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);
+  });
+}

docs/ARCHITECTURE.md

@@ -561,6 +561,12 @@ opening/initializing the selected video_out") is opened again once,
 500 ms later (`render: the video output was not up for the open`); the
 player error it raised clears when the media loads.
 
+A player whose HTTP stream lost the file's size (ffmpeg's "Seek failed
+(to N, size -38)": a request of its that got no headers in time, from a
+server without the relay's header rule, or the connection reuse above)
+fails every seek until a fresh open, so the coordinator runs "Reload
+video" once per media for it (`mpvStreamLost`, `onStreamLost`).
+
 Render mode: media_kit's Linux/Windows video output creates mpv's render
 context (OpenGL, else its software renderer) when the `VideoController` is
 created, and nothing in its API says which. The host therefore requests
@@ -899,7 +905,7 @@ How bytes move:
   kept; segments are evicted by distance (behind costs 4x), sizes
   estimated from the bytes per second of the pieces in so far (before
   that 4 Mbit/s video, 160 kbit/s audio, which also splits the budget).
-  A failed pull is retried after 1 s. Measured (mpv 0.41 through the
+  A failed pull is retried with backoff, as for bytes. Measured (mpv 0.41 through the
   dedicated server, loopback, ~30 MB/s upstream): Mux's TS HLS (1080p,
   one muxed playlist) opened in 0.8 s, seeks 0.07 to 1.1 s; Akamai's DASH
   (1080p + aac, two playlists, EDL) 1.3 s, seeks 0.03 to 1.4 s; an X
@@ -908,7 +914,11 @@ How bytes move:
   opened in 0.02 s from the buffer.
 - Filling a byte buffer: a viewer blocked on a slice first (oldest wait
   first, handing the source over at slice boundaries), then the pinned
-  slices, then read-ahead from the slowest viewer. A viewer reads the
+  slices, then read-ahead from the slowest viewer. A failed pull (the
+  source's upstream dropping, a CDN refusing) is followed by the next
+  one after 250 ms, doubling up to 8 s (`relayRetryDelay`), back to none
+  once a slice or a pull completes: a failing CDN is not asked again
+  every round trip (that drew Bilibili's 412/403). A viewer reads the
   slice being filled as its bytes arrive, not once it is whole (mpv's
   demuxer, blocked in a read, takes a seek only once that read returns).
   Nothing is fetched while nobody watches through the relay (a Jellyfin
@@ -969,7 +979,23 @@ How bytes move:
   its source before opening its own player, and the source reads its
   tracks' sizes at once.
 - Source away (D6): viewers keep playing from the buffer; a request
-  waits up to 60 s for missing bytes. The source member going `gone`
+  waits up to 60 s for missing bytes.
+- A viewer's byte answer is written on the raw connection (relic
+  `Hijack`, `_rangeResponse`): its headers go out with the first bytes,
+  or after 4 s without them (`relayHeaderWaitMs`). ffmpeg gives up on a
+  request whose headers do not come within mpv's 5 s `network-timeout`
+  and then has lost the file's size for good (8.x and 9.0 do not restore
+  it): every later seek fails with "Seek failed (to N, size -38)". With
+  the headers in time, a stalled read only times out and is reconnected
+  (`reconnect_*`, see "Media pipeline"), and plays on once the bytes come.
+  The answer always says `connection: close` and closes after its body:
+  ffmpeg asks for close, and ffmpeg 8.x, not told so, sends its next seek
+  on the connection after reading a range to its end (an mp4 with a box
+  after `mdat`, like X's clips: the 58-byte tail, then back to the
+  start) and fails it the same way. Other answers say so too when asked
+  (`closeWhenAsked`, every server route). A swap that ends the relay
+  closes its viewers' answers plainly (`RelayEnded`, not logged as a
+  broken read). The source member going `gone`
   does not end the relay: it lives through the source's grace
   (`media.relay.sourceGone`, `ServerHub.refreshRelaySource`). The source
   (`RelaySource`, kept by the media pipeline's `RelayFeeder` through the
@@ -1150,9 +1176,11 @@ all three link modes. Nothing is bundled or installed by hand.
   track's playlist must be readable, else the swap is refused ("Live
   streams can't be passed on", DRM, or nothing a relay can read) with a
   hint to use "Everyone fetches it". The swap names the tracks; its relay
-  source reads them with yt-dlp's headers (`PageRelayMedia`). On 403/410
-  (expired format URLs, 6 h on YouTube; signed segment URLs) from any
-  track it resolves the page again once for all tracks, finds the same
+  source reads them with yt-dlp's headers (`PageRelayMedia`). On
+  403/410/412 (expired format URLs, 6 h on YouTube; signed segment URLs;
+  Bilibili's 412) from any track it resolves the page again once for all
+  tracks (not again within 60 s: a further refusal fails the read and the
+  server's backoff asks later), finds the same
   format ids with the same sizes and piece counts and roles (playlists
   re-read and re-mapped by index; else "the video changed on the page")
   and goes on from where it stopped. Format URLs are bound to the

