Relay HLS/DASH-only pages as pieces tracks (protocol 11)

A track can be served as pieces: the server writes a VOD playlist whose entries are its own piece URLs (segments, init maps, AES-128 keys; byte ranges folded in; yt-dlp's DASH fragments become the same list). A piece buffer per track fills like the byte buffer. Picks a single file, then a split byte pair, then pieces. DRM, live, master playlists are refused. Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
2026-10-09 23:35:31 +03:00
1 parent 05f9e794a9
commit fe5d6c3b3f
@@ -342,10 +342,12 @@ computer and are used by its yt-dlp only (your own playback, "through
 me", and a server you host). Cookies log yt-dlp in as you on those
 sites: keep the file private, and for YouTube consider a throwaway
 account (Google may flag accounts used with yt-dlp).
-YouTube works in all three too. The last two never show the link to
+YouTube works in all three too, and so do pages and links that only
-anyone; they can't pass on live streams or pages that only offer streaming
+offer streaming playlists (HLS `.m3u8`, DASH `.mpd`, the HLS of X videos).
-playlists (use "Everyone fetches it" for those). Your upload has to beat the video's
+The last two never show the link to anyone; they can't pass on live
-bitrate: a 25-minute 2 GB episode is about 11 Mbit/s.
+streams or DRM-protected video (use "Everyone fetches it" for live).
 Your upload has to beat the video's bitrate: a 25-minute 2 GB episode is
 about 11 Mbit/s.
 ## Video quality and shaders
@@ -544,9 +544,10 @@ class SessionCoordinator {
    }
  }
-  /// This viewer's `/r/` URL; an mpv EDL of every track when there are
+  /// This viewer's `/r/` URL; with tracks, a pieces track's playlist, or
-  /// several (a page's separate video and audio).
+  /// an mpv EDL of every track when there are several (a page's separate
-  String _relayMediaUrlFor(String name, int mediaId, [int tracks = 1]) =>
+  /// video and audio).
  String _relayMediaUrlFor(String name, int mediaId, [List<RelayTrackSpec> tracks = const []]) =>
      relayViewerUrl('http://$_wsHost:$_wsPort/r/default/${sync.token}/$mediaId', name, tracks: tracks);
  /// Serves [media] for media [mediaId] on this member's relay channel.
@@ -687,7 +688,7 @@ class SessionCoordinator {
            _awaitLiveFor = r.mediaId;
            health.value = const HealthStatus(HealthSeverity.info, 'the server is getting the video…');
          } else {
-            await _openMedia(_relayMediaUrlFor(name, r.mediaId, relay.tracks.length));
+            await _openMedia(_relayMediaUrlFor(name, r.mediaId, relay.tracks));
          }
        } else {
          var url = stateMediaUrl(r);
@@ -707,7 +708,7 @@ class SessionCoordinator {
    } else if (_awaitLiveFor == r.mediaId && (stateMediaRelay(r)?.live ?? false)) {
      _awaitLiveFor = -1;
      await _openMedia(
-          _relayMediaUrlFor((r.media?['name'] as String?) ?? '', r.mediaId, stateMediaRelay(r)!.tracks.length));
+          _relayMediaUrlFor((r.media?['name'] as String?) ?? '', r.mediaId, stateMediaRelay(r)!.tracks));
    }
    _jellyfinTransport(r);
    _relayStatus(r);
@@ -1262,21 +1263,27 @@ class SessionCoordinator {
      return null;
    }
    if (relay == LinkRelay.me) {
-      // This computer reads it: a file, or a page's formats (one file, or
+      // This computer reads it: a file, or a page's formats (one file,
-      // separate video and audio tracks).
+      // separate video and audio tracks, or segmented playlists).
      if (probe == null) return 'cannot pass this link on: $probeError';
      final problem = probe.relayProblem;
      if (problem != null) return problem;
      final y = await ytDlp.ytDlp;
      final check = LinkRelayMedia(uri, ytDlp: y, probe: probe);
      var size = 0;
-      final List<String> kinds;
+      final List<RelayTrackSpec> specs;
      try {
        final tracks = await check.open().timeout(const Duration(seconds: 15));
        for (final t in tracks) {
-          size += await t.bytes.size().timeout(const Duration(seconds: 15));
+          // A playlist is read (DRM and live refused) but has no size.
          final pieces = t.pieces;
          if (pieces != null) {
            await pieces.list().timeout(const Duration(seconds: 15));
          } else {
            size += await t.bytes!.size().timeout(const Duration(seconds: 15));
          }
-        kinds = [for (final t in tracks) t.kind];
+        }
        specs = relayTrackSpecs(tracks);
      } catch (e) {
        return 'cannot pass this link on: $e';
      } finally {
@@ -1284,11 +1291,11 @@ class SessionCoordinator {
      }
      _pendingOrigin = _LinkOrigin(url, probe, y);
      _swap(HelloMedia(
-          name: name, size: size, duration: 0, relay: MediaRelay(tracks: kinds.length > 1 ? kinds : const [])));
+          name: name, size: size, duration: 0, relay: MediaRelay(tracks: specs)));
      return null;
    }
    // The server reads it with its own yt-dlp; only a page this app knows
-    // it cannot relay (live, no video file) is refused here (the server
+    // it cannot relay (live, nothing a relay can read) is refused here (the server
    // may reach what this computer cannot).
    if (probe != null && probe.isPage && probe.info != null && probe.relayProblem != null) {
      return probe.relayProblem;
@@ -423,7 +423,8 @@ errors leave it out).
 A member's media through the server's memory, for viewers who cannot
 reach it themselves (`packages/core/lib/src/relay.dart` for the wire and
-the source, `packages/server/lib/src/relay_buffer.dart`,
+the source, `relay_pieces.dart` for segmented streams,
 `packages/server/lib/src/relay_buffer.dart`, `piece_buffer.dart`,
 `relay_routes.dart`; wire in [PROTOCOL.md](PROTOCOL.md) "Relay").
 What gets relayed (`SessionCoordinator.swapPicked`):
@@ -450,6 +451,27 @@ streams in sync across seeks; one URL keeps the app's "one media URL"
 path (sync, swap, reopen) as is. The source member plays the page
 itself (yt-dlp hook) like any other link.
 A page with only segmented streams (HLS `m3u8_native`, DASH
 `http_dash_segments`, e.g. X's HLS, generic `.m3u8`/`.mpd`) is relayed as
 **pieces** tracks (`"as":"pieces"`, one muxed playlist or video + audio):
 the fetcher parses the upstream media playlist (`parseHlsPlaylist`), or
 yt-dlp's DASH `fragments` (`dashPieces`: a first fragment without a
 duration is the init section, so no MPD parser), into a
 `RelayPieceList`: each segment, `EXT-X-MAP` init section and AES-128 key
 is a piece, one upstream GET, with `EXT-X-BYTERANGE` folded into the
 piece. The server serves a VOD media playlist it writes itself from that
 list (`RelayPieceList.playlist`: durations, media sequence,
 discontinuities, MAP, AES-128 keys with their IVs; every URI its own
 `p/<i>`), so a source can never point viewers anywhere else. One URL per
 piece also avoids mpv's seek hang on byte-range fMP4 playlists (Apple's
 `bipbop` hung on the first seek read directly). Refused: DRM (SAMPLE-AES,
 `skd:`, any non-identity key format, `EXT-X-SESSION-KEY`, yt-dlp's
 `has_drm`), a live playlist (no `EXT-X-ENDLIST`; yt-dlp's `is_live`
 first), a master or I-frame playlist. A remux on the fetcher (ffmpeg
 `-c copy` into one pipe) and yt-dlp's own download + merge were rejected:
 the size is unknown until the end, a seek past the written part cannot be
 served, and it needs ffmpeg or disk.
 How bytes move:
 - The source dials `/r/.../source` (outbound, so NAT/CGNAT are fine) and
  answers the server's pulls from the file or the URL (`RelaySource` over
@@ -467,8 +489,8 @@ How bytes move:
  8.6 to 12.5 MB/s); chunked, a 1080p relay seeks in under 0.8 s (4 to
  6.6 s unchunked) and even beats mpv reading YouTube directly, since the
  buffer reads ahead at full speed.
- The server keeps one `RelayBuffer` per track of a relayed media (one
+- The server keeps one buffer per track of a relayed media (one for a
-  for a file): 1 MiB slices in
+  file), a `RelayBuffer` for bytes: 1 MiB slices in
  memory, filled by one sequential pull at a time. Kept: the first and
  last 2 MiB (container indexes; mpv reads both on open and on seeks),
  64 MiB behind the slowest viewer and 256 MiB past the furthest, the
@@ -480,9 +502,27 @@ How bytes move:
  each, which keeps both windows at the same playback distance (1080p
  video + aac: about 246 + 10 MiB ahead); each pins its own head and
  tail. A viewer is its token in every track's buffer.
- Filling: a viewer blocked on a slice first (oldest wait first, handing
+- A pieces track gets a `PieceBuffer`: whole pieces by index, each one
-  the source over at slice boundaries), then the pinned slices, then
+  pull (a piece names its index on the channel), served whole with Range
-  read-ahead from the slowest viewer. Nothing is fetched while nobody
+  inside a piece. A viewer's position is its newest requested segment
  (opening the playlist puts a new viewer at the first one, so its first
  pieces are fetched before it asks). Filled: a blocked viewer's piece
  (oldest wait first; a pull nobody waits on gives way at once), the init
  and key pieces the viewers' segments use, then segments ahead of each
  viewer up to the ahead budget, slowest first. Init and key pieces are
  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
  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
  video's HLS video + audio 0.3 to 0.75 s, seeks 0.03 to 0.4 s; A/V sync
  within 0.2 ms after every seek; a second viewer of the same pieces
  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. Nothing is fetched while nobody
  watches through the relay (a Jellyfin source with only account holders
  uploads nothing).
 - mpv leaves superseded requests open but unread after a seek: only a
@@ -498,7 +538,8 @@ How bytes move:
  stop. A crash frees it with the process.
 - Source away: viewers keep playing from the buffer; a request waits up
  to 60 s for missing bytes. The source reconnects with backoff (same
-  token after a resume); a different size drops that track's buffer.
+  token after a resume); a different size (or piece count and roles)
  drops that track's buffer.
  `state.media.relay.live` follows the channel, and viewers show "waiting
  for <name> to pass the video on" while it is down.
@@ -581,7 +622,12 @@ all three link modes. Nothing is bundled or installed by hand.
    and the server buffer, and mpv decodes them all), then bitrate; audio the original language, not a
    `-drc` variant unless alone, m4a/aac before opus, then bitrate.
    YouTube has only these (299 + 140 for a 1080p60 video).
-  - Neither (only HLS/DASH playlists) or a live stream: nothing to relay.
+  - Neither: segmented streams (`pickPieces`: `m3u8_native`/`m3u8` with a
    media playlist URL, or `http_dash_segments` with http(s) `fragments`;
    not `has_drm`): one muxed playlist (video and audio not known to be
    absent) unless a video-only + audio-only pair is taller, with the
    same height, codec and audio rules. Relayed as pieces (see "Relay").
  - A live stream, or nothing of the above: nothing to relay.
 - **Probe** (`probeLink`, `link_probe.dart`): a `Range: bytes=0-0` GET; a
  video/audio type, or a generic one on a media file name, is a file
  (`looksLikeMediaFile`); anything else goes to yt-dlp, and when yt-dlp
@@ -595,14 +641,16 @@ all three link modes. Nothing is bundled or installed by hand.
  (`script-opts=ytdl_hook-ytdl_path=...`, `ytdl-format=bv*[height<=1080]+ba/b`,
  once per player), waiting for the download unless the URL names a media
  file. A hook error shows as "can't open this link: ...".
