Sessions leave redacted logs; viewers wait for a member relay's source before opening it

Logs: one redaction pass (query strings to names, member tokens,
passwords, cookies, auth headers) for every line; daily files with size
and age caps in the app (hosted server included) and the dedicated
server (--log-dir); swaps, relay channels, per-member /r /m /j outcomes,
server fetches, mpv load errors, yt-dlp runs. Settings > About: Copy
diagnostics, Open logs folder.

Bilibili through someone else: the viewer opened /r/ before the source
dialled in; the server holds that request and mpv gives up after 5 s
("could not be opened"). Viewers now open a member relay once it is
live, the putter starts its source before its own player, and track
sizes are read in parallel.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

apps/desktop/lib/main.dart

@@ -9,6 +9,7 @@ import 'package:window_manager/window_manager.dart';
 
 import 'app.dart';
 import 'src/playback/yt_dlp_app.dart';
+import 'src/platform/app_log.dart';
 import 'src/platform/desktop_entry.dart';
 import 'src/platform/window_memory.dart';
 import 'src/screens/home/ui.dart';
@@ -41,6 +42,7 @@ Future<void> main(List<String> args) async {
         ),
       );
   WidgetsFlutterBinding.ensureInitialized();
+  await startAppLogging();
   await Hive.initFlutter();
   await AppSettings.instance.ensureLoaded();
   await windowManager.ensureInitialized();

apps/desktop/lib/src/platform/app_log.dart

@@ -0,0 +1,98 @@
+import 'dart:async';
+import 'dart:io';
+
+import 'package:flutter/foundation.dart';
+import 'package:path_provider/path_provider.dart';
+import 'package:sync_relay_core/core.dart';
+
+import '../playback/yt_dlp_app.dart';
+import '../screens/home/ui.dart' show appName, displayVersion;
+import '../state/session_state.dart';
+
+// The app's log on disk (<app support>/logs/app-YYYY-MM-DD.log; core's
+// logging.dart writes, rotates and redacts it), debugPrint routed into it,
+// and the "Copy diagnostics" text of Settings → About.
+
+final _appLog = createLogger('app');
+
+/// The logs folder, once [startAppLogging] ran.
+String? appLogDir;
+
+/// Opens the log files and routes `debugPrint` and uncaught errors into
+/// the log. Needs Flutter's binding (path_provider).
+Future<void> startAppLogging() async {
+  try {
+    final base = await getApplicationSupportDirectory();
+    final dir = '${base.path}${Platform.pathSeparator}logs';
+    if (startLogFiles(dir) != null) appLogDir = dir;
+  } catch (e) {
+    _appLog.warning('no log files: $e');
+  }
+  // Everything the app prints for diagnosis lands in the file too (the
+  // log's console output replaces the print).
+  debugPrint = (String? message, {int? wrapWidth}) {
+    if (message == null || message.isEmpty) return;
+    final bad = RegExp(r'fail|error|crash|exception', caseSensitive: false).hasMatch(message);
+    bad ? _appLog.warning(message) : _appLog.info(message);
+  };
+  PlatformDispatcher.instance.onError = (error, stack) {
+    _appLog.severe('uncaught: $error', error, stack);
+    return false;
+  };
+  _appLog.info('$appName ${displayVersion()} on ${Platform.operatingSystem} '
+      '${Platform.operatingSystemVersion}, pid $pid, Dart ${Platform.version.split(' ').first}');
+}
+
+/// The text "Copy diagnostics" gives: versions, then the last [since] of
+/// the app log (its hosted server's records included), already redacted.
+Future<String> diagnosticsText({Duration since = const Duration(hours: 6)}) async {
+  final b = StringBuffer()
+    ..writeln('$appName ${displayVersion()}')
+    ..writeln('OS: ${Platform.operatingSystem} ${Platform.operatingSystemVersion}')
+    ..writeln('Dart: ${Platform.version.split(' ').first}');
+  try {
+    final mpv = await SessionState.instance.coordinator.playback.mpvVersion().timeout(const Duration(seconds: 2));
+    b.writeln('mpv: ${mpv ?? 'no player yet'}');
+  } catch (_) {
+    b.writeln('mpv: unknown');
+  }
+  try {
+    final y = await AppYtDlp.instance.ytDlp.timeout(const Duration(seconds: 2));
+    final cookies = y.cookies.read(count: false);
+    b.writeln('yt-dlp: ${y.version ?? 'not ready'}${y.lastError == null ? '' : ' (${y.lastError})'}, '
+        'cookies: ${cookies == null ? 'none' : 'set'}');
+  } catch (_) {
+    b.writeln('yt-dlp: unknown');
+  }
+  final files = logFiles;
+  b.writeln('Log: ${files?.currentPath ?? 'none (console only)'}');
+  b.writeln('--- last ${since.inHours} h of the log ---');
+  b.write(files?.recent(since) ?? '');
+  return redactLog(b.toString());
+}
+
+/// Saves [text] next to the logs as `diagnostics-<time>.txt`; its path,
+/// or null when it could not be written.
+Future<String?> saveDiagnostics(String text) async {
+  final dir = appLogDir;
+  if (dir == null) return null;
+  final t = DateTime.now();
+  String two(int n) => n.toString().padLeft(2, '0');
+  final path = '$dir${Platform.pathSeparator}diagnostics-${t.year}${two(t.month)}${two(t.day)}-'
+      '${two(t.hour)}${two(t.minute)}${two(t.second)}.txt';
+  try {
+    // Older ones go after the log files' week.
+    for (final e in Directory(dir).listSync()) {
+      if (e is File &&
+          e.uri.pathSegments.last.startsWith('diagnostics-') &&
+          t.difference(e.statSync().modified) > const Duration(days: 7)) {
+        e.deleteSync();
+      }
+    }
+    await File(path).writeAsString(text);
+    return path;
+  } catch (e) {
+    _appLog.warning('cannot save diagnostics: $e');
+    return null;
+  }
+}