docs/DEVELOPMENT.md

@@ -227,11 +227,12 @@ Traps:
   so shell tests run the real player layout without it; otherwise test pure
   logic (`StateDiff`, coordinator without widgets).
 - A `SyncServer` started inside a test: relic reports an error writing a
-  response (a relay response cut short when its relay is dropped at a
-  swap or stop) to the zone the server started in, which fails the
-  test; outside tests it is only logged. Start it in `runZonedGuarded`
-  and fail on anything else (`queue_switch_e2e`, `relay_e2e`): a viewer's
-  read is still in flight at the `stop` in tearDown.
+  response (one cut short at `stop`: `/m`, `/j`; the relay's byte
+  answers are written on the raw connection and end on their own) to
+  the zone the server started in, which fails the test; outside tests it
+  is only logged. Start it in `runZonedGuarded` and fail on anything
+  else (`queue_switch_e2e`, `relay_e2e`): a viewer's read can still be
+  in flight at the `stop` in tearDown.
 - The Link page probes a filled-in link (network, yt-dlp), which never
   answers on the widget-test clock: pass `AppShell(linkProbe:)` /
   `LibraryTab(probe:)` a fake, or `pumpAndSettle` never returns.

docs/PROTOCOL.md

@@ -543,8 +543,12 @@ anything else means one file.
   one-file URL of a media with `tracks`) or piece (or `p/` of a byte
   track), 410 unknown session, 503 + `retry-after` the source never
   connected or the server could not read the link (60 s), or a piece did
-  not arrive within 60 s. A byte request whose bytes do not arrive within
-  60 s ends early. Bytes count toward `bytesServed`.
+  not arrive within 60 s. A byte request's headers come with its first
+  bytes, or after 4 s without them (under mpv's 5 s timeout); its body
+  then waits for missing bytes, up to 60 s for each, else the connection
+  is reset. Every byte answer says `connection: close` and closes after
+  its body; a swap that ends the relay closes it early without a reset.
+  Bytes count toward `bytesServed`.
 - `GET /r/<sid>/<token>/source` (WebSocket): the source's relay channel,
   dialled by the source (works behind NAT); `?q=<queueId>` for the warm
   relay of that item (or the media's relay started from it). Only that

docs/ROADMAP.md

@@ -58,6 +58,14 @@ Problems are numbered; delete a line when its fix ships.
 - P36 After a reconnect the app re-dials the warm relay of an Up next
   item that was already played ("relay channel refused: not the source of
   Up next item q8"): harmless, the feeder should end with its item.
+- P37 A relay viewer whose read waits on bytes the source does not send
+  for longer than ffmpeg's reconnect budget (4 tries per read, about
+  40 s with the 4 s header rule) gets an I/O error that mpv takes as the
+  end of the file. Measured with mpv 0.41 / ffmpeg 9.0.2 and the source
+  away about a minute across a seek: it played on; one read stalled past
+  the budget ends the file. Options: a longer `reconnect_delay_max` for
+  relay URLs only, or the end-of-file path reopening a relay viewer
+  that stopped short while the server plays.
 
 **Restructure** (in order; reviewed against the code). Truth lives in
 three places: the server (members, presence, gate, transport, queue incl.

packages/core/lib/src/constants.dart

@@ -31,6 +31,13 @@ abstract class RelayConstants {
   /// A viewer request waits this long for its bytes (source away) before
   /// the server gives up on it.
   static const int relayWaitSecs = 60;
+  /// Failed pulls in a row back off: 250 ms doubling up to this, back to
+  /// none once bytes come again.
+  static const int relayRetryMaxMs = 8000;
+  /// A relay viewer's response headers wait at most this long for its
+  /// first bytes, under mpv's 5 s `network-timeout`: a header that never
+  /// comes in time costs ffmpeg the file's size for good.
+  static const int relayHeaderWaitMs = 4000;
   /// The warm relay (Up next's first relayed item, before it goes up)
   /// keeps this much ahead of and behind its viewers' preloads, besides
   /// the pins: about what a preload player reads ahead (48 MiB), never the

packages/core/lib/src/link_probe.dart

@@ -282,9 +282,12 @@ Future<String> _readText(HttpClientResponse res) async {
 /// audio, or segmented playlists). A byte track is range-read with
 /// yt-dlp's headers in GETs of the format's `http_chunk_size`; a pieces
 /// track's playlist (or DASH fragments) is read on first use, then each
-/// piece is one GET. Format URLs expire: on 403/410 from any track the page
-/// is resolved again once for all tracks (same format ids, same sizes and
-/// piece counts) and the read goes on from where it stopped.
+/// piece is one GET. Format URLs expire: on 403/410/412 from any track the
+/// page is resolved again once for all tracks (same format ids, same sizes
+/// and piece counts) and the read goes on from where it stopped. Another
+/// expiry within [refreshGap] of that resolve fails the read instead (the
+/// server backs off and asks again), so a site refusing the fresh URLs too
+/// is not resolved again and again.
 class PageRelayMedia implements RelayMedia {
   final Uri page;
   final YtDlp ytDlp;
@@ -296,7 +299,10 @@ class PageRelayMedia implements RelayMedia {
   final _http = HttpClient();
   var _gen = 0;
   Future<void>? _refreshing;
-  PageRelayMedia(this.page, this.ytDlp, {List<YtDlpTrack>? tracks}) : _picked = tracks;
+  DateTime? _refreshedAt;
+  final Duration refreshGap;
+  PageRelayMedia(this.page, this.ytDlp, {List<YtDlpTrack>? tracks, this.refreshGap = const Duration(seconds: 60)})
+      : _picked = tracks;
 