- **Through me**: the putter's probe must give tracks, else the swap is
+- **Through me**: the putter's probe must give tracks, and a pieces
-  refused ("Live streams can't be passed on", or no video file: only
+  track's playlist must be readable, else the swap is refused ("Live
-  streaming playlists) with a hint to use "Everyone fetches it". The swap
+  streams can't be passed on", DRM, or nothing a relay can read) with a
-  names the tracks; its relay source reads them with yt-dlp's headers
+  hint to use "Everyone fetches it". The swap names the tracks; its relay
-  (`PageRelayMedia`). On 403/410 (expired format URLs, 6 h on YouTube) from
+  source reads them with yt-dlp's headers (`PageRelayMedia`). On 403/410
-  any track it resolves the page again once for all tracks, finds the
+  (expired format URLs, 6 h on YouTube; signed segment URLs) from any
-  same format ids with the same sizes (else "the video changed on the
+  track it resolves the page again once for all tracks, finds the same
-  page") and goes on from where it stopped. Format URLs are bound to the
+  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
  resolving machine's address, so they never leave it.
 - **Server fetches it**: the server probes the link itself
  (`LinkRelayMedia`) with its own yt-dlp (off with `--no-yt-dlp` /
@@ -235,17 +235,22 @@ flow control, cases) is in [ARCHITECTURE.md](ARCHITECTURE.md), "Relay".
 swap  media.relay = {}               the swapper's machine serves the bytes
      media.relay = {"tracks":[{"kind":"video"},{"kind":"audio"}]}
                                     the same, as two tracks played together
      media.relay = {"tracks":[{"kind":"video","as":"pieces"}]}
                                     the same, as one segmented playlist
      media.relay = {"fetch":true}   the server reads media.url itself
 state media.relay = {"source":"m2","live":true}   or {"fetch":true,"live":true}
      media.relay = {"fetch":true,"live":true,"tracks":[{"kind":"video"},{"kind":"audio"}]}
 ```
-Tracks: a relayed media is one file, or several byte tracks (a page's
+Tracks: a relayed media is one file, or tracks that viewers play as one
-separate video and audio files) that viewers play as one. `tracks` lists
+(a page's separate video and audio). `tracks` lists them in order, up to
-them in order, 2 to 4 entries of `kind` `video` or `audio`; absent (or
+4 entries of `kind` `video` or `audio`, each a Range-readable byte stream,
-anything else) means one file. Every track is a Range-readable byte
+or with `"as":"pieces"` a segmented stream (an HLS media playlist or DASH
-stream; other kinds of track (e.g. segmented playlists) would add a field
+fragments) relayed piece by piece and served as a VOD media playlist. A
-to the entry.
+piece is one segment, init section (`EXT-X-MAP`) or AES-128 key, fetched
 upstream whole with one GET. At least 2 entries, or 1 pieces entry (a
 muxed playlist, `kind` `video`); absent, an unknown `kind` or `as`, or
 anything else means one file.
 - `swap`: `relay` with `jellyfin` keeps the ref (the swapper serves it to
  viewers without an account; `fetch` is ignored there). `relay` without
@@ -269,19 +274,27 @@ to the entry.
  Clients build it after a swap like the `/m` one:
  `http://<host>/r/default/<token>/<mediaId>/<name>` (name with `/` and
  `\` made `_`, then URL-encoded; the server ignores it). With `tracks`,
-  it is an mpv EDL of every track's URL
+  each track's URL is `http://<host>/r/default/<token>/<mediaId>/<i>/<name>`
-  `http://<host>/r/default/<token>/<mediaId>/<i>/<name>`, each framed by
+  (a pieces track: `<name>.m3u8`); one track is opened as is, several as
-  its byte length:
+  an mpv EDL of them all, each framed by its byte length:
  `edl://!no_clip;%<len>%<track 0 url>;!new_stream;!no_clip;%<len>%<track 1 url>`.
 - `GET /r/<sid>/<token>/<mediaId>/<name>`: the media (one file), Range
  table as for `/m`. `GET /r/<sid>/<token>/<mediaId>/<i>/<name>`: track
  `i` (from 0) of a media with `tracks` (track 0 of a one-file media
-  too). Errors: 401 bad token, 404 not the current relayed
+  too); for a pieces track (any name), its media playlist
-  media (a player still on an older `mediaId` gets 404, never the new
+  (`application/vnd.apple.mpegurl`): `#EXT-X-PLAYLIST-TYPE:VOD`,
-  bytes) or no such track (the one-file URL of a media with `tracks`), 410
+  `EXT-X-MEDIA-SEQUENCE`, `EXTINF` durations, `EXT-X-DISCONTINUITY`,
-  unknown session, 503 + `retry-after` the source never connected or the
+  `EXT-X-MAP` and `EXT-X-KEY:METHOD=AES-128` (with `IV` when given) as
-  server could not read the link (60 s). A request whose bytes do not
+  upstream, `EXT-X-ENDLIST`; every URI is `p/<j>`. `GET
-  arrive within 60 s ends early. Bytes count toward `bytesServed`.
+  /r/<sid>/<token>/<mediaId>/<i>/p/<j>`: piece `j` of pieces track `i`,
  whole, once it is in (a Range inside the piece answers 206). Errors: 401
  bad token, 404 not the current relayed media (a player still on an
  older `mediaId` gets 404, never the new bytes), no such track (the
  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`.
 - `GET /r/<sid>/<token>/source` (WebSocket): the source's relay channel,
  dialled by the source (works behind NAT). Only the current source's
  token is accepted; anyone else, or the source after the next swap, is
@@ -293,18 +306,31 @@ source -> server  {"t":"info","size":<bytes>}            first frame, one file
                  {"t":"info","tracks":[{"kind":"video","size":<bytes>},
                                        {"kind":"audio","size":<bytes>}]}
                                                         first frame, tracks
                  {"t":"info","tracks":[{"kind":"video","as":"pieces","seq":5,
                     "pieces":[{"init":true},{"key":true},
                               {"d":6.0,"map":0,"key":1,"iv":"0x..."},
                               {"d":4.5,"map":0,"disc":true}]}]}
                                                         a pieces track
                  binary: [u32 big-endian pull id][bytes]  in order per pull
                  {"t":"end","id":1}  {"t":"fail","id":1,"reason":"..."}
 server -> source  {"t":"pull","id":1,"track":0,"start":0,"end":1048575}
                                                         inclusive; no track: 0
                  {"t":"pull","id":2,"track":1,"piece":17} piece 17, whole
                  {"t":"ack","id":1,"n":65536}             bytes arrived
                  {"t":"cancel","id":1}                    stop that pull
 ```
  Pull ids are unique across the tracks of a connection; `track` is sent
-  only for a media with `tracks`. An `info` whose track count differs
+  only for a media with `tracks`. A pieces track's `pieces` are its
-  from the swap's `tracks` closes the channel with 4001. A track `size`
+  pieces in order: `{"init":true}` an init section, `{"key":true}` an
-  different from the one before (a reconnect) drops that track's buffer.
+  AES-128 key, else a segment of `d` seconds whose `map` / `key` name the
  init and key pieces it uses, `iv` an explicit IV (`0x` + hex), `disc` a
  discontinuity before it; `seq` (default 0) the media sequence of the
  first segment. A piece pull's bytes end with `end` like a range pull.
  An `info` whose tracks differ from the swap's `tracks` (count, bytes or
  pieces), or a malformed piece list, closes the channel with 4001. A
  track `size` or piece count/roles different from the one before (a
  reconnect) drops that track's buffer.
 ## Join and reconnect
@@ -19,22 +19,18 @@ off by deleting it in the same commit as the code. Wire changes also edit
   through media_kit; if not, resolve in the app and open the relay
   tracks' formats. YouTube: yt-dlp wants a JS runtime (deno) for full
   extraction. Formats that need yt-dlp's cookies.
   - Relay "pieces" tracks for pages with only HLS/DASH (`m3u8_native`,
     `http_dash_segments`): a track served as a synthesized VOD media
     playlist whose entries are pieces (one segment, init `EXT-X-MAP` or
     AES-128 key = one upstream GET, served whole, Range inside a piece
     honoured), with a piece buffer per track (pieces by index, filled
     like `RelayBuffer`: a blocked viewer, the init piece, then ahead of
     the slowest viewer; evicted by distance); `EXT-X-BYTERANGE` folded
     into pieces (it also avoids mpv's seek hang on byte-range fMP4
     playlists); yt-dlp's DASH `fragments` turned into the same playlist,
     so no MPD parser. Wire: a track entry gains `"as":"pieces"`, `info`
     lists piece durations, a pull names a piece. Refuse DRM
     (`skd:`/SAMPLE-AES/Widevine). Measured in a spike: piece relay of
     Mux/Apple/Akamai HLS/DASH opened in 1.5 to 2.2 s, seeks 0.02 to 2.4 s.
   - Live streams through the relay (today refused: the sync model needs a
     fixed duration and seeking).
-   - `tool/relay_e2e.dart`: a two-track case over a shaped source link.
+   - `tool/relay_e2e.dart`: a two-track case (bytes and pieces) over a
     shaped source link. Pieces on a slow source: a cold piece can take
     longer than media_kit's 5 s `network-timeout`; if it does, fetch the
     first pieces before setting `relay.live`.
   - Pieces, unverified so far: the 403/410 re-resolve against a real
     expiring HLS (signed segment URLs), HLS discontinuities / ad
     splicing, Windows' bundled ffmpeg with extensionless piece URLs
     (`p/<i>`; fine with ffmpeg 9 on Linux).
   - Thumbnails and scrubbing previews of a relayed media with tracks
     should open the video track's URL alone, not the EDL.
 4. **AniList account**: sign in (OAuth via a localhost callback), "My list"
   filter on the Schedule; today's "Following" (AnimeFlow's monitored
@@ -5,6 +5,7 @@ export 'src/clock_sync.dart';
 export 'src/config.dart';
 export 'src/protocol.dart';
 export 'src/relay.dart';
 export 'src/relay_pieces.dart';
 export 'src/link_probe.dart';
 export 'src/yt_dlp.dart';
 export 'src/cookies.dart';
@@ -5,6 +5,7 @@ import 'dart:typed_data';
 import 'constants.dart';
 import 'relay.dart';
 import 'relay_pieces.dart';
 import 'yt_dlp.dart';
 // What a pasted link is: a media file (range-readable as is) or a web page
@@ -71,12 +72,8 @@ String _unescape(String s) => s
 String notAVideoText(String? ytDlpError) =>
    ytDlpError == null ? 'Not a video' : "Not a video (and yt-dlp couldn't read it: $ytDlpError)";
-/// The text for a page with no video file a relay can read.
+/// The text for a page with no video a relay can read.
-const noRelayFormatText = 'This page has no video file to pass on (only streaming playlists). '
+const noRelayFormatText = "This page has no video a relay can read. Choose 'Everyone fetches it' instead.";
    "Choose 'Everyone fetches it' instead.";
 /// The text for a live stream.
 const liveStreamText = "Live streams can't be passed on. Choose 'Everyone fetches it' instead.";
 class LinkProbeException implements Exception {
  final String message;
@@ -190,18 +187,22 @@ Future<String> _readText(HttpClientResponse res) async {
  return utf8.decode(out.takeBytes(), allowMalformed: true);
 }
-/// A page's relay tracks (see [YtDlpInfo.tracks]: one file, or video +
+/// A page's relay tracks (see [YtDlpInfo.tracks]: one file, video +
-/// audio), range-read with yt-dlp's headers in GETs of the format's
+/// audio, or segmented playlists). A byte track is range-read with
-/// `http_chunk_size`. Format URLs expire: on 403/410 from any track the
+/// yt-dlp's headers in GETs of the format's `http_chunk_size`; a pieces
-/// page is resolved again once for all tracks (same formats, same sizes)
+/// track's playlist (or DASH fragments) is read on first use, then each
-/// and the read goes on from where it stopped.