apps/desktop/lib/src/playback/playback_controller.dart

@@ -8,7 +8,7 @@ import 'package:flutter/widgets.dart';
 import 'package:media_kit/media_kit.dart';
 import 'package:media_kit_video/media_kit_video.dart';
 import 'package:signals/signals_flutter.dart';
-import 'package:sync_relay_core/core.dart' show JellyfinStreamType;
+import 'package:sync_relay_core/core.dart' show JellyfinStreamType, LogThrottle, createLogger;
 
 import '../jellyfin/track_prefs.dart' show MpvTrack;
 import '../shaders/shader_assets.dart';
@@ -361,6 +361,7 @@ class PlaybackController {
   // an error there while shaders are on is a shader that failed to load
   // or compile (mpv then renders without it).
   void _onLog(PlayerLog log) {
+    _toAppLog(log);
     if (log.prefix == 'ytdl_hook' && (log.level == 'error' || log.level == 'fatal')) {
       final text = log.text.trim().replaceFirst(RegExp(r'^ERROR:\s*'), '');
       if (text.isNotEmpty) {
@@ -380,6 +381,8 @@ class PlaybackController {
     if (!_loadErrorSent && _loadFailed(log, duration.peek())) {
       _loadErrorSent = true;
       final http = _httpError;
+      _mpvLog.warning('cannot open the media: [${log.prefix}] ${log.text.trim()}'
+          '${http == null ? ' (no HTTP error logged; a request that timed out after 5 s looks like this)' : ' ($http)'}');
       onLoadError?.call(http == null ? 'the video stream could not be opened' : 'the video server answered $http');
       return;
     }
@@ -402,6 +405,23 @@ class PlaybackController {
     }
   }
 
+  // mpv's warnings and errors (the session player's; ffmpeg's through
+  // whichever player logs them) go to the app log, a few per kind at a
+  // time: a refused request makes ffmpeg retry and warn on every try.
+  static final _mpvLog = createLogger('mpv');
+  static final _mpvThrottle = LogThrottle(burst: 5);
+
+  static void _toAppLog(PlayerLog log, {String player = ''}) {
+    final level = log.level;
+    if (level != 'warn' && level != 'error' && level != 'fatal') return;
+    final text = log.text.trim();
+    if (text.isEmpty) return;
+    final pass = _mpvThrottle.allow('$player${log.prefix} $level ${text.length > 40 ? text.substring(0, 40) : text}');
+    if (pass == null) return;
+    final line = '$pass$player[${log.prefix}] $text';
+    level == 'warn' ? _mpvLog.info(line) : _mpvLog.warning(line);
+  }
+
   /// [log] says the media could not be opened: before a [duration] is
   /// known, mpv's fatal "nothing to play" or a demuxer error, or a yt-dlp
   /// hook error.
@@ -438,6 +458,19 @@ class PlaybackController {
     }
   }
 