   Future<YtDlpInfo> _info() async {
     final info = await ytDlp.resolve(page.toString());
@@ -335,14 +341,22 @@ class PageRelayMedia implements RelayMedia {
     return fetchHlsPlaylist(_http, f.url, f.headers);
   }
 
-  static bool _expired(Object e) => e is UpstreamHttpException && (e.status == 403 || e.status == 410);
+  /// A format URL the site no longer takes: 403, 410, or Bilibili's 412.
+  static bool _expired(Object e) =>
+      e is UpstreamHttpException && (e.status == 403 || e.status == 410 || e.status == 412);
 
   /// Resolves the page again: the same format ids with the same sizes and
   /// piece shapes, else the video changed.
-  Future<void> _refresh(int gen) {
+  Future<void> _refresh(int gen, Object cause) {
     if (gen != _gen) return Future.value();
+    final last = _refreshedAt;
+    if (_refreshing == null && last != null && DateTime.now().difference(last) < refreshGap) {
+      // Resolved a moment ago and refused again: not once more.
+      return Future.error(cause);
+    }
     return _refreshing ??= () async {
-      _log.info('relay of ${logUrl(page)}: a format URL expired (403/410), resolving the page again');
+      _refreshedAt = DateTime.now();
+      _log.info('relay of ${logUrl(page)}: a format URL expired (HTTP ${cause is UpstreamHttpException ? cause.status : '?'}), resolving the page again');
       try {
         const changed = LinkProbeException('the video changed on the page');
         final info = await _info();
@@ -384,7 +398,7 @@ class PageRelayMedia implements RelayMedia {
       return _sizes[i] = await _open(i).size();
     } catch (e) {
       if (!_expired(e)) rethrow;
-      await _refresh(gen);
+      await _refresh(gen, e);
       return _sizes[i] = await _open(i).size();
     }
   }
@@ -397,7 +411,7 @@ class PageRelayMedia implements RelayMedia {
       return _pieces[i] = await _loadPieces(_picked![i].format);
     } catch (e) {
       if (!_expired(e)) rethrow;
-      await _refresh(gen);
+      await _refresh(gen, e);
       return _pieces[i] ??= await _loadPieces(_picked![i].format);
     }
   }
@@ -416,7 +430,7 @@ class PageRelayMedia implements RelayMedia {
       } catch (e) {
         if (retried || !_expired(e)) rethrow;
         retried = true;
-        await _refresh(gen);
+        await _refresh(gen, e);
       }
     }
   }
@@ -435,7 +449,7 @@ class PageRelayMedia implements RelayMedia {
       } catch (e) {
         if (retried || !_expired(e)) rethrow;
         retried = true;
-        await _refresh(gen);
+        await _refresh(gen, e);
       }
     }
   }