+/// 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.
 class PageRelayMedia implements RelayMedia {
  final Uri page;
  final YtDlp ytDlp;
  List<YtDlpTrack>? _picked;
  List<HttpRelayBytes?> _inner = const [];
  List<UpstreamPieces?> _pieces = const [];
  List<RelayTrack>? _tracks;
  final _sizes = <int, int>{};
  final _http = HttpClient();
  var _gen = 0;
  Future<void>? _refreshing;
  PageRelayMedia(this.page, this.ytDlp, {List<YtDlpTrack>? tracks}) : _picked = tracks;
@@ -218,7 +219,13 @@ class PageRelayMedia implements RelayMedia {
    final picked = _picked ??= (await _info()).tracks;
    if (picked.isEmpty) throw const LinkProbeException(noRelayFormatText);
    _inner = List.filled(picked.length, null, growable: false);
-    return _tracks = [for (var i = 0; i < picked.length; i++) RelayTrack(picked[i].kind, _PageTrack(this, i))];
+    _pieces = List.filled(picked.length, null, growable: false);
    return _tracks = [
      for (var i = 0; i < picked.length; i++)
        picked[i].format.isPieces
            ? RelayTrack.pieces(picked[i].kind, _PagePieces(this, i))
            : RelayTrack(picked[i].kind, _PageTrack(this, i)),
    ];
  }
  HttpRelayBytes _open(int i) => _inner[i] ??= () {
@@ -227,26 +234,45 @@ class PageRelayMedia implements RelayMedia {
            headers: f.headers, chunkSize: f.chunkSize ?? RelayConstants.relayUpstreamChunkBytes);
      }();
  /// Reads track [i]'s playlist (or DASH fragments) into pieces.
  Future<UpstreamPieces> _loadPieces(YtDlpFormat f) async {
    final frags = f.fragments;
    if (f.protocol == 'http_dash_segments') {
      if (frags == null) throw const RelayPiecesException('no DASH fragments');
      return dashPieces(frags);
    }
    return fetchHlsPlaylist(_http, f.url, f.headers);
  }
  static bool _expired(Object e) => e is UpstreamHttpException && (e.status == 403 || e.status == 410);
-  /// Resolves the page again: the same format ids with the same sizes,
+  /// Resolves the page again: the same format ids with the same sizes and
-  /// else the video changed.
+  /// piece shapes, else the video changed.
  Future<void> _refresh(int gen) {
    if (gen != _gen) return Future.value();
    return _refreshing ??= () async {
      try {
        const changed = LinkProbeException('the video changed on the page');
        final info = await _info();
        final next = <YtDlpTrack>[];
        for (final t in _picked!) {
          final f = info.formats[t.format.id];
-          if (f == null) throw const LinkProbeException('the video changed on the page');
+          if (f == null || f.isPieces != t.format.isPieces) throw changed;
          next.add((kind: t.kind, format: f));
        }
        final pieces = List<UpstreamPieces?>.filled(next.length, null, growable: false);
        for (var i = 0; i < next.length; i++) {
          final had = _pieces[i];
          if (had == null) continue;
          final p = pieces[i] = await _loadPieces(next[i].format);
          if (!p.list.sameShape(had.list)) throw changed;
        }
        final old = _inner;
        _picked = next;
        _pieces = pieces;
        _inner = List.filled(next.length, null, growable: false);
        for (final e in _sizes.entries) {
-          if (await _open(e.key).size() != e.value) throw const LinkProbeException('the video changed on the page');
+          if (await _open(e.key).size() != e.value) throw changed;
        }
        _gen++;
        for (final b in old) {
@@ -271,6 +297,19 @@ class PageRelayMedia implements RelayMedia {
    }
  }
  Future<UpstreamPieces> _piecesOf(int i) async {
    final known = _pieces[i];
    if (known != null) return known;
    final gen = _gen;
    try {
      return _pieces[i] = await _loadPieces(_picked![i].format);
    } catch (e) {
      if (!_expired(e)) rethrow;
      await _refresh(gen);
      return _pieces[i] ??= await _loadPieces(_picked![i].format);
    }
  }
  Stream<List<int>> _read(int i, int start, int end) async* {
    var pos = start;
    var retried = false;
@@ -290,11 +329,31 @@ class PageRelayMedia implements RelayMedia {
    }
  }
  /// Piece [index] of track [i]; on expiry once more after a re-resolve
  /// (nothing was sent yet: an expired URL fails before its first byte).
  Stream<List<int>> _readPiece(int i, int index) async* {
    var retried = false;
    while (true) {
      final gen = _gen;
      final p = await _piecesOf(i);
      if (index >= p.sources.length) throw RangeError.index(index, p.sources, 'piece');
      try {
        yield* readPiece(_http, p.sources[index], _picked![i].format.headers);
        return;
      } catch (e) {
        if (retried || !_expired(e)) rethrow;
        retried = true;
        await _refresh(gen);
      }
    }
  }
  @override
  Future<void> close() async {
    for (final b in _inner) {
      await b?.close();
    }
    _http.close(force: true);
  }
 }
@@ -314,6 +373,22 @@ class _PageTrack implements RelayBytes {
  Future<void> close() async {}
 }
 class _PagePieces implements RelayPieces {
  final PageRelayMedia media;
  final int index;
  _PagePieces(this.media, this.index);
  @override
  Future<RelayPieceList> list() async => (await media._piecesOf(index)).list;
  @override
  Stream<List<int>> read(int piece) => media._readPiece(index, piece);
  /// The media closes its tracks.
  @override
  Future<void> close() async {}
 }
 /// A link of either kind, probed on first [open] (or given [probe]): a
 /// file as one track ([HttpRelayBytes]), a page as its picked formats
 /// ([PageRelayMedia]). Throws [LinkProbeException] with the reason when it
@@ -144,46 +144,75 @@ class HelloMedia {
  }
 }
 /// One track of a relayed media: [kind] `video` or `audio`; a byte stream
 /// (a Range-readable file) unless [pieces] (a segmented playlist relayed
 /// piece by piece, served as a media playlist). JSON `{"kind":"video"}`,
 /// `{"kind":"audio","as":"pieces"}`.
 class RelayTrackSpec {
  final String kind;
  final bool pieces;
  const RelayTrackSpec(this.kind, {this.pieces = false});
  Map<String, Object?> toJson() => {'kind': kind, if (pieces) 'as': 'pieces'};
  /// Null when [j] is not a track entry this side knows.
  static RelayTrackSpec? fromJson(Object? j) {
    if (j is! Map || !MediaRelay.trackKinds.contains(j['kind'])) return null;
    final as = j['as'];
    if (as != null && as != 'bytes' && as != 'pieces') return null;
    return RelayTrackSpec(j['kind'] as String, pieces: as == 'pieces');
  }
  @override
  bool operator ==(Object other) => other is RelayTrackSpec && other.kind == kind && other.pieces == pieces;
  @override
  int get hashCode => Object.hash(kind, pieces);
  @override
  String toString() => pieces ? '$kind pieces' : kind;
 }
 /// Relayed media: the bytes go through the server's buffer (`/r/...`).
 /// Either from the [source] member's machine (it dials the relay channel),
 /// or, with [fetch], the server reads the swap's link itself. A swap sends
 /// `{}` or `{"fetch":true}`; the server fills in [source] (the swapper's
 /// member id, empty with [fetch]) and whether bytes can flow ([live]).
-/// [tracks]: the kinds (`video`, `audio`) of a media relayed as several
+/// [tracks]: the tracks of a media relayed as several played together, or
-/// tracks played together; empty for one file. A member swap names them,
+/// as one segmented playlist; empty for one file. A member swap names
-/// the server fills them in for [fetch] once it has read the link.
+/// them, the server fills them in for [fetch] once it has read the link.
 class MediaRelay {
  final String source;
  final bool fetch;
  final bool live;
-  final List<String> tracks;
+  final List<RelayTrackSpec> tracks;
  const MediaRelay({this.source = '', this.fetch = false, this.live = false, this.tracks = const []});
  static const trackKinds = {'video', 'audio'};
  static const maxTracks = 4;
-  MediaRelay copyWith({bool? live, List<String>? tracks}) =>
+  /// [tracks] as sent: several, or one pieces track; else one file.
  static bool validTracks(List<RelayTrackSpec> t) =>
      t.length <= maxTracks && (t.length > 1 || (t.length == 1 && t.single.pieces));
  MediaRelay copyWith({bool? live, List<RelayTrackSpec>? tracks}) =>
      MediaRelay(source: source, fetch: fetch, live: live ?? this.live, tracks: tracks ?? this.tracks);
  Map<String, Object?> toJson() => {
        if (source.isNotEmpty) 'source': source,
        if (fetch) 'fetch': true,
        if (live) 'live': true,
-        if (tracks.length > 1) 'tracks': [for (final k in tracks) {'kind': k}],
+        if (validTracks(tracks)) 'tracks': [for (final t in tracks) t.toJson()],
      };
-  /// Unknown kinds, a single entry or too many make it one file.
+  /// Unknown kinds, a single byte track or too many make it one file.
  factory MediaRelay.fromJson(Map<String, Object?> json) {
    final t = json['tracks'];
-    final kinds = t is List
+    final specs = t is List ? [for (final e in t) RelayTrackSpec.fromJson(e)] : const <RelayTrackSpec?>[];
-        ? [for (final e in t) e is Map && e['kind'] is String ? e['kind'] as String : '']
+    final ok = !specs.contains(null) && validTracks(specs.cast<RelayTrackSpec>());
        : <String>[];
    final ok = kinds.length > 1 && kinds.length <= maxTracks && kinds.every(trackKinds.contains);
    return MediaRelay(
        source: json['source'] is String ? json['source'] as String : '',
        fetch: json['fetch'] == true,
        live: json['live'] == true,
-        tracks: ok ? List.unmodifiable(kinds) : const []);
+        tracks: ok ? List.unmodifiable(specs.cast<RelayTrackSpec>()) : const []);
  }
 }
@@ -7,20 +7,26 @@ import 'dart:typed_data';
 import 'constants.dart';
 import 'logging.dart';
 import 'protocol.dart';
 import 'relay_pieces.dart';
 // The relay channel: a member's machine (the source) serves the current
 // media's bytes to the server, which buffers them for every viewer (`/r/...`).
 // The source dials out (works behind NAT/CGNAT) on its own WebSocket, so
 // video bytes never queue behind `/sync` messages. A media is one track (a
-// file) or several (a page's separate video and audio).
+// file) or several (a page's separate video and audio); a track is a
 // byte stream or pieces (a segmented playlist, see relay_pieces.dart).
 //
 //   source -> server  {"t":"info","size":N}               first frame, one track
 //                     {"t":"info","tracks":[{"kind":"video","size":N},...]}
 //                     {"t":"info","tracks":[{"kind":"video","as":"pieces",
 //                                            "pieces":[{"init":true},{"d":6.0,"map":0},...]}]}
 //                     binary [u32 BE pull id][bytes]      in order per pull
 //                     {"t":"end","id":1}                  pull complete
 //                     {"t":"fail","id":1,"reason":"..."}  cannot read
 //   server -> source  {"t":"pull","id":1,"track":0,"start":0,"end":N}
 //                                                         bytes start..end
 //                     {"t":"pull","id":2,"track":0,"piece":17}  one piece, whole
 //                     {"t":"ack","id":1,"n":65536}         n bytes arrived
 //                     {"t":"cancel","id":1}               stop this pull
 //
@@ -37,11 +43,14 @@ String relayUrlName(String name) => Uri.encodeComponent(name.replaceAll(RegExp(r
 /// What a viewer opens for relayed media at [base]
 /// (`http://<host>/r/<sid>/<token>/<mediaId>`): `<base>/<name>` for one
-/// track, else an mpv EDL playing `<base>/<i>/<name>` of every track
+/// file; with [tracks], each track's URL (`<base>/<i>/<name>`, a pieces
-/// together (see [mpvEdl]).
+/// track's playlist `<base>/<i>/<name>.m3u8`), several played together as
-String relayViewerUrl(String base, String name, {int tracks = 1}) {
+/// an mpv EDL (see [mpvEdl]).
