败是静默的
+    _subscriptions.add(
+      player.stream.error.listen((error) {
+        debugPrint('❌ [AudioService] player error: $error');
+        _lastPlaybackError = error;
+        notifyListeners();
+      }),
+    );
+
+    // 缓冲状态：定位"流量很高但缓冲为 0"时要靠它
+    _subscriptions.add(
+      player.stream.buffering.listen((buffering) {
+        debugPrint('🌊 [AudioService] buffering=$buffering');
+        _stallTimer?.cancel();
+        _stallTimer = null;
+        if (!buffering) return;
+        // 持续缓冲超过阈值：多半是网络卡死，主动重连一次当前歌曲
+        _stallTimer = Timer(_stallThreshold, _recoverFromStall);
+      }),
+    );
+
     // ⭐ 完整生命周期校验的 completed 监听
     _subscriptions.add(
       player.stream.completed.listen((_) {
@@ -885,6 +967,8 @@ class AudioService extends ChangeNotifier {
   @override
   void dispose() {
     // 先保存再清理
+    _stallTimer?.cancel();
+    _stallTimer = null;
     savePlaybackState();
     _stopListening();
     positionNotifier.dispose();

lib/services/playback_service.dart

@@ -1,4 +1,6 @@
 // lib/services/playback_service.dart
+import 'dart:async';
+
 import 'package:media_kit/media_kit.dart';
 import 'package:flutter/foundation.dart';
 
@@ -11,6 +13,12 @@ class PlaybackService {
     debugPrint('🎯 [PlaybackService] 单例创建, serviceId=$_serviceId');
   }
 
+  /// 弱网/源站抖动时，mpv 的 open 可能长时间不返回。
+  /// 不设上限的话 _playCurrent() 的 finally 永远不执行，
+  /// _isChangingTrack 会永久为 true，整个切歌链路假死。
+  static const Duration _openTimeout = Duration(seconds: 20);
+  static const Duration _controlTimeout = Duration(seconds: 5);
+
   Player? _player;
   bool _initialized = false;
   late final String _serviceId; // ⭐ 服务实例 ID
@@ -41,15 +49,41 @@ class PlaybackService {
   // ---------- 播放 ----------
   Future<void> play(String url, {Map<String, String>? headers}) async {
     debugPrint(
-        '▶️ [PlaybackService] play() called, serviceId=$_serviceId, player=${_player?.hashCode}');
+        '▶️ [PlaybackService] play() called, serviceId=$_serviceId, player=${_player?.hashCode}, url=${_shortUrl(url)}');
     if (!_initialized) init();
 
     final media = (headers != null && headers.isNotEmpty)
         ? Media(url, httpHeaders: headers)
         : Media(url);
 
-    await _player!.open(media);
-    await _player!.play();
+    try {
+      await _player!.open(media).timeout(_openTimeout);
+      await _player!.play().timeout(_controlTimeout);
+    } on TimeoutException {
+      debugPrint(
+          '❌ [PlaybackService] open timeout (${_openTimeout.inSeconds}s): ${_shortUrl(url)}');
+      await _safeStop();
+      rethrow;
+    } catch (e) {
+      debugPrint('❌ [PlaybackService] play failed: $e');
+      await _safeStop();
+      rethrow;
+    }
+  }
+
+  /// 失败时尽量把播放器拉回可用的空闲状态，避免下一次 open 继续卡住。
+  Future<void> _safeStop() async {
+    try {
+      await _player?.stop().timeout(const Duration(seconds: 3));
+    } catch (_) {
+      // 忽略：stop 本身失败也不应影响上层解锁
+    }
+  }
+
+  /// 日志里不打完整 URL（WebDAV 路径很长，且可能带签名参数）
+  String _shortUrl(String url) {
+    final hash = url.hashCode.toRadixString(16);
+    return url.length <= 48 ? url : '${url.substring(0, 48)}…#$hash';
   }
 
   // ---------- 控制 ----------
@@ -57,7 +91,7 @@ class PlaybackService {
     debugPrint(
         '⏸️ [PlaybackService] pause() called, serviceId=$_serviceId, player=${_player?.hashCode}');
     if (_initialized && _player != null) {
-      await _player!.pause();
+      await _player!.pause().timeout(_controlTimeout);
     }
   }
 
@@ -65,7 +99,7 @@ class PlaybackService {
     debugPrint(
         '▶️ [PlaybackService] resume() called, serviceId=$_serviceId, player=${_player?.hashCode}');
     if (_initialized && _player != null) {
-      await _player!.play();
+      await _player!.play().timeout(_controlTimeout);
     }
   }
 
@@ -73,7 +107,7 @@ class PlaybackService {
     debugPrint(
         '⏹️ [PlaybackService] stop() called, serviceId=$_serviceId, player=${_player?.hashCode}');
     if (_initialized && _player != null) {
-      await _player!.stop();
+      await _player!.stop().timeout(_controlTimeout);
     }
   }
 
@@ -81,7 +115,7 @@ class PlaybackService {
     debugPrint(
         '⏩ [PlaybackService] seek() called, serviceId=$_serviceId, player=${_player?.hashCode}');
     if (_initialized && _player != null) {
-      await _player!.seek(position);
+      await _player!.seek(position).timeout(_controlTimeout);
     }
   }