packages/core/test/page_relay_test.dart

@@ -0,0 +1,107 @@
+import 'dart:io';
+import 'dart:typed_data';
+
+import 'package:sync_relay_core/core.dart';
+import 'package:test/test.dart';
+
+// A page relayed by its formats: an expired format URL (403/410/412) is
+// resolved again once, and not again and again when the site keeps
+// refusing.
+
+/// yt-dlp that resolves the page to the origin's current format URL.
+class _CountingYtDlp extends YtDlp {
+  final Uri Function() url;
+  var resolves = 0;
+  _CountingYtDlp(super.dir, this.url);
+
+  @override
+  Future<YtDlpInfo> resolve(String page, {Duration timeout = const Duration(seconds: 25)}) async {
+    resolves++;
+    final f = YtDlpFormat(id: 'v', url: url());
+    return YtDlpInfo(title: 'page', tracks: [(kind: '', format: f)], formats: {'v': f});
+  }
+}
+
+void main() {
+  late HttpServer origin;
+  late Directory tmp;
+  // Only `/v<gen>.mp4` of the current generation is served; older ones get
+  // [refusal] (a format URL that expired).
+  var gen = 1;
+  var refusal = 412;
+  final bytes = Uint8List.fromList(List.generate(100000, (i) => i % 251));
+
+  setUp(() async {
+    gen = 1;
+    refusal = 412;
+    tmp = await Directory.systemTemp.createTemp('page-relay');
+    origin = await HttpServer.bind(InternetAddress.loopbackIPv4, 0);
+    origin.listen((req) async {
+      if (req.uri.path != '/v$gen.mp4') {
+        req.response.statusCode = refusal;
+        await req.response.close();
+        return;
+      }
+      final m = RegExp(r'bytes=(\d+)-(\d+)').firstMatch(req.headers.value('range') ?? '')!;
+      final start = int.parse(m.group(1)!), end = int.parse(m.group(2)!);
+      req.response
+        ..statusCode = 206
+        ..headers.contentType = ContentType('video', 'mp4')
+        ..headers.set('content-range', 'bytes $start-$end/${bytes.length}')
+        ..add(bytes.sublist(start, end + 1));
+      await req.response.close();
+    });
+  });
+
+  tearDown(() async {
+    await origin.close(force: true);
+    await tmp.delete(recursive: true);
+  });
+
+  Uri current() => Uri.parse('http://127.0.0.1:${origin.port}/v$gen.mp4');
+
+  Future<List<int>> read(RelayTrack t, int start, int end) async =>
+      [for (final c in await t.bytes!.read(start, end).toList()) ...c];
+
+  for (final status in [403, 410, 412]) {
+    test('HTTP $status: the page is resolved again once and the read goes on', () async {
+      refusal = status;
+      final y = _CountingYtDlp(tmp.path, current);
+      final media = PageRelayMedia(Uri.parse('https://example.com/watch'), y);
+      addTearDown(media.close);
+      final t = (await media.open()).single;
+      expect(await t.bytes!.size(), bytes.length);
+      expect(await read(t, 0, 999), bytes.sublist(0, 1000));
+      gen = 2; // the first URL expired
+      expect(await read(t, 5000, 5999), bytes.sublist(5000, 6000));
+      expect(y.resolves, 2);
+    });
+  }
+
+  test('refused again right after a resolve: the read fails, no resolve storm', () async {
+    final y = _CountingYtDlp(tmp.path, current);
+    final media = PageRelayMedia(Uri.parse('https://example.com/watch'), y);
+    addTearDown(media.close);
+    final t = (await media.open()).single;
+    await t.bytes!.size();
+    gen = 2;
+    expect(await read(t, 0, 9), bytes.sublist(0, 10));
+    gen = 3; // the fresh URL is refused too
+    for (var i = 0; i < 5; i++) {
+      await expectLater(read(t, 0, 9), throwsA(isA<UpstreamHttpException>().having((e) => e.status, 'status', 412)));
+    }
+    expect(y.resolves, 2);
+  });
+
+  test('another expiry after the gap resolves again', () async {
+    final y = _CountingYtDlp(tmp.path, current);
+    final media = PageRelayMedia(Uri.parse('https://example.com/watch'), y, refreshGap: Duration.zero);
+    addTearDown(media.close);
+    final t = (await media.open()).single;
+    await t.bytes!.size();
+    for (gen = 2; gen <= 4; gen++) {
+      expect(await read(t, 100, 199), bytes.sublist(100, 200));
+    }
+    expect(y.resolves, 4);
+  });
+}

packages/server/lib/src/media_endpoint.dart

@@ -249,3 +249,23 @@ MimeType _mimeOf(String? contentType) {
     return MimeType.octetStream;
   }
 }
+
+/// Says `connection: close` on a response to a request that asked for it
+/// (ffmpeg always does). dart:io closes such a connection after the
+/// response but, behind relic, without saying so; ffmpeg (8.x) then sends
+/// its next seek on that dead connection after reading a range to its end
+/// (an mp4's tail) and the seek fails for good ("size -38").
+Handler closeWhenAsked(Handler next) => (req) async {
+      final res = await next(req);
+      if (res is! Response) return res;
+      bool asked;
+      try {
+        asked = req.headers.connection?.isClose ?? false;
+      } catch (_) {
+        asked = false;
+      }
+      if (!asked) return res;
+      return res.copyWith(
+          headers: res.headers.transform(
+              (h) => h.connection = const ConnectionHeader.directives([ConnectionHeaderType.close])));
+    };

packages/server/lib/src/piece_buffer.dart

@@ -81,6 +81,7 @@ class PieceBuffer implements TrackBuffer {
   /// null before a minute of it has arrived.
   double? get bytesPerSec => _knownSecs >= 60 ? _knownBytes / _knownSecs : null;
   DateTime? _retryAt;
+  var _failures = 0; // failed pulls in a row (backoff)
   var _disposed = false;
   Completer<void> _kick = Completer();
   final Completer<void> _ready = Completer();
@@ -167,6 +168,7 @@ class PieceBuffer implements TrackBuffer {
     final pl = _pull;
     if (pl == null || pl.id != id || _disposed) return;
     _pull = null;
+    _failures = 0;
     final data = pl.buf.takeBytes();
     final i = pl.piece;
     _pieces[i] = data;
@@ -182,12 +184,18 @@ class PieceBuffer implements TrackBuffer {
     _wake();
   }
 