 String relayViewerUrl(String base, String name, {List<RelayTrackSpec> tracks = const []}) {
  final n = relayUrlName(name);
-  return tracks <= 1 ? '$base/$n' : mpvEdl([for (var i = 0; i < tracks; i++) '$base/$i/$n']);
+  if (tracks.isEmpty) return '$base/$n';
  final urls = [for (var i = 0; i < tracks.length; i++) '$base/$i/$n${tracks[i].pieces ? '.m3u8' : ''}'];
  return urls.length == 1 ? urls.single : mpvEdl(urls);
 }
 /// An mpv `edl://` URL playing [urls] as the streams of one file (a video
@@ -244,14 +253,40 @@ class RelayWindow {
  }
 }
-/// One byte stream of a relayed media: [kind] is `video` or `audio` when
+/// Where a source reads a pieces track from (see relay_pieces.dart).
-/// a media has several, empty for a single file.
+abstract interface class RelayPieces {
  /// The track's shape; a playlist is fetched on the first call. Throws
  /// [RelayPiecesException] when it cannot be relayed (DRM, live).
  Future<RelayPieceList> list();
  /// Piece [index], whole, in order. Cancelling stops reading.
  Stream<List<int>> read(int index);
  Future<void> close();
 }
 /// One track of a relayed media: [kind] is `video` or `audio` when a
 /// media has several (or is pieces), empty for a single file. Read as
 /// [bytes] (a Range-readable stream) or as [pieces].
 class RelayTrack {
  final String kind;
-  final RelayBytes bytes;
+  final RelayBytes? bytes;
-  const RelayTrack(this.kind, this.bytes);
+  final RelayPieces? pieces;
  const RelayTrack(this.kind, RelayBytes this.bytes) : pieces = null;
  const RelayTrack.pieces(this.kind, RelayPieces this.pieces) : bytes = null;
  RelayTrackSpec get spec => RelayTrackSpec(kind.isEmpty ? 'video' : kind, pieces: pieces != null);
 }
 /// The wire's `relay.tracks` for [tracks]: empty for one file.
 List<RelayTrackSpec> relayTrackSpecs(List<RelayTrack> tracks) =>
    tracks.length == 1 && tracks.single.pieces == null ? const [] : [for (final t in tracks) t.spec];
 /// A track's `info` entry: its kind and size, or its pieces.
 Future<Map<String, Object?>> relayTrackInfo(RelayTrack t) async => t.pieces != null
    ? {...t.spec.toJson(), ...(await t.pieces!.list()).toJson()}
    : {'kind': t.kind, 'size': await t.bytes!.size()};
 /// What a relay serves: one track, or several played together (a page's
 /// separate video and audio).
 abstract interface class RelayMedia {
@@ -317,16 +352,11 @@ class RelaySource {
  /// when it cannot be read.
  Future<void> start() async {
    final tracks = await media.open();
-    final sizes = [for (final t in tracks) await t.bytes.size()];
+    final infos = [for (final t in tracks) await relayTrackInfo(t)];
    _tracks = tracks;
    unawaited(_loop({
      't': 'info',
-      if (tracks.length == 1)
+      if (relayTrackSpecs(tracks).isEmpty) 'size': infos.single['size'] else 'tracks': infos,
        'size': sizes.single
      else
        'tracks': [
          for (var i = 0; i < tracks.length; i++) {'kind': tracks[i].kind, 'size': sizes[i]},
        ],
    }));
  }
@@ -373,12 +403,20 @@ class RelaySource {
    if (id is! int) return;
    switch (j['t']) {
      case 'pull':
-        final start = j['start'], end = j['end'], track = j['track'] ?? 0;
+        final start = j['start'], end = j['end'], track = j['track'] ?? 0, piece = j['piece'];
        if (start is! int || end is! int || start < 0 || end < start) return;
        if (track is! int || track < 0 || track >= _tracks.length) return;
        final t = _tracks[track];
        final Stream<List<int>> Function() read;
        if (piece is int) {
          if (t.pieces == null || piece < 0) return;
          read = () => t.pieces!.read(piece);
        } else {
          if (start is! int || end is! int || start < 0 || end < start || t.bytes == null) return;
          read = () => t.bytes!.read(start, end);
        }
        final p = _Pull(id);
        _pulls[id] = p;
-        unawaited(_serve(ws, p, _tracks[track].bytes, start, end));
+        unawaited(_serve(ws, p, read));
      case 'ack':
        final p = _pulls[id];
        final n = j['n'];
@@ -409,8 +447,8 @@ class RelaySource {
    _pulls.clear();
  }
-  Future<void> _serve(WebSocket ws, _Pull p, RelayBytes bytes, int start, int end) async {
+  Future<void> _serve(WebSocket ws, _Pull p, Stream<List<int>> Function() read) async {
-    final it = p.it = StreamIterator(bytes.read(start, end));
+    final it = p.it = StreamIterator(read());
    final pending = BytesBuilder(copy: false);
    const frame = RelayConstants.relayFrameBytes;
    try {
@@ -0,0 +1,385 @@
 import 'dart:async';
 import 'dart:convert';
 import 'dart:io';
 import 'dart:math';
 import 'relay.dart';
 // Pieces tracks: a segmented stream (an HLS media playlist, or yt-dlp's
 // DASH fragments) relayed piece by piece. A piece is one upstream GET
 // served whole: a segment, an init section (`EXT-X-MAP`) or an AES-128
 // key. The fetcher (the source member, or the server for a link it reads
 // itself) parses the upstream playlist into a [RelayPieceList] (no URLs);
 // the server turns that into the VOD media playlist viewers open, whose
 // entries are its own `p/<i>` piece URLs (see [RelayPieceList.playlist]).
 /// Why a segmented stream cannot be relayed.
 class RelayPiecesException implements Exception {
  final String message;
  const RelayPiecesException(this.message);
  @override
  String toString() => message;
 }
 /// The text for DRM-protected media.
 const drmText = "This video is DRM-protected; it can't be relayed.";
 /// The text for a live stream.
 const liveStreamText = "Live streams can't be passed on. Choose 'Everyone fetches it' instead.";
 enum RelayPieceRole { segment, init, key }
 /// One piece of a [RelayPieceList]. A segment has its duration [d], the
 /// init piece [map] and AES-128 key piece [key] (with an explicit [iv])
 /// that apply to it, and [disc] when an `EXT-X-DISCONTINUITY` precedes it.
 class RelayPiece {
  final RelayPieceRole role;
  final double d;
  final int? map, key;
  final String? iv;
  final bool disc;
  const RelayPiece.segment(this.d, {this.map, this.key, this.iv, this.disc = false})
      : role = RelayPieceRole.segment;
  const RelayPiece.init()
      : role = RelayPieceRole.init,
        d = 0,
        map = null,
        key = null,
        iv = null,
        disc = false;
  const RelayPiece.key()
      : role = RelayPieceRole.key,
        d = 0,
        map = null,
        key = null,
        iv = null,
        disc = false;
  bool get isSegment => role == RelayPieceRole.segment;
  Map<String, Object?> toJson() => switch (role) {
        RelayPieceRole.init => {'init': true},
        RelayPieceRole.key => {'key': true},
        RelayPieceRole.segment => {
            'd': d,
            if (map != null) 'map': map,
            if (key != null) 'key': key,
            if (iv != null) 'iv': iv,
            if (disc) 'disc': true,
          },
      };
 }
 final _ivRe = RegExp(r'^0[xX][0-9a-fA-F]{1,32}$');
 /// The shape of a pieces track: what the server needs to serve it (no
 /// upstream URLs). Sent in the relay channel's `info` (see PROTOCOL.md).
 class RelayPieceList {
  final List<RelayPiece> pieces;
  /// `EXT-X-MEDIA-SEQUENCE` of the first segment (AES-128 without an
  /// explicit IV uses the segment's sequence number).
  final int seq;
  const RelayPieceList(this.pieces, {this.seq = 0});
  static const maxPieces = 200000;
  int get length => pieces.length;
  double get duration => pieces.fold(0.0, (a, p) => a + p.d);
  /// Same piece count and roles (a re-resolve or a reconnect of the same
  /// media).
  bool sameShape(RelayPieceList o) {
    if (o.length != length) return false;
    for (var i = 0; i < length; i++) {
      if (o.pieces[i].role != pieces[i].role) return false;
    }
    return true;
  }
  Map<String, Object?> toJson() => {
        if (seq != 0) 'seq': seq,
        'pieces': [for (final p in pieces) p.toJson()],
      };
  /// Parses and checks a track entry's `seq` / `pieces`; null when
  /// malformed (an index out of range or of the wrong role, a bad IV).
  static RelayPieceList? fromJson(Map j) {
    final raw = j['pieces'];
    final seq = j['seq'] ?? 0;
    if (raw is! List || raw.isEmpty || raw.length > maxPieces || seq is! int || seq < 0) return null;
    final roles = <RelayPieceRole>[];
    for (final e in raw) {
      if (e is! Map) return null;
      roles.add(e['init'] == true
          ? RelayPieceRole.init
          : e['key'] == true
              ? RelayPieceRole.key
              : RelayPieceRole.segment);
    }
    int? ref(Object? v, RelayPieceRole want) {
      if (v == null) return null;
      if (v is! int || v < 0 || v >= roles.length || roles[v] != want) throw const FormatException();
      return v;
    }
    final out = <RelayPiece>[];
    try {
      for (var i = 0; i < raw.length; i++) {
        final e = raw[i] as Map;
        switch (roles[i]) {
          case RelayPieceRole.init:
            out.add(const RelayPiece.init());
          case RelayPieceRole.key:
            out.add(const RelayPiece.key());
          case RelayPieceRole.segment:
            final d = e['d'], iv = e['iv'];
            if (d is! num || !d.isFinite || d < 0 || d > 3600) return null;
            if (iv != null && (iv is! String || !_ivRe.hasMatch(iv))) return null;
            out.add(RelayPiece.segment(d.toDouble(),
                map: ref(e['map'], RelayPieceRole.init),
                key: ref(e['key'], RelayPieceRole.key),
                iv: iv as String?,
                disc: e['disc'] == true));
        }
      }
    } on FormatException {
      return null;
    }
    if (!out.any((p) => p.isSegment)) return null;
    return RelayPieceList(List.unmodifiable(out), seq: seq);
  }
  /// The VOD media playlist viewers open: every entry is `p/<i>`
  /// (relative to the playlist's URL), init and key pieces included.
  String playlist() {
    final segs = pieces.where((p) => p.isSegment);
    final target = max(1, segs.map((p) => p.d).fold(0.0, max).ceil());
    final b = StringBuffer()
      ..writeln('#EXTM3U')
      ..writeln('#EXT-X-VERSION:6')
      ..writeln('#EXT-X-TARGETDURATION:$target')
      ..writeln('#EXT-X-MEDIA-SEQUENCE:$seq')
      ..writeln('#EXT-X-PLAYLIST-TYPE:VOD');
    int? map, key;
    String? iv;
    for (var i = 0; i < pieces.length; i++) {
      final p = pieces[i];
      if (!p.isSegment) continue;
      if (p.disc) b.writeln('#EXT-X-DISCONTINUITY');
      if (p.map != null && p.map != map) b.writeln('#EXT-X-MAP:URI="p/${p.map}"');
      if (p.key != key || p.iv != iv) {
        b.writeln(p.key == null
            ? '#EXT-X-KEY:METHOD=NONE'
            : '#EXT-X-KEY:METHOD=AES-128,URI="p/${p.key}"${p.iv == null ? '' : ',IV=${p.iv}'}');
      }
      map = p.map;
      key = p.key;
      iv = p.iv;
      b
        ..writeln('#EXTINF:${p.d.toStringAsFixed(6)},')
        ..writeln('p/$i');
    }
    b.writeln('#EXT-X-ENDLIST');
    return b.toString();
  }
 }
 /// Where the fetcher reads piece `i`: one GET of [url], of [length] bytes
 /// at [offset] when it is a byte range of a bigger file.
 typedef PieceSource = ({Uri url, int? offset, int? length});
 /// A parsed upstream stream: the shape plus where each piece lives.
 typedef UpstreamPieces = ({RelayPieceList list, List<PieceSource> sources});
 Map<String, String> _attrs(String s) {
  final out = <String, String>{};
  for (final m in RegExp(r'([A-Z0-9-]+)=("[^"]*"|[^,]*)').allMatches(s)) {
    var v = m.group(2)!;
    if (v.length >= 2 && v.startsWith('"') && v.endsWith('"')) v = v.substring(1, v.length - 1);
    out[m.group(1)!] = v;
  }
  return out;
 }
 /// An HLS media playlist at [base] as pieces: every segment, `EXT-X-MAP`
 /// and AES-128 `EXT-X-KEY` becomes a piece; `EXT-X-BYTERANGE` (and a MAP's
 /// `BYTERANGE`) is folded into the piece (the offset defaults to the end
 /// of the previous range of the same URI). Throws [RelayPiecesException]
 /// for DRM (SAMPLE-AES, `skd:`, any non-identity key format), a live
 /// playlist (no `EXT-X-ENDLIST`), a master playlist or no segments.