+  /// `mpv-version` and `ffmpeg-version` of the session player, for
+  /// diagnostics; null when there is no player.
+  Future<String?> mpvVersion() async {
+    final p = _player;
+    if (p == null) return null;
+    try {
+      final dynamic platform = p.platform;
+      return '${await platform?.getProperty('mpv-version')}, ffmpeg ${await platform?.getProperty('ffmpeg-version')}';
+    } catch (_) {
+      return null;
+    }
+  }
+
   /// Opens [url] paused. [headers] go with every request for it (mpv
   /// `http-header-fields`): a Jellyfin token travels there, not in the URL.
   /// Each media gets its own player: when [preload] holds the same [url]
@@ -708,6 +741,7 @@ class PlaybackController {
       // that is gone (a promotion disposes it) the next one created, which
       // can be this preload. Its HTTP errors still name the session's.
       if (log.prefix == 'ffmpeg') return _onLog(log);
+      _toAppLog(log, player: 'preload ');
       if (_loadFailed(log, p.state.duration)) pre.fail();
     });
     try {

apps/desktop/lib/src/screens/home/about_page.dart

@@ -1,12 +1,14 @@
 import 'package:flutter/material.dart';
 
+import '../../platform/app_log.dart';
 import '../../update/update_controller.dart';
 import '../../widgets/copy_open.dart';
 import 'settings_page.dart';
 import 'ui.dart';
 import 'update_indicator.dart';
 
-/// Name, version, update state and the source link (Settings → About).
+/// Name, version, update state, the source link and the logs (Settings →
+/// About).
 class AboutPanel extends StatelessWidget {
   final UpdateController? updates;
   const AboutPanel({super.key, this.updates});
@@ -41,8 +43,45 @@ class AboutPanel extends StatelessWidget {
                   ),
                 ],
               ),
+              const SizedBox(height: kGap),
+              Text('Something went wrong? Copy diagnostics and send them to whoever helps you: '
+                  'versions and the last hours of the log, passwords, tokens and cookies left out.',
+                  style: t.bodySmall?.copyWith(color: Colors.grey)),
+              const SizedBox(height: 8),
+              Wrap(
+                spacing: 8,
+                runSpacing: 8,
+                children: [
+                  FilledButton.tonalIcon(
+                    onPressed: _copyDiagnostics,
+                    icon: const Icon(Icons.bug_report_outlined, size: 18),
+                    label: const Text('Copy diagnostics'),
+                  ),
+                  OutlinedButton.icon(
+                    onPressed: appLogDir == null ? null : () => _openLogs(),
+                    icon: const Icon(Icons.folder_open, size: 18),
+                    label: const Text('Open logs folder'),
+                  ),
+                ],
+              ),
             ],
           ),
         );
   }
+
+  static Future<void> _copyDiagnostics() async {
+    final text = await diagnosticsText();
+    final saved = await saveDiagnostics(text);
+    await copyToClipboard(text,
+        done: saved == null ? 'Diagnostics copied' : 'Diagnostics copied (also saved in the logs folder)');
+  }
+
+  static Future<void> _openLogs() async {
+    final dir = appLogDir;
+    if (dir == null) return;
+    // The file manager opens a folder path the way a browser opens a link.
+    if (!await openInBrowser(Uri.directory(dir).toString())) {
+      await copyToClipboard(dir, done: "Couldn't open it — folder path copied");
+    }
+  }
 }

apps/desktop/lib/src/session/session_coordinator.dart

@@ -33,6 +33,8 @@ import 'now_playing.dart';
 import 'now_playing_info.dart';
 import 'switch_trace.dart';
 