+  static final _pullFailThrottle = LogThrottle(burst: 3);
+
   @override
   void feedFail(int id, String reason) {
     if (_pull?.id != id) return;
-    _log.warning('relay piece pull $id failed: $reason');
     _pull = null;
-    _retryAt = DateTime.now().add(const Duration(seconds: 1));
+    final delay = relayRetryDelay(++_failures);
+    _retryAt = DateTime.now().add(delay);
+    final pass = _pullFailThrottle.allow('${identityHashCode(this)}');
+    if (pass != null) {
+      _log.warning('${pass}relay piece pull $id failed ($_failures in a row, next in ${delay.inMilliseconds} ms): $reason');
+    }
     _wake();
   }
 

packages/server/lib/src/relay_buffer.dart

@@ -9,6 +9,21 @@ import 'relay_stats.dart';
 
 final _log = createLogger('relay');
 
+/// A viewer's read ended because its relay went (a swap, server stop):
+/// not a failure, the response just ends.
+class RelayEnded implements Exception {
+  const RelayEnded();
+  @override
+  String toString() => 'relay ended';
+}
+
+/// The wait before the next pull after [failures] failed ones in a row:
+/// 250 ms doubling to [RelayConstants.relayRetryMaxMs] (a source whose
+/// upstream keeps failing must not be asked again and again at once).
+Duration relayRetryDelay(int failures) => failures <= 0
+    ? Duration.zero
+    : Duration(milliseconds: min(RelayConstants.relayRetryMaxMs, 250 * (1 << min(failures - 1, 10))));
+
 /// One track's buffer on the server: a byte stream ([RelayBuffer]) or
 /// pieces ([PieceBuffer]).
 abstract interface class TrackBuffer {
@@ -209,6 +224,9 @@ class RelayBuffer implements TrackBuffer {
 
   var _reqSeq = 0;
   var _disposed = false;
+  // Failed pulls in a row, and when the next pull may start (backoff).
+  var _failures = 0;
+  DateTime? _retryAt;
   Completer<void> _kick = Completer();
   final Completer<void> _ready = Completer();
 
@@ -292,7 +310,11 @@ class RelayBuffer implements TrackBuffer {
       off += take;
       pl.pos += take;
       final done = pl.pos == _sliceEnd(s) + 1;
-      if (done) _slices[s] = pl.takeSlice();
+      if (done) {
+        _slices[s] = pl.takeSlice();
+        // A whole slice came: the source works again.
+        _failures = 0;
+      }
       // Whole, or more of it: whoever waits on it reads on.
       for (final c in _waiters.remove(s) ?? const <Completer<void>>[]) {
         c.complete();
@@ -309,16 +331,26 @@ class RelayBuffer implements TrackBuffer {
     final pl = _pull;
     if (pl != null && pl.id == id) {
       _pull = null;
+      _failures = 0;
       _wake();
     }
   }
 
   @override
   void feedFail(int id, String reason) {
-    if (_pull?.id != id) return;
-    _log.warning('relay pull $id failed: $reason');
-    feedEnd(id);
+    final pl = _pull;
+    if (pl == null || pl.id != id) return;
+    _pull = null;
+    final delay = relayRetryDelay(++_failures);
+    _retryAt = DateTime.now().add(delay);
+    final pass = _pullFailThrottle.allow('${identityHashCode(this)}');
+    if (pass != null) {
+      _log.warning('${pass}relay pull $id failed ($_failures in a row, next in ${delay.inMilliseconds} ms): $reason');
     }
+    _wake();
+  }
+
+  static final _pullFailThrottle = LogThrottle(burst: 3);
 
   // ------------------------------------------------------------- scheduling
 
@@ -412,6 +444,12 @@ class RelayBuffer implements TrackBuffer {
           await _sleep();
           continue;
         }
+        final retry = _retryAt;
+        if (retry != null && DateTime.now().isBefore(retry)) {
+          await _sleep(retry.difference(DateTime.now()));
+          continue;
+        }
+        _retryAt = null;
         _evict();
         final s = _nextMissing();
         if (s == null) {
@@ -431,10 +469,10 @@ class RelayBuffer implements TrackBuffer {
     }
   }
 
-  Future<void> _sleep() async {
+  Future<void> _sleep([Duration limit = const Duration(seconds: 5)]) async {
     _kick = Completer();
     // Also wake periodically: viewers age out of [_active].
-    await _kick.future.timeout(const Duration(seconds: 5), onTimeout: () {});
+    await _kick.future.timeout(limit, onTimeout: () {});
   }
 
   /// The bytes of slice [s] from [from] (offset in the slice) as soon as
@@ -499,7 +537,7 @@ class RelayBuffer implements TrackBuffer {
 
   /// Streams [start]..[end] to [viewer]. Cancelling the subscription (the
   /// viewer went away) ends it; a slice that never arrives ends it with an
-  /// error after [waitLimit].
+  /// error after [waitLimit]; the buffer's [dispose] with [RelayEnded].
   Stream<Uint8List> read(String viewer, int start, int end, {void Function(int n)? onSent}) async* {
     final v = _viewers.putIfAbsent(viewer, _Viewer.new);
     final tail = start >= size - pin;
@@ -520,6 +558,7 @@ class RelayBuffer implements TrackBuffer {
       while (off <= end) {
         final s = off ~/ slice;
         final data = await _waitBytes(v, s, off - s * slice, me, () => tail || v.newest == me);
+        if (_disposed) throw const RelayEnded();
         if (data == null) throw StateError('relay: bytes at $off did not arrive');
         final n = min(data.length, end - off + 1);
         if (n <= 0) continue;

packages/server/lib/src/relay_routes.dart

@@ -1,5 +1,6 @@
 import 'dart:async';
 import 'dart:convert';
+import 'dart:io' show HttpDate, IOSink, Socket;
 import 'dart:math';
 import 'dart:typed_data';
 