 UpstreamPieces parseHlsPlaylist(String text, Uri base) {
  final lines = const LineSplitter().convert(text).map((l) => l.trim()).toList();
  if (lines.isEmpty || !lines.first.startsWith('#EXTM3U')) {
    throw const RelayPiecesException('not an HLS playlist');
  }
  final pieces = <RelayPiece>[];
  final sources = <PieceSource>[];
  final lastEnd = <String, int>{};
  final shared = <String, int>{}; // init/key piece by "url offset length"
  var seq = 0;
  var ended = false;
  double? extinf;
  int? brLen, brOff, map, key;
  String? iv;
  var disc = false;
  int add(RelayPiece p, Uri url, int? len, int? off) {
    if (len != null) {
      off ??= lastEnd['$url'] ?? 0;
      lastEnd['$url'] = off + len;
    }
    pieces.add(p);
    sources.add((url: url, offset: len == null ? null : off, length: len));
    return pieces.length - 1;
  }
  int sharedPiece(RelayPiece p, String uri, (int, int?)? range) {
    final url = base.resolve(uri);
    if (!url.isScheme('http') && !url.isScheme('https')) throw const RelayPiecesException(drmText);
    final k = '${p.role} $url ${range?.$2} ${range?.$1}';
    return shared[k] ??= add(p, url, range?.$1, range?.$2);
  }
  (int, int?) range(String v) {
    final parts = v.split('@');
    final len = int.tryParse(parts[0]);
    final off = parts.length > 1 ? int.tryParse(parts[1]) : null;
    if (len == null || len <= 0 || (parts.length > 1 && (off == null || off < 0))) {
      throw const RelayPiecesException('bad byte range in the playlist');
    }
    return (len, off);
  }
  for (final line in lines) {
    if (line.isEmpty) continue;
    if (!line.startsWith('#')) {
      final url = base.resolve(line);
      if (!url.isScheme('http') && !url.isScheme('https')) {
        throw const RelayPiecesException('the playlist has a segment that is not http(s)');
      }
      add(RelayPiece.segment(extinf ?? 0, map: map, key: key, iv: iv, disc: disc), url, brLen, brOff);
      extinf = null;
      brLen = brOff = null;
      disc = false;
      continue;
    }
    final colon = line.indexOf(':');
    final tag = colon < 0 ? line : line.substring(0, colon);
    final value = colon < 0 ? '' : line.substring(colon + 1);
    switch (tag) {
      case '#EXT-X-STREAM-INF':
        throw const RelayPiecesException('a master playlist, not a media playlist');
      case '#EXT-X-I-FRAMES-ONLY':
        throw const RelayPiecesException('an I-frame playlist');
      case '#EXT-X-SESSION-KEY':
        throw const RelayPiecesException(drmText);
      case '#EXT-X-MEDIA-SEQUENCE':
        seq = int.tryParse(value) ?? 0;
      case '#EXT-X-ENDLIST':
        ended = true;
      case '#EXT-X-DISCONTINUITY':
        disc = true;
      case '#EXTINF':
        extinf = double.tryParse(value.split(',').first.trim());
      case '#EXT-X-BYTERANGE':
        (brLen, brOff) = range(value);
      case '#EXT-X-MAP':
        final a = _attrs(value);
        final uri = a['URI'];
        if (uri == null) throw const RelayPiecesException('EXT-X-MAP without a URI');
        map = sharedPiece(const RelayPiece.init(), uri, a['BYTERANGE'] == null ? null : range(a['BYTERANGE']!));
      case '#EXT-X-KEY':
        final a = _attrs(value);
        final method = (a['METHOD'] ?? '').toUpperCase();
        if (method == 'NONE') {
          key = null;
          iv = null;
          continue;
        }
        final format = a['KEYFORMAT'];
        final uri = a['URI'] ?? '';
        if (method != 'AES-128' || (format != null && format != 'identity') || uri.isEmpty) {
          throw const RelayPiecesException(drmText);
        }
        final v = a['IV'];
        if (v != null && !_ivRe.hasMatch(v)) throw const RelayPiecesException('bad IV in the playlist');
        key = sharedPiece(const RelayPiece.key(), uri, null);
        iv = v;
    }
  }
  if (!ended) throw const RelayPiecesException(liveStreamText);
  if (!pieces.any((p) => p.isSegment)) throw const RelayPiecesException('the playlist has no segments');
  if (pieces.length > RelayPieceList.maxPieces) throw const RelayPiecesException('the playlist is too long');
  return (list: RelayPieceList(List.unmodifiable(pieces), seq: seq), sources: List.unmodifiable(sources));
 }
 /// yt-dlp's DASH `fragments` (`http_dash_segments`) as pieces: a first
 /// fragment without a duration is the init section, every other one a
 /// segment (so no MPD parser of our own).
 UpstreamPieces dashPieces(List<({Uri url, double? duration})> fragments) {
  if (fragments.isEmpty) throw const RelayPiecesException('no fragments');
  final pieces = <RelayPiece>[];
  final sources = <PieceSource>[];
  int? map;
  for (var i = 0; i < fragments.length; i++) {
    final f = fragments[i];
    if (i == 0 && f.duration == null) {
      map = 0;
      pieces.add(const RelayPiece.init());
    } else {
      pieces.add(RelayPiece.segment(f.duration ?? 0, map: map));
    }
    sources.add((url: f.url, offset: null, length: null));
  }
  if (!pieces.any((p) => p.isSegment)) throw const RelayPiecesException('no fragments');
  return (list: RelayPieceList(List.unmodifiable(pieces)), sources: List.unmodifiable(sources));
 }
 /// Fetches an HLS media playlist with [headers] and parses it
 /// ([parseHlsPlaylist]).
 Future<UpstreamPieces> fetchHlsPlaylist(HttpClient client, Uri url, Map<String, String> headers) async {
  final req = await client.getUrl(url).timeout(const Duration(seconds: 20));
  headers.forEach(req.headers.set);
  final res = await req.close().timeout(const Duration(seconds: 20));
  if (res.statusCode != 200) {
    await res.drain<void>().catchError((_) {});
    throw UpstreamHttpException(res.statusCode, url);
  }
  final text = await res.transform(utf8.decoder).join().timeout(const Duration(seconds: 30));
  // Relative entries resolve against where the playlist ended up.
  final at = res.redirects.isEmpty ? url : url.resolveUri(res.redirects.last.location);
  return parseHlsPlaylist(text, at);
 }
 /// Reads one piece: a GET of its URL (with its byte range), whole.
 Stream<List<int>> readPiece(HttpClient client, PieceSource s, Map<String, String> headers) async* {
  final req = await client.getUrl(s.url).timeout(const Duration(seconds: 20));
  headers.forEach(req.headers.set);
  req.headers.set(HttpHeaders.acceptEncodingHeader, 'identity');
  final off = s.offset, len = s.length;
  if (len != null) req.headers.set(HttpHeaders.rangeHeader, 'bytes=${off ?? 0}-${(off ?? 0) + len - 1}');
  final res = await req.close().timeout(const Duration(seconds: 20));
  if (res.statusCode != 200 && res.statusCode != 206) {
    await res.drain<void>().catchError((_) {});
    throw UpstreamHttpException(res.statusCode, s.url);
  }
  if (len == null) {
    yield* res;
    return;
  }
  // A server ignoring Range sends the whole file: cut the piece out.
  var skip = res.statusCode == 200 ? (off ?? 0) : 0;
  var left = len;
  await for (final c in res) {
    var from = 0;
    if (skip > 0) {
      from = min(skip, c.length);
      skip -= from;
    }
    final take = min(c.length - from, left);
    if (take > 0) yield from == 0 && take == c.length ? c : c.sublist(from, from + take);
    left -= take;
    if (left <= 0) return;
  }
  if (left > 0) throw HttpException('upstream ended early', uri: s.url);
 }
@@ -25,7 +25,8 @@ class YtDlpException implements Exception {
 }
 /// One playable format of a resolved page: a URL and the headers yt-dlp
-/// says to send with it.
+/// says to send with it. A segmented format ([isPieces]) is an HLS media
 /// playlist at [url], or DASH [fragments].
 class YtDlpFormat {
  final String id;
  final Uri url;
@@ -35,8 +36,23 @@ class YtDlpFormat {
  /// yt-dlp's `downloader_options.http_chunk_size`: the longest GET the
  /// site serves at full speed.
  final int? chunkSize;
  /// yt-dlp's `protocol`: `https`/`http` (a file), `m3u8_native`/`m3u8`
  /// (an HLS media playlist), `http_dash_segments` ([fragments]).
  final String protocol;
  /// DASH fragments (a first one without a duration is the init section).
  final List<({Uri url, double? duration})>? fragments;
  const YtDlpFormat(
-      {required this.id, required this.url, this.headers = const {}, this.height, this.ext = '', this.chunkSize});
+      {required this.id,
      required this.url,
      this.headers = const {},
      this.height,
      this.ext = '',
      this.chunkSize,
      this.protocol = 'https',
      this.fragments});
  /// Relayed as pieces (see relay_pieces.dart), not as a byte stream.
  bool get isPieces => _piecesProtocols.contains(protocol);
  static YtDlpFormat _of(Map f) {
    final headers = f['http_headers'];
@@ -50,10 +66,34 @@ class YtDlpFormat {
      height: h == 0 ? null : h,
      ext: '${f['ext']}',
      chunkSize: chunk is num && chunk > 0 ? chunk.toInt() : null,
      protocol: '${f['protocol']}',
      fragments: f['protocol'] == 'http_dash_segments' ? _fragments(f) : null,
    );
  }
  /// yt-dlp's `fragments` (`url`, or `path` under `fragment_base_url`),
  /// or null when one is not plain http(s).
  static List<({Uri url, double? duration})>? _fragments(Map f) {
    final raw = f['fragments'];
    final base = Uri.tryParse('${f['fragment_base_url'] ?? f['url'] ?? ''}');
    if (raw is! List || raw.isEmpty) return null;
    final out = <({Uri url, double? duration})>[];
    for (final fr in raw) {
      if (fr is! Map) return null;
      final u = fr['url'] is String
          ? Uri.tryParse(fr['url'] as String)
          : fr['path'] is String && base != null
              ? base.resolve(fr['path'] as String)
              : null;
      if (u == null || (!u.isScheme('http') && !u.isScheme('https'))) return null;
      out.add((url: u, duration: fr['duration'] is num ? (fr['duration'] as num).toDouble() : null));
    }
    return out;
  }
 }
 const _piecesProtocols = {'m3u8_native', 'm3u8', 'http_dash_segments'};
 /// A format a relay plays as one of a media's tracks ([kind] `video` or
 /// `audio`; empty for a single file holding both).
 typedef YtDlpTrack = ({String kind, YtDlpFormat format});
@@ -69,11 +109,11 @@ class YtDlpInfo {
  final String? thumbnail;
  /// A live stream (never relayed).
  final bool isLive;
-  /// What a relay range-reads (see [pickTracks]): one file, or a video and
+  /// What a relay reads (see [pickTracks]): one file, a video and an
-  /// an audio file; empty when the page has neither (only streaming
+  /// audio file, or segmented playlists; empty when there is nothing (or
-  /// playlists, or live).