+final _log = createLogger('session');
+
 /// Thrown when a connect is superseded by cancel/leave before it finished.
 class ConnectCancelled implements Exception {
   const ConnectCancelled();
@@ -387,7 +389,7 @@ class SessionCoordinator {
   }
 
   void _setPlayerError(String raw) {
-    debugPrint('player cannot open media $_mediaId: $raw');
+    _log.warning('player cannot open media $_mediaId: $raw');
     _playerErrorFor = _mediaId;
     _playerErrorDuration = playback.duration.peek();
     playerError.value = playErrorText(raw);
@@ -563,6 +565,8 @@ class SessionCoordinator {
   /// the player held before (a reconnect) keeps it.
   Future<bool> _openMedia(String url, {Map<String, String>? headers}) async {
     if (url == _openUrl) return true;
+    _log.info('media $_mediaId: open ${_urlKind(url, headers)}: ${logUrl(url)}'
+        '${headers == null || headers.isEmpty ? '' : ' (headers: ${headers.keys.join(', ')})'}');
     final keepPlayer = _playerMediaId == _openedFor;
     _openedFor = _playerMediaId;
     _openUrl = url;
@@ -610,6 +614,23 @@ class SessionCoordinator {
     _sendReport(r);
   }
 
+  /// What [url] is, for the log.
+  String _urlKind(String url, Map<String, String>? headers) {
+    if (url.startsWith('edl://')) return 'relay (tracks)';
+    final u = Uri.tryParse(url);
+    if (u == null || !(u.isScheme('http') || u.isScheme('https'))) return 'local file';
+    if (u.host == _wsHost && u.port == _wsPort && u.pathSegments.isNotEmpty) {
+      return switch (u.pathSegments.first) {
+        'r' => 'relay',
+        'm' => "the host's file",
+        'j' => "Jellyfin through the server's login",
+        _ => 'server URL',
+      };
+    }
+    if (headers != null && headers.isNotEmpty) return 'Jellyfin, own sign-in';
+    return 'link, fetched here';
+  }
+
   /// A web link (not this server's): mpv's yt-dlp hook gets the app's
   /// managed yt-dlp, downloaded first if needed; a media file name does
   /// not wait for that.
@@ -656,6 +677,12 @@ class SessionCoordinator {
   /// The server-fetched media this viewer opens once it is live.
   int _awaitLiveFor = -1;
 
+  /// A relay nobody feeds yet: the server's fetch before it is read, or a
+  /// member's before its channel is up while that member is still here
+  /// (once it left, the server's buffer is all there is: open that).
+  bool _waitForSource(StateBroadcast r, MediaRelay relay) =>
+      !relay.live && (relay.fetch || r.members.any((m) => m.id == relay.source));
+
   /// Relayed through a member whose channel is down: say who everyone is
   /// waiting for (the buffer still plays what it has).
   void _relayStatus(StateBroadcast r) {
@@ -881,21 +908,27 @@ class SessionCoordinator {
               memberRelayUrl:
                   relay != null && !mine ? _relayMediaUrlFor(name, r.mediaId, const [], relay.queueId) : null);
         } else if (origin is _FileOrigin) {
-          await _openMedia(origin.path);
+          // The source first: viewers wait on it, not on this player.
           _startRelaySource(r.mediaId, RelayMedia.single(FileRelayBytes(File(origin.path))),
               announce: true, queueId: relay?.queueId ?? '');
+          await _openMedia(origin.path);
         } else if (origin is _LinkOrigin) {
-          await _openMedia(origin.url);
           _startRelaySource(
               r.mediaId, LinkRelayMedia(Uri.parse(origin.url), ytDlp: origin.ytDlp, probe: origin.probe),
               announce: true, queueId: relay?.queueId ?? '');
+          await _openMedia(origin.url);
         } else if (relay != null && !mine) {
-          if (relay.fetch && !relay.live) {
-            // The server is still getting it (yt-dlp takes seconds; mpv
-            // gives up on a silent request after 5): open once it is live.
+          if (_waitForSource(r, relay)) {
+            // The server is still getting it (yt-dlp takes seconds), or the
+            // member passing it on has not dialled in yet (its app opens the
+            // link first): mpv gives up on a silent request after 5 s, so
+            // open once it is live.
             _openUrl = null;
             _awaitLiveFor = r.mediaId;
-            health.value = const HealthStatus(HealthSeverity.info, 'the server is getting the video…');
+            final who = r.members.where((m) => m.id == relay.source).firstOrNull?.name;
+            _log.info('media ${r.mediaId}: waiting for ${relay.fetch ? 'the server' : who} before opening the relay');
+            health.value = HealthStatus(HealthSeverity.info,
+                relay.fetch ? 'the server is getting the video…' : 'waiting for $who to pass the video on…');
           } else {
             await _openMedia(_relayMediaUrlFor(name, r.mediaId, relay.tracks, relay.queueId));
           }
@@ -915,7 +948,7 @@ class SessionCoordinator {
       _seekGate.reset();
       _clearPendingSeek();
       if (_mediaId == r.mediaId) settledMediaId.value = r.mediaId;
-    } else if (_awaitLiveFor == r.mediaId && (stateMediaRelay(r)?.live ?? false)) {
+    } else if (_awaitLiveFor == r.mediaId && stateMediaRelay(r) != null && !_waitForSource(r, stateMediaRelay(r)!)) {
       _awaitLiveFor = -1;
       final relay = stateMediaRelay(r)!;
       await _openMedia(