@@ -491,7 +492,7 @@ void mountRelayRoutes(
     });
   });
 
-  Future<Response> viewer(Request req, {required bool tracked, bool piece = false}) async {
+  Future<Result> viewer(Request req, {required bool tracked, bool piece = false}) async {
     final stopwatch = Stopwatch()..start();
     final sid = req.pathParameters.raw[#sid] ?? '';
     final token = req.pathParameters.raw[#token] ?? '';
@@ -506,17 +507,18 @@ void mountRelayRoutes(
     }
     // Every outcome goes to the log (throttled per member there); a 503
     // after a long wait is a source that never came up.
-    Response logged(Response res, {int bytes = 0}) {
-      onLog?.call(
+    void note(int status, {int bytes = 0}) => onLog?.call(
         token,
         MediaLog(
           method: req.method.name,
           path: '/r/$sid/$key/${track == null ? '' : '$track/'}${pieceIndex == null ? '' : 'p/$pieceIndex'}',
           rangeHeader: rangeHeader,
-            status: res.statusCode,
+          status: status,
           bytesWritten: bytes,
           durationMs: stopwatch.elapsedMilliseconds,
         ));
+    Response logged(Response res, {int bytes = 0}) {
+      note(res.statusCode, bytes: bytes);
       return res;
     }
 
@@ -571,23 +573,16 @@ void mountRelayRoutes(
       return logged(Response(res.status, body: Body.fromString('status ${res.status}'), headers: headers));
     }
     final end = res.end ?? size - 1;
-    final Stream<Uint8List> body = buffer
-        .read(token, res.start, end, onSent: (n) => session.addBytes(token, n))
-        .transform(StreamTransformer.fromHandlers(handleError: (e, st, sink) {
-      _readFails(token, '${r.name} track ${track ?? 0} from ${res.start}: $e');
-      sink.addError(e, st);
-    }));
-    MimeType mime;
-    try {
-      mime = MimeType.parse(res.headers['content-type'] ?? '');
-    } catch (_) {
-      mime = MimeType.octetStream;
-    }
-    return logged(
-        Response(res.status,
-            body: Body.fromDataStream(body, mimeType: mime, contentLength: end - res.start + 1),
-            headers: headers),
-        bytes: end - res.start + 1);
+    final length = end - res.start + 1;
+    note(res.status, bytes: length);
+    final what = '${r.name} track ${track ?? 0} from ${res.start}';
+    return _rangeResponse(res.status, res.headers, length,
+        req.method == Method.head
+            ? null
+            : buffer.read(token, res.start, end, onSent: (n) => session.addBytes(token, n)),
+        onBroke: (e) => e is RelayEnded
+            ? _log.fine('relay ended under a viewer\'s read: $what')
+            : _readFails(token, '$what: $e'));
   }
 
   // `:seg` is the name of a single file, the track of several (the router
@@ -597,6 +592,74 @@ void mountRelayRoutes(
   app.get('/r/:sid/:token/:mediaId/:seg/p/:piece', (final req) => viewer(req, tracked: true, piece: true));
 }
 