+  /// live).
  final List<YtDlpTrack> tracks;
-  /// Every plain http(s) format by id, to find [tracks] again after a
+  /// Every relayable format by id, to find [tracks] again after a
  /// re-resolve.
  final Map<String, YtDlpFormat> formats;
  const YtDlpInfo({
@@ -110,7 +150,7 @@ class YtDlpInfo {
      tracks: isLive ? const [] : pickTracks(formats),
      formats: {
        for (final f in formats)
-          if (_httpUrl(f) && f is Map) '${f['format_id'] ?? ''}': YtDlpFormat._of(f),
+          if ((_httpUrl(f) || _piecesUrl(f)) && f is Map) '${f['format_id'] ?? ''}': YtDlpFormat._of(f),
      },
    );
  }
@@ -129,6 +169,16 @@ bool _httpUrl(Object? f) {
  return uri != null && (uri.isScheme('http') || uri.isScheme('https'));
 }
 /// A segmented format (HLS playlist or DASH fragments) over http(s), not
 /// DRM-protected (the playlist's keys are checked when it is read).
 bool _piecesUrl(Object? f) {
  if (f is! Map || f['has_drm'] == true || !_piecesProtocols.contains(f['protocol'])) return false;
  final url = f['url'];
  final uri = url is String ? Uri.tryParse(url) : null;
  if (uri == null || (!uri.isScheme('http') && !uri.isScheme('https'))) return false;
  return f['protocol'] != 'http_dash_segments' || YtDlpFormat._fragments(f) != null;
 }
 int _height(Map f) => f['height'] is num ? (f['height'] as num).toInt() : 0;
 double _tbr(Map f) =>
    f['tbr'] is num ? (f['tbr'] as num).toDouble() : (f['abr'] is num ? (f['abr'] as num).toDouble() : 0);
@@ -180,19 +230,22 @@ int _videoCodecRank(Map f) {
 /// shortest above), HEVC before AV1 before VP9 before h264 (smallest
 /// first, see [_videoCodecRank]), then bitrate. Audio: the original language, not a `-drc` variant, m4a/aac
 /// before others (opus), then bitrate.
-(YtDlpFormat, YtDlpFormat)? pickSplit(List<Object?> formats, {int maxHeight = 1080}) {
+(YtDlpFormat, YtDlpFormat)? pickSplit(List<Object?> formats, {int maxHeight = 1080}) => _pickPair(
-  final video = [
+      [
        for (final f in formats)
          if (_httpUrl(f) && (f as Map)['acodec'] == 'none' && f['vcodec'] is String && f['vcodec'] != 'none' &&
              _singleFileExts.contains(f['ext']))
            f,
-  ];
+      ],
-  final audio = [
+      [
        for (final f in formats)
          if (_httpUrl(f) && (f as Map)['vcodec'] == 'none' && f['acodec'] is String && f['acodec'] != 'none' &&
              _audioExts.contains(f['ext']))
            f,
-  ];
+      ],
      maxHeight);
 (YtDlpFormat, YtDlpFormat)? _pickPair(List<Map> video, List<Map> audio, int maxHeight) {
  if (video.isEmpty || audio.isEmpty) return null;
  video.sort((a, b) {
    final h = _byHeight(_height(a), _height(b), maxHeight);
@@ -213,20 +266,44 @@ int _videoCodecRank(Map f) {
  return (YtDlpFormat._of(video.first), YtDlpFormat._of(audio.first));
 }
 /// The pair if its video is taller than [single] (by the [maxHeight] rule).
 bool _pairWins((YtDlpFormat, YtDlpFormat)? pair, YtDlpFormat? single, int maxHeight) =>
    pair != null &&
    (single == null ||
        (single.height != null &&
            pair.$1.height != null &&
            _byHeight(pair.$1.height!, single.height!, maxHeight) < 0));
 /// Segmented formats (HLS / DASH): one muxed playlist (video and audio
 /// not known to be absent) unless a video-only + audio-only pair is
 /// taller; the same height, codec and audio rules as files.
 List<YtDlpTrack> pickPieces(List<Object?> formats, {int maxHeight = 1080}) {
  final ok = [for (final f in formats) if (_piecesUrl(f)) f as Map];
  final muxed = [for (final f in ok) if (f['vcodec'] != 'none' && f['acodec'] != 'none') f]..sort((a, b) {
      final h = _byHeight(_height(a), _height(b), maxHeight);
      return h != 0 ? h : _tbr(b).compareTo(_tbr(a));
    });
  final pair = _pickPair([for (final f in ok) if (f['acodec'] == 'none' && f['vcodec'] != 'none') f],
      [for (final f in ok) if (f['vcodec'] == 'none' && f['acodec'] != 'none') f], maxHeight);
  final single = muxed.isEmpty ? null : YtDlpFormat._of(muxed.first);
  if (_pairWins(pair, single, maxHeight)) {
    return [(kind: 'video', format: pair!.$1), (kind: 'audio', format: pair.$2)];
  }
  return single == null ? const [] : [(kind: 'video', format: single)];
 }
 /// What a relay reads of [formats]: the single file ([pickSingleFile])
 /// unless a video + audio pair ([pickSplit]) is taller (both by the
-/// [maxHeight] rule); empty when there is neither.
+/// [maxHeight] rule); with neither, segmented playlists ([pickPieces]);
 /// empty when there is nothing.
 List<YtDlpTrack> pickTracks(List<Object?> formats, {int maxHeight = 1080}) {
  final single = pickSingleFile(formats, maxHeight: maxHeight);
  final split = pickSplit(formats, maxHeight: maxHeight);
-  if (split != null &&
+  if (_pairWins(split, single, maxHeight)) {
-      (single == null ||
+    return [(kind: 'video', format: split!.$1), (kind: 'audio', format: split.$2)];
          (single.height != null &&
              split.$1.height != null &&
              _byHeight(split.$1.height!, single.height!, maxHeight) < 0))) {
    return [(kind: 'video', format: split.$1), (kind: 'audio', format: split.$2)];
  }
-  return single == null ? const [] : [(kind: '', format: single)];
+  if (single != null) return [(kind: '', format: single)];
  return pickPieces(formats, maxHeight: maxHeight);
 }
 /// The official standalone build for this platform, or null if there is
@@ -0,0 +1,130 @@
 // Pieces tracks: HLS playlist parsing (byte ranges folded, init and key
 // pieces, refusals), the synthesized playlist, and the track wire.
 import 'package:sync_relay_core/core.dart';
 import 'package:test/test.dart';
 void main() {
  final base = Uri.parse('https://cdn.example/v/index.m3u8');
  test('byte ranges fold into pieces; MAP, AES-128 key and sequence carry over to the served playlist', () {
    final p = parseHlsPlaylist('''
 #EXTM3U
 #EXT-X-VERSION:7
 #EXT-X-TARGETDURATION:6
 #EXT-X-MEDIA-SEQUENCE:5
 #EXT-X-MAP:URI="main.mp4",BYTERANGE="700@0"
 #EXT-X-KEY:METHOD=AES-128,URI="https://keys.example/k1",IV=0x00000000000000000000000000000001
 #EXTINF:6.0,
 #EXT-X-BYTERANGE:1000@700
 main.mp4
 #EXTINF:5.5,
 #EXT-X-BYTERANGE:2000
 main.mp4
 #EXT-X-KEY:METHOD=NONE
 #EXT-X-DISCONTINUITY
 #EXTINF:4,
 seg/3.ts?tok=a
 #EXT-X-ENDLIST
 ''', base);
    expect([for (final s in p.sources) (s.url.toString(), s.offset, s.length)], [
      ('https://cdn.example/v/main.mp4', 0, 700),
      ('https://keys.example/k1', null, null),
      ('https://cdn.example/v/main.mp4', 700, 1000),
      ('https://cdn.example/v/main.mp4', 1700, 2000), // offset: end of the previous range
      ('https://cdn.example/v/seg/3.ts?tok=a', null, null),
    ]);
    // Through the wire and back: same shape, same playlist.
    final list = RelayPieceList.fromJson(p.list.toJson())!;
    expect(list.sameShape(p.list), isTrue);
    expect(list.playlist(), '''
 #EXTM3U
 #EXT-X-VERSION:6
 #EXT-X-TARGETDURATION:6
 #EXT-X-MEDIA-SEQUENCE:5
 #EXT-X-PLAYLIST-TYPE:VOD
 #EXT-X-MAP:URI="p/0"
 #EXT-X-KEY:METHOD=AES-128,URI="p/1",IV=0x00000000000000000000000000000001
 #EXTINF:6.000000,
 p/2
 #EXTINF:5.500000,
 p/3
 #EXT-X-DISCONTINUITY
 #EXT-X-KEY:METHOD=NONE
 #EXTINF:4.000000,
 p/4
 #EXT-X-ENDLIST
 ''');
  });
  test('refused: DRM, live, master; a malformed piece list', () {
    String pl(String body, {bool end = true}) => '#EXTM3U\n$body\n#EXTINF:4,\na.ts\n${end ? '#EXT-X-ENDLIST' : ''}';
    Matcher refused(String text) =>
        throwsA(isA<RelayPiecesException>().having((e) => e.message, 'message', text));
    expect(() => parseHlsPlaylist(pl('#EXT-X-KEY:METHOD=SAMPLE-AES,URI="skd://x"'), base), refused(drmText));
    expect(() => parseHlsPlaylist(pl('#EXT-X-KEY:METHOD=AES-128,URI="skd://x"'), base), refused(drmText));
    expect(
        () => parseHlsPlaylist(
            pl('#EXT-X-KEY:METHOD=AES-128,URI="k",KEYFORMAT="urn:uuid:edef8ba9-79d6-4ace-a3c8-27dcd51d21ed"'), base),
        refused(drmText));
    expect(() => parseHlsPlaylist(pl(''), base).list, returnsNormally);
    expect(() => parseHlsPlaylist(pl('', end: false), base), refused(liveStreamText));
    expect(() => parseHlsPlaylist('#EXTM3U\n#EXT-X-STREAM-INF:BANDWIDTH=1\nv.m3u8\n', base),
        throwsA(isA<RelayPiecesException>()));
    expect(
        RelayPieceList.fromJson({
          'pieces': [
            {'d': 4.0, 'map': 0},
          ],
        }),
        isNull,
        reason: 'map must name an init piece');
    expect(
        RelayPieceList.fromJson({
          'pieces': [
            {'d': 4.0, 'iv': '0x1"\n#EXT'},
          ],
        }),
        isNull);
  });
  test('track wire: bytes entries as before, pieces with "as"; one pieces track is enough', () {
    final two = MediaRelay.fromJson({
      'source': 'm2',
      'tracks': [
        {'kind': 'video', 'as': 'pieces'},
        {'kind': 'audio', 'as': 'pieces'},
      ],
    });
    expect(two.tracks, const [RelayTrackSpec('video', pieces: true), RelayTrackSpec('audio', pieces: true)]);
    expect(two.toJson()['tracks'], [
      {'kind': 'video', 'as': 'pieces'},
      {'kind': 'audio', 'as': 'pieces'},
    ]);
    final one = MediaRelay.fromJson({
      'tracks': [
        {'kind': 'video', 'as': 'pieces'},
      ],
    });
    expect(one.tracks.single.pieces, isTrue);
    // One byte track is one file; an unknown "as" drops the list.