apps/desktop/lib/src/state/server_isolate.dart

@@ -57,9 +57,9 @@ class ServerIsolate {
       if (msg is! Map) return;
       switch (msg['t']) {
         case 'log':
-          final err = msg['error'];
+          final err = msg['error'], stack = msg['stack'];
           onLog('${msg['level']} ${msg['logger']}: ${msg['message']}'
-              '${err == null ? '' : ' ($err)'}');
+              '${err == null ? '' : ' ($err)'}${stack == null ? '' : '\n$stack'}');
         case 'ready':
           if (!startup.isCompleted) {
             startup.complete((msg['port'] as int, msg['stop'] as SendPort));

apps/desktop/lib/src/state/session_state.dart

@@ -379,7 +379,8 @@ class SessionState {
             ytDlpDir: await _sharedYtDlpDir(),
           ),
           loopbackOnly: soloSession,
-          onLog: (line) => debugPrint('[server] $line'),
+          // Its records join the app's log as `server.<logger>`.
+          onLog: (line) => relogLine('server', line),
           onCrash: (cause) => _onServerCrash(server, cause),
         );
       } catch (e, st) {

apps/desktop/test/relay_e2e_test.dart

@@ -1,4 +1,5 @@
 import 'dart:async';
+import 'dart:convert';
 import 'dart:io';
 
 import 'package:flutter_test/flutter_test.dart';
@@ -169,6 +170,46 @@ void main() {
     }
   });
 
+  test('a source that dials in after mpv would have given up (5 s): viewers wait for it, then play',
+      timeout: const Timeout(Duration(minutes: 2)), () async {
+    final file = await _media();
+    final alice = await member('alice');
+    // Bob is a bare client: he puts a file up and starts passing it on
+    // only 7 s later (a slow first upstream read, a slow own player).
+    final ws = await WebSocket.connect('ws://127.0.0.1:${server.boundPort}/sync');
+    final welcome = Completer<Map>();
+    ws.listen((f) {
+      final j = jsonDecode('$f') as Map;
+      if (j['t'] == 'welcome' && !welcome.isCompleted) welcome.complete(j);
+    }, onError: (_) {});
+    ws.add(jsonEncode(const HelloMessage(
+            proto: RelayConstants.protoVersion,
+            secret: 'pw',
+            user: HelloUser(name: 'bob'),
+            media: HelloMedia(name: '', size: 0, duration: 0))
+        .toJson()));
+    final token = (await welcome.future.timeout(const Duration(seconds: 5)))['token'] as String;
+    final size = await File(file).length();
+    ws.add(jsonEncode({
+      't': 'swap',
+      'media': {'name': 'late.mkv', 'size': size, 'duration': 0, 'relay': <String, Object?>{}},
+    }));
+    await _poll(() => stateMediaRelay(alice.serverState.peek()!)?.source.isNotEmpty ?? false, 'relay state');
+    await Future.delayed(const Duration(seconds: 7));
+    expect(alice.playerError.peek(), isEmpty, reason: 'no "could not be opened" while the source is on its way');
+    expect(alice.health.peek().message, contains('waiting for bob'));
+    final src = RelaySource(Uri.parse('ws://127.0.0.1:${server.boundPort}/r/default/$token/source'),
+        RelayMedia.single(FileRelayBytes(File(file))));
+    await src.start();
+    try {
+      await _poll(() => alice.playback.durationSecs > 19, 'alice opened it once the source was up');
+      expect(alice.playerError.peek(), isEmpty);
+    } finally {
+      await src.close();
+      await ws.close();
+    }
+  });
+
   test('the source drops uncleanly and resumes: it keeps its copy and keeps serving',
       timeout: const Timeout(Duration(minutes: 2)), () async {
     final file = await _media();