+/// A relayed byte range written on the raw connection: the status line
+/// and headers go out with the first body bytes, or after [headerWait]
+/// without them (dart:io would hold them until body bytes come, and an
+/// ffmpeg whose request gets no headers within its 5 s timeout has lost
+/// the file's size: every later seek fails with "size -38"). Then the
+/// body as the buffer has it; a read waiting on missing bytes keeps the
+/// connection open meanwhile. Always `connection: close` and closed after
+/// the body: ffmpeg asks for close, and one that is not told so reuses the
+/// connection the server closed for its next seek and fails it the same
+/// way. A relay that ends ([RelayEnded]) closes the connection plainly;
+/// any other failure ([onBroke]) resets it. The client going away cancels
+/// the read.
+Hijack _rangeResponse(int status, Map<String, String> headers, int length, Stream<Uint8List>? body,
+    {required void Function(Object e) onBroke, Duration headerWait = const Duration(milliseconds: RelayConstants.relayHeaderWaitMs)}) {
+  return Hijack((ch) async {
+    final out = ch.sink;
+    final head = StringBuffer('HTTP/1.1 $status ${status == 206 ? 'Partial Content' : 'OK'}\r\n')
+      ..write('date: ${HttpDate.format(DateTime.now())}\r\n');
+    headers.forEach((k, v) {
+      if (k != 'content-length' && k != 'connection') head.write('$k: $v\r\n');
+    });
+    head.write('content-length: $length\r\nconnection: close\r\n\r\n');
+    final it = body == null ? null : StreamIterator(body);
+    // A read cancelled while it waits ends with what it was waiting for
+    // (its relay ending, say): moot then.
+    Future<void> stop() async => it?.cancel().catchError((_) {});
+    // The client closing (a seek elsewhere, the player gone) ends the read.
+    final input = ch.stream.listen(null,
+        onDone: () => unawaited(stop()), onError: (_) => unawaited(stop()), cancelOnError: true);
+    var reset = false;
+    // The first bytes go out with the headers when they come in time.
+    Future<bool>? first = it?.moveNext();
+    if (first != null) {
+      await Future.any([first.then((_) {}, onError: (_) {}), Future<void>.delayed(headerWait)]);
+    }
+    try {
+      out.add(latin1.encode('$head'));
+      if (out is IOSink) await out.flush();
+      while (it != null) {
+        final bool more;
+        try {
+          final next = first ?? it.moveNext();
+          first = null;
+          more = await next;
+        } catch (e) {
+          onBroke(e);
+          reset = e is! RelayEnded;
+          break;
+        }
+        if (!more) break;
+        out.add(it.current);
+        if (out is IOSink) await out.flush();
+      }
+    } catch (_) {
+      // The client went away mid-write: nothing to say.
+      reset = true;
+    } finally {
+      await input.cancel().catchError((_) {});
+      await stop();
+      if (reset && out is Socket) {
+        out.destroy();
+      } else {
+        await out.close().catchError((_) {});
+      }
+    }
+  });
+}
+
 Response _plain(int status) => Response(status, body: Body.fromString('status $status'));
 
 final _readFailThrottle = LogThrottle(burst: 3);

packages/server/lib/src/sync_server.dart

@@ -121,6 +121,7 @@ class SyncServer {
         })),
       );
     });
+    _app.use('/', closeWhenAsked);
     final requests = MediaRequestLog();
     void logRequest(String token, MediaLog log) => requests(hub.who(token), log);
     mountSessionMediaRoute(_app, sessions, onLog: logRequest);

packages/server/test/relay_test.dart

@@ -393,6 +393,64 @@ void main() {
       expect(carol.welcome.mediaUrl, mpvEdl(['$carolBase/0/$name.m3u8', '$carolBase/1/$name']));
     });
 
+    test('a slow source: the headers still come in time, the bytes once they arrive', () async {
+      final alice = await join('alice');
+      final bob = await join('bob');
+      await swapRelay(alice);
+      final gated = _GatedBytes(FileRelayBytes(file));
+      await startSource(alice, from: RelayMedia.single(gated));
+      await _until(() => server.hub.relay!.live, 'live');
+      final url = Uri.parse(viewerUrl(bob, mediaIdOf(alice)));
+      final start = 2 * RelayBuffer.slice + 10;
+      final sw = Stopwatch()..start();
+      final res = await _RawResponse.get(url, range: 'bytes=$start-');
+      // No bytes: the headers go out on their own before mpv's 5 s timeout.
+      final head = await res.head.timeout(const Duration(seconds: 6));
+      expect(sw.elapsedMilliseconds, lessThan(5000));
+      expect(head, startsWith('HTTP/1.1 206'));
+      expect(head, contains('content-range: bytes $start-${bytes.length - 1}/${bytes.length}'));
+      expect(head, contains('content-length: ${bytes.length - start}'));
+      expect(head, contains('connection: close'));
+      expect(res.body, isEmpty);
+      gated.open();
+      expect(await res.done.timeout(const Duration(seconds: 10)), isTrue, reason: 'closed plainly');
+      expect(res.body, bytes.sublist(start));
+    });
+
+    test('a range read to its end says connection: close (ffmpeg 8 reuses it otherwise)', () async {
+      final alice = await join('alice');
+      final bob = await join('bob');
+      await swapRelay(alice);
+      await startSource(alice);
+      final url = Uri.parse(viewerUrl(bob, mediaIdOf(alice)));
+      // The 58-byte tail box of an mp4 whose moov comes first, as ffmpeg asks.
+      final res = await _RawResponse.get(url, range: 'bytes=${bytes.length - 58}-');
+      expect(await res.head, contains('connection: close'));
+      expect(await res.done.timeout(const Duration(seconds: 5)), isTrue);
+      expect(res.body, bytes.sublist(bytes.length - 58));
+      // Any other answer to a request that asks for close says so too.
+      final health = await _RawResponse.get(Uri.parse('http://127.0.0.1:${server.boundPort}/health'));
+      expect(await health.head, contains('connection: close'));
+      await health.done;
+    });
+
+    test('a swap ends a viewer\'s waiting read plainly', () async {
+      final alice = await join('alice');
+      final bob = await join('bob');
+      await swapRelay(alice);
+      await startSource(alice, from: RelayMedia.single(_GatedBytes(FileRelayBytes(file))));
+      await _until(() => server.hub.relay!.live, 'live');
+      final res = await _RawResponse.get(Uri.parse(viewerUrl(bob, mediaIdOf(alice))), range: 'bytes=1000-');
+      await res.head.timeout(const Duration(seconds: 6));
+      alice.send({
+        't': 'swap',
+        'media': {'name': 'x', 'size': 0, 'duration': 0, 'url': 'http://example.invalid/x.mkv'},
+      });
+      // Closed, not reset.
+      expect(await res.done.timeout(const Duration(seconds: 5)), isTrue);
+      expect(res.body, isEmpty);
+    });
+
     test('Jellyfin swap can carry a relay', () async {
       final alice = await join('alice');
       final before = alice.raw.length;
@@ -704,6 +762,28 @@ void main() {
       await old.cancel();
     });
 