    expect(
        MediaRelay.fromJson({
          'tracks': [
            {'kind': 'video'},
          ],
        }).tracks,
        isEmpty);
    expect(
        MediaRelay.fromJson({
          'tracks': [
            {'kind': 'video', 'as': 'frames'},
            {'kind': 'audio'},
          ],
        }).tracks,
        isEmpty);
    expect(const MediaRelay(tracks: [RelayTrackSpec('video'), RelayTrackSpec('audio')]).toJson()['tracks'], [
      {'kind': 'video'},
      {'kind': 'audio'},
    ]);
  });
 }
@@ -101,7 +101,8 @@ void main() {
      final single = _f('22', 'https', 'avc1', 'mp4a', 720);
      expect(pickTracks([single, ...youtube]).map((x) => x.kind), ['video', 'audio']);
      expect(pickTracks([_f('18', 'https', 'avc1', 'mp4a', 1080), ...youtube]).single.format.id, '18');
-      expect(pickTracks([_f('96', 'm3u8_native', 'avc1', 'mp4a', 1080)]), isEmpty);
+      expect(pickTracks([_f('96', 'm3u8_native', 'avc1', 'mp4a', 1080)]).single.format.isPieces, isTrue);
      expect(pickTracks([_f('96', 'rtmp', 'avc1', 'mp4a', 1080)]), isEmpty);
      final live = YtDlpInfo.parse({'title': 'x', 'is_live': true, 'formats': youtube});
      expect(live.isLive, isTrue);
      expect(live.tracks, isEmpty);
@@ -117,8 +118,10 @@ void main() {
  test('viewer URL: one track plain, several an mpv EDL with %length% framing', () {
    expect(relayViewerUrl('http://h:1/r/default/t/3', 'a/b c'), 'http://h:1/r/default/t/3/a_b%20c');
    const v = 'http://h:1/r/default/t/3/0/x', a = 'http://h:1/r/default/t/3/1/x';
-    expect(relayViewerUrl('http://h:1/r/default/t/3', 'x', tracks: 2),
+    expect(relayViewerUrl('http://h:1/r/default/t/3', 'x', tracks: const [RelayTrackSpec('video'), RelayTrackSpec('audio')]),
        'edl://!no_clip;%${v.length}%$v;!new_stream;!no_clip;%${a.length}%$a');
    expect(relayViewerUrl('http://h:1/r/default/t/3', 'x', tracks: const [RelayTrackSpec('video', pieces: true)]),
        'http://h:1/r/default/t/3/0/x.m3u8');
    expect(mpvEdl(['http://x/a;b,c']), 'edl://!no_clip;%14%http://x/a;b,c');
  });
@@ -191,7 +194,7 @@ void main() {
    expect(probe.title, startsWith('a girl.'));
    expect(probe.relayProblem, isNull);
    final media = LinkRelayMedia(url, ytDlp: y, probe: probe);
-    final bytes = (await media.open()).single.bytes;
+    final bytes = (await media.open()).single.bytes!;
    final size = await bytes.size();
    var n = 0;
    await for (final c in bytes.read(0, size - 1)) {
@@ -201,6 +204,33 @@ void main() {
    expect(n, size);
  }, timeout: const Timeout(Duration(minutes: 3)), skip: Platform.environment['YTDLP_LIVE'] == '1' ? false : 'YTDLP_LIVE not set');
  test('pieces only without a file: the tweet\'s HLS video + audio; DASH fragments; muxed HLS', () {
    final tweet = [
      _f('hls-audio-128000-Audio', 'm3u8_native', 'none', null, null, tbr: 128),
      _f('hls-430', 'm3u8_native', 'avc1.64001F', 'none', 598, tbr: 430),
    ];
    expect([for (final t in pickTracks(tweet)) (t.kind, t.format.id, t.format.isPieces)],
        [('video', 'hls-430', true), ('audio', 'hls-audio-128000-Audio', true)]);
    // A byte file (even a shorter one) comes first.
    expect(pickTracks([...tweet, _f('http-832', 'https', null, null, 360)]).single.format.id, 'http-832');
    final dash = _f('v', 'http_dash_segments', 'avc1', 'none', 1080, extra: {
      'fragment_base_url': 'https://cdn.example/b/',
      'fragments': [
        {'path': 'init.mp4'},
        {'path': 's1.m4s', 'duration': 4.0},
        {'url': 'https://other.example/s2.m4s', 'duration': 2.5},
      ],
    });
    final t = pickTracks([dash, _f('a', 'http_dash_segments', 'none', 'mp4a', null, extra: {'fragments': [{'url': 'https://cdn.example/a', 'duration': 6.5}]})]);
    final p = dashPieces(t.first.format.fragments!);
    expect(p.sources.map((s) => '${s.url}'), ['https://cdn.example/b/init.mp4', 'https://cdn.example/b/s1.m4s', 'https://other.example/s2.m4s']);
    expect((p.list.pieces.first.role, p.list.pieces[2].map, p.list.duration), (RelayPieceRole.init, 0, 6.5));
    // A muxed playlist wins unless the pair is taller.
    final muxed = _f('m', 'm3u8_native', 'avc1', 'mp4a', 720);
    expect(pickTracks([muxed, ...tweet]).single.format.id, 'm');
    expect(pickTracks([muxed, ...tweet]).single.kind, 'video');
  });
  test('codec order: HEVC, then AV1, then VP9, then h264 at the same height', () {
    final audio = _f('140', 'https', 'none', 'mp4a.40.2', null);
    String pick(List<String> codecs) => pickSplit([
@@ -0,0 +1,368 @@
 import 'dart:async';
 import 'dart:math';
 import 'dart:typed_data';
 import 'package:sync_relay_core/core.dart';
 import 'relay_buffer.dart';
 final _log = createLogger('relay');
 class _PiecePull {
  final int id, piece;
  final buf = BytesBuilder(copy: false);
  _PiecePull(this.id, this.piece);
 }
 class _Viewer {
  int pos = -1; // its newest requested segment
  int? waiting; // that segment, while it is missing
  int waitSince = 0;
  int open = 0;
  DateTime lastActive = DateTime.now();
 }
 /// The server side of a pieces track (see relay_pieces.dart): pieces by
 /// index in memory, each filled by one whole pull, served whole (a Range
 /// inside a piece is the route's job).
 ///
 /// - Fill order: a viewer blocked on a piece (oldest wait first), the init
 ///   and key pieces the viewers' segments use, then segments ahead of each
 ///   viewer, slowest first, up to [ahead] bytes.
 /// - A viewer's position is its newest requested segment; a superseded
 ///   request still gets its piece but no longer drives the feed, and a pull
 ///   nobody waits on gives way to a blocked viewer at once.
 /// - Kept: every init and key piece, plus segments by distance from the
 ///   viewers ([ahead] + [back] bytes; behind a viewer costs 4x), never LRU.
 ///   Sizes before a piece arrives are estimated from the bytes per second
 ///   of the ones that did, else [rate].
 class PieceBuffer implements TrackBuffer {
  @override
  int ahead, back;
  final Duration waitLimit;
  /// Bytes per second assumed before any segment has arrived.
  double rate;
  final int Function() _newId;
  PieceBuffer({
    this.ahead = RelayConstants.relayAheadBytes,
    this.back = RelayConstants.relayBackBytes,
    this.waitLimit = const Duration(seconds: RelayConstants.relayWaitSecs),
    this.rate = 500000,
    int Function()? nextId,
  }) : _newId = nextId ?? _counter();
  static int Function() _counter() {
    var n = 0;
    return () => ++n;
  }
  RelayFeed? _feed;
  /// The track's shape, once the feed has connected.
  RelayPieceList? list;
  String? _playlist;
  final _pieces = <int, Uint8List>{};
  final _sizes = <int, int>{}; // known sizes, kept after eviction
  var _knownSecs = 0.0;
  var _knownBytes = 0;
  final _waiters = <int, List<Completer<void>>>{};
  final _aux = <int, int>{}; // init/key piece a request waits on -> since (us)
  final _viewers = <String, _Viewer>{};
  _PiecePull? _pull;
  DateTime? _retryAt;
  var _disposed = false;
  Completer<void> _kick = Completer();
  final Completer<void> _ready = Completer();
  @override
  int bytesFromFeed = 0, bytesToViewers = 0;
  @override
  bool get live => _feed != null && list != null;
  @override
  int get cachedBytes => _pieces.values.fold(0, (a, p) => a + p.length);
  /// The media playlist viewers open (see [RelayPieceList.playlist]).
  String? get playlist => _playlist ??= list?.playlist();
  /// Attaches a (re)connected feed for a track of [list]'s shape. A
  /// different shape means different content: the buffer starts over.
  void attach(RelayFeed feed, RelayPieceList list) {
    if (_disposed) return;
    final old = this.list;
    if (old != null && !old.sameShape(list)) {
      _log.info('relay source changed pieces ${old.length} -> ${list.length}, dropping buffer');
      _pieces.clear();
      _sizes.clear();
      _knownSecs = 0;
      _knownBytes = 0;
      _viewers.clear();
      _aux.clear();
    }
    this.list = list;
    _playlist = null;
    _feed = feed;
    _pull = null;
    if (!_ready.isCompleted) _ready.complete();
    if (_fillerRunning) {
      _wake();
    } else {
      unawaited(_filler());
    }
  }
  @override
  void detach(RelayFeed feed) {
    if (!identical(_feed, feed)) return;
    _feed = null;
    _pull = null;
    _wake();
  }
  @override
  void dispose() {
    _disposed = true;
    _feed = null;
    _pull = null;
    _pieces.clear();
    for (final list in _waiters.values) {
      for (final c in list) {
        if (!c.isCompleted) c.complete();
      }
    }
    _waiters.clear();
    if (!_ready.isCompleted) _ready.complete();
    _wake();
  }
  void _wake() {
    if (!_kick.isCompleted) _kick.complete();
  }
  // ------------------------------------------------------------ feed input
  @override
  void feedBytes(int id, Uint8List data) {
    final pl = _pull;
    if (pl == null || pl.id != id || _disposed) return;
    _feed?.arrived(id, data.length);
    bytesFromFeed += data.length;
    pl.buf.add(data);
  }
  @override
  void feedEnd(int id) {
    final pl = _pull;
    if (pl == null || pl.id != id || _disposed) return;
    _pull = null;
    final data = pl.buf.takeBytes();
    final i = pl.piece;
    _pieces[i] = data;
    final p = list!.pieces[i];
    if (p.isSegment && !_sizes.containsKey(i)) {
      _knownSecs += p.d;
      _knownBytes += data.length;
    }
    _sizes[i] = data.length;
    for (final c in _waiters.remove(i) ?? const <Completer<void>>[]) {
      c.complete();
    }
    _wake();
  }
  @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));
    _wake();
  }
  // ------------------------------------------------------------- scheduling
  double _est(int i) {
    final known = _sizes[i];
    if (known != null) return known.toDouble();
    final p = list!.pieces[i];
    if (!p.isSegment) return 0;
    return (_knownSecs > 0 ? _knownBytes / _knownSecs : rate) * p.d;
  }
  Iterable<_Viewer> get _active {
    final now = DateTime.now();
    return _viewers.values.where(
        (v) => v.pos >= 0 && (v.open > 0 || now.difference(v.lastActive) < const Duration(seconds: 60)));
  }
  /// Estimated byte offset of every piece (prefix sums), for distances.
