Relay: viewers keep the file size (headers within 4 s, connection: close), retries back off, 412 re-resolves

Two server behaviours left ffmpeg (8.1/9.0) without the file size, after
which every seek failed with "size -38" until a reload:
- dart:io holds response headers until body bytes exist: a stalled
  source meant no headers within mpv's 5 s timeout. Relay byte responses
  are now written on the raw connection, headers with the first bytes or
  after 4 s, and a swap ends waiting reads with a plain close.
- every response closed its socket without saying connection: close, so
  ffmpeg 8.x reused a dead connection for its next seek (pol's X clips).
  The server now says it whenever the request asks.
A failed pull waits 250 ms doubling to 8 s instead of re-pulling at once
(the ~130 ms storm that drew Bilibili's 412/403); 412 counts as an expired
URL and re-resolves the page, at most once per 60 s. The app reloads a
player once per media when ffmpeg reports the lost size anyway. P37: a
read that waits past ffmpeg's retry budget still ends the file early.

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

apps/desktop/lib/src/platform/ports/player_host.dart

@@ -184,6 +184,13 @@ abstract interface class PlayerHost {
   int get live;
 }
 
+/// [log] says ffmpeg's HTTP stream lost the file's size and so every
+/// seek on it fails ("Seek failed (to 3417735, size -38)": -38 is ENOSYS,
+/// what ffmpeg answers for the size once a reconnect's or a seek's request
+/// failed). Only a fresh open gets it back.
+bool mpvStreamLost(PlayerLogLine log) =>
+    log.prefix == 'ffmpeg' && log.text.contains('Seek failed') && log.text.contains('size -38)');
+
 /// [log] says the media could not be opened: before a [duration] is known,
 /// mpv's fatal "nothing to play" or a demuxer error.
 bool mpvLoadFailed(PlayerLogLine log, Duration duration) {

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

@@ -63,6 +63,10 @@ class PlaybackController {
   /// e.g. a CDN's 403), at most once per [beginOpen].
   void Function(String error)? onLoadError;
 
+  /// The current media's HTTP stream lost its size (see [mpvStreamLost]):
+  /// at most once per player and media; the coordinator opens it again.
+  void Function()? onStreamLost;
+
   /// Seeks from the controls (seek bar, the video widget's own) go here
   /// instead of to mpv.
   void Function(Duration position)? onUserSeek;
@@ -71,6 +75,7 @@ class PlaybackController {
   // load error was reported.
   String? _httpError;
   bool _loadErrorSent = false;
+  bool _streamLostSent = false;
 
   /// The current player. Only while [alive].
   HostedPlayer get player => _player!;
@@ -144,6 +149,7 @@ class PlaybackController {
     _chapterJson = '';
     _httpError = null;
     _loadErrorSent = false;
+    _streamLostSent = false;
     _shaders = '';
     _loaded = loaded && p.state.duration > Duration.zero;
     _opened = loaded;
@@ -196,6 +202,7 @@ class PlaybackController {
     _chapterJson = '';
     _httpError = null;
     _loadErrorSent = false;
+    _streamLostSent = false;
     _loaded = false;
     chapters.value = const [];
     tracks.value = const [];
@@ -286,7 +293,14 @@ class PlaybackController {
   // error while shaders are on is a shader that failed to load or compile
   // (mpv then renders without it).
   void _onLog(PlayerLogLine log) {
-    if (log.prefix == 'ffmpeg') return;
+    if (log.prefix == 'ffmpeg') {
+      if (!_streamLostSent && mpvStreamLost(log)) {
+        _streamLostSent = true;
+        _log.warning('the media stream lost its size, every seek fails: [ffmpeg] ${log.text.trim()}');
+        onStreamLost?.call();
+      }
+      return;
+    }
     if (!_loadErrorSent && mpvLoadFailed(log, duration.peek())) {
       _loadErrorSent = true;
       final http = _httpError;

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

@@ -426,6 +426,7 @@ class SessionCoordinator {
     if (_playerReady) return;
     try {
       playback.onLoadError = _setPlayerError;
+      playback.onStreamLost = _streamLost;
       playback.onUserSeek = (d) => _userSeek(d.inMicroseconds / 1e6);
       _bufferSub ??= playback.buffered.subscribe((_) => _onBuffered());
       await media.start();
@@ -479,6 +480,19 @@ class SessionCoordinator {
   }
 
   bool _reloadingVideo = false;
+  // The media whose lost stream was reloaded already (once per media).
+  int _streamReloadedFor = -1;
+
+  /// The player's HTTP stream lost the file's size (an old server, a
+  /// request that timed out): every seek on it would fail until a fresh
+  /// open. "Reload video" once per media, as the user would.
+  void _streamLost() {
+    if (_streamReloadedFor == _mediaId || _mediaId < 0) return;
+    _streamReloadedFor = _mediaId;
+    _log.info('media $_mediaId: the stream lost its size; reloading the video once');
+    unawaited(reloadVideo());
+  }
+
   void Function()? _bufferSub;
 
   /// "Reload video" (button, Ctrl+R, status menu): fresh

apps/desktop/test/stream_lost_test.dart

@@ -0,0 +1,20 @@
+import 'package:flutter_test/flutter_test.dart';
+import 'package:starsync/src/platform/ports/player_host.dart';
+
+// ffmpeg's lost stream (its HTTP context lost the file's size, every seek
+// fails until a fresh open): the lines the coordinator reloads the video
+// on, once per media.
+void main() {
+  test('a seek that failed for want of the size is a lost stream', () {
+    expect(mpvStreamLost(const PlayerLogLine('ffmpeg', 'error', 'Seek failed (to 3417735, size -38)')), isTrue);
+    expect(mpvStreamLost(const PlayerLogLine('ffmpeg', 'warn', 'Seek failed (to 11997, size -38)\n')), isTrue);
+  });
+
+  test('anything else is not', () {
+    // A seek that failed with the size known, a reconnect, another prefix.
+    expect(mpvStreamLost(const PlayerLogLine('ffmpeg', 'error', 'Seek failed (to 20851, size 1023081)')), isFalse);
+    expect(mpvStreamLost(const PlayerLogLine('ffmpeg', 'warn', 'http: Will reconnect at 3407872 in 0 second(s)')),
+        isFalse);
+    expect(mpvStreamLost(const PlayerLogLine('lavf', 'warn', 'Seek failed (to 1, size -38)')), isFalse);
+  });
+}