+    test('a source that keeps failing is asked again with backoff, at once again after bytes', () async {
+      final failing = _FailingBytes(FileRelayBytes(file));
+      final b = RelayBuffer(ahead: 2 * RelayBuffer.slice, back: RelayBuffer.slice, pin: RelayBuffer.slice);
+      final f = LocalRelayFeed(RelayTrack('', failing), b);
+      b.attach(f, bytes.length);
+      addTearDown(() async {
+        b.dispose();
+        await f.close();
+      });
+      final reader = b.read('a', 0, 99).toList();
+      await Future<void>.delayed(const Duration(milliseconds: 2500));
+      // 0, 0.25, 0.75, 1.75 s: not one pull per round trip.
+      expect(failing.reads, inInclusiveRange(3, 5));
+      failing.failing = false;
+      final got = await reader.timeout(const Duration(seconds: 10));
+      expect([for (final c in got) ...c], bytes.sublist(0, 100));
+      expect(relayRetryDelay(0), Duration.zero);
+      expect(relayRetryDelay(1), const Duration(milliseconds: 250));
+      expect(relayRetryDelay(3), const Duration(seconds: 1));
+      expect(relayRetryDelay(50), const Duration(milliseconds: RelayConstants.relayRetryMaxMs));
+    });
+
     test('dispose frees everything and ends waiting readers', () async {
       await read('a', 0, 10);
       buf.dispose();
@@ -762,3 +842,80 @@ class _FakeYtDlp extends YtDlp {
   Future<YtDlpInfo> resolve(String url, {Duration timeout = const Duration(seconds: 25)}) async =>
       YtDlpInfo(title: 'page', tracks: tracks, formats: {for (final t in tracks) t.format.id: t.format});
 }
+
+/// Bytes that come only once [open] is called (a source whose upstream
+/// stalls).
+class _GatedBytes implements RelayBytes {
+  final RelayBytes inner;
+  final _gate = Completer<void>();
+  _GatedBytes(this.inner);
+  void open() => _gate.complete();
+  @override
+  Future<int> size() => inner.size();
+  @override
+  Stream<List<int>> read(int start, int end) async* {
+    await _gate.future;
+    yield* inner.read(start, end);
+  }
+
+  @override
+  Future<void> close() => inner.close();
+}
+
+/// Reads that fail at once while [failing] (an upstream dropping every
+/// connection); [reads] counts them.
+class _FailingBytes implements RelayBytes {
+  final RelayBytes inner;
+  var failing = true;
+  var reads = 0;
+  _FailingBytes(this.inner);
+  @override
+  Future<int> size() => inner.size();
+  @override
+  Stream<List<int>> read(int start, int end) {
+    reads++;
+    if (failing) return Stream.error(const HttpException('Connection closed while receiving data'));
+    return inner.read(start, end);
+  }
+
+  @override
+  Future<void> close() => inner.close();
+}
+
+/// One HTTP/1.1 GET on its own socket, as ffmpeg sends it
+/// (`Connection: close`): the header block as soon as it comes, the body
+/// as it comes, and how the connection ended ([done]: true when the
+/// server closed it plainly, false on a reset).
+class _RawResponse {
+  final _head = Completer<String>();
+  Future<String> get head => _head.future;
+  final body = <int>[];
+  final _done = Completer<bool>();
+  Future<bool> get done => _done.future;
+
+  static Future<_RawResponse> get(Uri url, {String? range}) async {
+    final r = _RawResponse();
+    final s = await Socket.connect(url.host, url.port);
+    final buf = <int>[];
+    s.listen((d) {
+      if (r._head.isCompleted) {
+        r.body.addAll(d);
+        return;
+      }
+      buf.addAll(d);
+      final text = latin1.decode(buf);
+      final at = text.indexOf('\r\n\r\n');
+      if (at < 0) return;
+      r._head.complete(text.substring(0, at).toLowerCase().replaceFirst('http/1.1', 'HTTP/1.1'));
+      r.body.addAll(buf.sublist(at + 4));
+    }, onError: (_) {
+      if (!r._done.isCompleted) r._done.complete(false);
+    }, onDone: () {
+      if (!r._done.isCompleted) r._done.complete(true);
+      s.destroy();
+    });
+    s.write('GET ${url.path} HTTP/1.1\r\nHost: ${url.host}\r\n${range == null ? '' : 'Range: $range\r\n'}'
+        'Connection: close\r\n\r\n');
+    return r;
+  }
+}