  List<double> _offsets() {
    final l = list!;
    final out = List<double>.filled(l.length + 1, 0);
    for (var i = 0; i < l.length; i++) {
      out[i + 1] = out[i] + _est(i);
    }
    return out;
  }
  double _distance(int i, List<double> off, List<_Viewer> active) {
    var d = double.infinity;
    for (final v in active) {
      d = min(d, i >= v.pos ? off[i] - off[v.pos] : (off[v.pos] - off[i]) * 4);
    }
    return d;
  }
  bool _waited(int i) => _aux.containsKey(i) || _viewers.values.any((v) => v.waiting == i);
  /// Cached segments past the budget, farthest first; returns the
  /// distance of the farthest segment kept (a fill farther than it would
  /// only be evicted again).
  double _evict(List<double> off, List<_Viewer> active) {
    final l = list!;
    final cached = [for (final i in _pieces.keys) if (l.pieces[i].isSegment) i];
    var total = cached.fold(0, (a, i) => a + _pieces[i]!.length);
    cached.sort((a, b) => _distance(b, off, active).compareTo(_distance(a, off, active)));
    final limit = ahead + back;
    var k = 0;
    for (; k < cached.length && total > limit; k++) {
      final i = cached[k];
      if (_waited(i)) continue;
      total -= _pieces.remove(i)!.length;
    }
    _cachedSegBytes = total;
    return k < cached.length ? _distance(cached[k], off, active) : double.infinity;
  }
  var _cachedSegBytes = 0;
  int? _nextMissing() {
    final l = list!;
    final active = _active.toList()..sort((a, b) => a.pos.compareTo(b.pos));
    final off = _offsets();
    final keep = _evict(off, active);
    final blocked = <(int, int)>[
      for (final v in _viewers.values)
        if (v.waiting != null && !_pieces.containsKey(v.waiting)) (v.waiting!, v.waitSince),
      for (final e in _aux.entries)
        if (!_pieces.containsKey(e.key)) (e.key, e.value),
    ]..sort((a, b) => a.$2.compareTo(b.$2));
    if (blocked.isNotEmpty) return blocked.first.$1;
    if (active.isEmpty) return null; // nobody watches: fetch nothing
    for (final v in active) {
      var budget = 0.0;
      for (var i = v.pos; i < l.length; i++) {
        final p = l.pieces[i];
        if (!p.isSegment) continue;
        for (final dep in [p.map, p.key]) {
          if (dep != null && !_pieces.containsKey(dep)) return dep;
        }
        if (!_pieces.containsKey(i)) {
          final room = _cachedSegBytes + _est(i) <= ahead + back;
          if (i == v.pos || room || _distance(i, off, active) < keep) return i;
          break;
        }
        budget += _est(i);
        if (budget > ahead) break;
      }
    }
    return null;
  }
  var _fillerRunning = false;
  Future<void> _filler() async {
    _fillerRunning = true;
    try {
      while (!_disposed) {
        final feed = _feed;
        final retry = _retryAt;
        if (feed == null || _pull != null || list == null) {
          await _sleep();
          continue;
        }
        if (retry != null && DateTime.now().isBefore(retry)) {
          await _sleep(retry.difference(DateTime.now()));
          continue;
        }
        _retryAt = null;
        final i = _nextMissing();
        if (i == null) {
          await _sleep();
          continue;
        }
        final pl = _pull = _PiecePull(_newId(), i);
        feed.pullPiece(pl.id, i);
      }
    } finally {
      _fillerRunning = false;
    }
  }
  Future<void> _sleep([Duration limit = const Duration(seconds: 5)]) async {
    _kick = Completer();
    // Also wake periodically: viewers age out of [_active].
    await _kick.future.timeout(limit, onTimeout: () {});
  }
  // ---------------------------------------------------------------- viewers
  @override
  Future<bool> ready() async {
    if (list != null) return true;
    await _ready.future.timeout(waitLimit, onTimeout: () {});
    return list != null && !_disposed;
  }
  /// [viewer] opened the playlist: a new viewer starts at the first
  /// segment, so its first pieces are fetched before it asks.
  void opened(String viewer) {
    final l = list;
    if (l == null) return;
    final v = _viewers.putIfAbsent(viewer, _Viewer.new);
    if (v.pos < 0) v.pos = l.pieces.indexWhere((p) => p.isSegment);
    v.lastActive = DateTime.now();
    _wake();
  }
  /// Piece [index] for [viewer], once it is in; null when it does not
  /// arrive within [waitLimit] (or the buffer is gone).
  Future<Uint8List?> piece(String viewer, int index) async {
    final l = list;
    if (l == null || index < 0 || index >= l.length) return null;
    final v = _viewers.putIfAbsent(viewer, _Viewer.new);
    final segment = l.pieces[index].isSegment;
    final now = DateTime.now().microsecondsSinceEpoch;
    v.open++;
    v.lastActive = DateTime.now();
    if (segment) {
      v.pos = index;
      v.waiting = _pieces.containsKey(index) ? null : index;
      v.waitSince = now;
    } else if (!_pieces.containsKey(index)) {
      _aux.putIfAbsent(index, () => now);
    }
    // A pull nobody waits on gives way to this one at once.
    final pl = _pull;
    if (!_pieces.containsKey(index) && pl != null && pl.piece != index && !_waited(pl.piece)) {
      _pull = null;
      _feed?.cancel(pl.id);
    }
    _wake();
    try {
      final deadline = DateTime.now().add(waitLimit);
      while (!_pieces.containsKey(index)) {
        if (_disposed || DateTime.now().isAfter(deadline)) return null;
        final c = Completer<void>();
        _waiters.putIfAbsent(index, () => []).add(c);
        await c.future.timeout(const Duration(seconds: 2), onTimeout: () {});
      }
      return _pieces[index];
    } finally {
      v.open--;
      v.lastActive = DateTime.now();
      if (segment && v.waiting == index) v.waiting = null;
      if (!segment) _aux.remove(index);
    }
  }
 }
@@ -7,28 +7,54 @@ import 'package:sync_relay_core/core.dart';
 final _log = createLogger('relay');
-/// What feeds a [RelayBuffer]: a member's relay channel, or the server
+/// One track's buffer on the server: a byte stream ([RelayBuffer]) or
 /// pieces ([PieceBuffer]).
 abstract interface class TrackBuffer {
  /// Budgets, set by the relay when a media's tracks share them.
  abstract int ahead, back;
  bool get live;
  int get cachedBytes;
  int get bytesFromFeed;
  int get bytesToViewers;
  /// Waits for the feed's first connect, up to the wait limit.
  Future<bool> ready();
  void detach(RelayFeed feed);
  void dispose();
  void feedBytes(int id, Uint8List data);
  void feedEnd(int id);
  void feedFail(int id, String reason);
 }
 /// What feeds a [TrackBuffer]: a member's relay channel, or the server
 /// itself fetching a link ([LocalRelayFeed]).
 abstract class RelayFeed {
-  /// Ask for bytes [start]..[end]; they arrive via [RelayBuffer.feedBytes]
+  /// Ask for bytes [start]..[end]; they arrive via [TrackBuffer.feedBytes]
-  /// in order, then [RelayBuffer.feedEnd].
+  /// in order, then [TrackBuffer.feedEnd].
  void pull(int id, int start, int end);
  /// Ask for piece [index] of a pieces track, whole (arrives like [pull]).
  void pullPiece(int id, int index);
  void cancel(int id);
  /// Bytes of [id] reached the buffer (the remote feed acks them).
  void arrived(int id, int n) {}
 }
-/// Fetches from [bytes] in this process (the "server fetches it" mode).
+/// Fetches [track] in this process (the "server fetches it" mode).
 class LocalRelayFeed extends RelayFeed {
-  final RelayBytes bytes;
+  final RelayTrack track;
-  final RelayBuffer buffer;
+  final TrackBuffer buffer;
  final _subs = <int, StreamSubscription<List<int>>>{};
-  LocalRelayFeed(this.bytes, this.buffer);
+  LocalRelayFeed(this.track, this.buffer);
  @override
-  void pull(int id, int start, int end) {
+  void pull(int id, int start, int end) => _listen(id, track.bytes!.read(start, end));
-    _subs[id] = bytes.read(start, end).listen(
+
  @override
  void pullPiece(int id, int index) => _listen(id, track.pieces!.read(index));
  void _listen(int id, Stream<List<int>> data) {
    _subs[id] = data.listen(
          (c) => buffer.feedBytes(id, c is Uint8List ? c : Uint8List.fromList(c)),
          onError: (Object e) {
            unawaited(_subs.remove(id)?.cancel().catchError((_) {}));
@@ -51,7 +77,8 @@ class LocalRelayFeed extends RelayFeed {
      await s.cancel().catchError((_) {});
    }
    _subs.clear();
-    await bytes.close();
+    await track.bytes?.close();
    await track.pieces?.close();
  }
 }
@@ -66,6 +93,10 @@ class ChannelRelayFeed extends RelayFeed {
  void pull(int id, int start, int end) =>
      send(jsonEncode({'t': 'pull', 'id': id, if (track != null) 'track': track, 'start': start, 'end': end}));
  @override
  void pullPiece(int id, int index) =>
      send(jsonEncode({'t': 'pull', 'id': id, if (track != null) 'track': track, 'piece': index}));
  @override
  void cancel(int id) => send(jsonEncode({'t': 'cancel', 'id': id}));
@@ -102,8 +133,9 @@ class _Viewer {
 /// - Only a viewer's newest request moves its position or asks the feed
 ///   for anything; older ones still get bytes once they are buffered.
 /// Everything lives in memory: dropping the buffer is the cleanup.
-class RelayBuffer {
+class RelayBuffer implements TrackBuffer {
  /// Set by the relay when a media's tracks share the budget.
  @override
  int ahead, back;
  final int pin;
  final Duration waitLimit;
@@ -137,9 +169,12 @@ class RelayBuffer {
  final Completer<void> _ready = Completer();
  /// Totals, for logs and tests.
  @override
  int bytesFromFeed = 0, bytesToViewers = 0;
  @override
  bool get live => _feed != null && size > 0;
  @override
  int get cachedBytes => _slices.values.fold(0, (a, s) => a + s.length);
  int get _nSlices => (size + slice - 1) ~/ slice;
  int _sliceEnd(int s) => min((s + 1) * slice, size) - 1;
@@ -163,6 +198,7 @@ class RelayBuffer {
  }
  /// The feed went away; buffered slices keep serving.
  @override
  void detach(RelayFeed feed) {
    if (!identical(_feed, feed)) return;
    _feed = null;
@@ -170,6 +206,7 @@ class RelayBuffer {
    _wake();
  }
  @override
  void dispose() {
    _disposed = true;
    _feed = null;
@@ -191,6 +228,7 @@ class RelayBuffer {
  // ------------------------------------------------------------ feed input
  @override
  void feedBytes(int id, Uint8List data) {
    final pl = _pull;
    if (pl == null || pl.id != id || _disposed) return;
@@ -221,6 +259,7 @@ class RelayBuffer {
    }
  }
  @override
  void feedEnd(int id) {
    final pl = _pull;
    if (pl != null && pl.id == id) {
@@ -229,6 +268,7 @@ class RelayBuffer {
    }
  }
  @override
  void feedFail(int id, String reason) {
    if (_pull?.id != id) return;
    _log.warning('relay pull $id failed: $reason');
@@ -376,6 +416,7 @@ class RelayBuffer {
  // ---------------------------------------------------------------- viewers
  /// Waits for the size (the feed's first connect), up to [waitLimit].
  @override
  Future<bool> ready() async {
    if (size > 0) return true;
    await _ready.future.timeout(waitLimit, onTimeout: () {});
@@ -8,6 +8,7 @@ import 'package:sync_relay_core/core.dart';
 import 'package:web_socket/web_socket.dart';
 import 'media_endpoint.dart';
 import 'piece_buffer.dart';
 import 'relay_buffer.dart';
 import 'session.dart';
@@ -25,6 +26,15 @@ List<(int, int)> relayBudgets(List<int> sizes,
  return [for (final n in sizes) (share(ahead, n), share(back, n))];
 }
 /// Bytes per second a pieces track is assumed to need before its first
 /// pieces arrive (its share of the budget, its distances): 4 Mbit/s for
 /// video, 160 kbit/s for audio.
 double _piecesRate(String kind) => kind == 'audio' ? 20000 : 500000;
 /// One track as the source (or the fetcher) described it: a byte size,
 /// or a piece list.
 typedef _TrackInfo = ({int? size, RelayPieceList? pieces});
 /// The relay of the current media: one buffer per track and where the
 /// bytes come from. One per relayed swap; [dispose] on the next swap or
 /// server stop (the buffers are memory only, so that is the whole cleanup).
@@ -35,11 +45,11 @@ class ActiveRelay {
  /// server fetches the link itself.
  final String? sourceToken;
  final void Function() onLiveChanged;
-  /// One per track, in track order; empty until the server has read a
+  /// One per track, in track order ([RelayBuffer] for bytes, [PieceBuffer]
-  /// fetched link.
+  /// for pieces); empty until the server has read a fetched link.
-  List<RelayBuffer> buffers = const [];
+  List<TrackBuffer> buffers = const [];
-  /// `media.relay.tracks`: the kinds when there are several tracks.
+  /// `media.relay.tracks`: empty for one file.
-  List<String> kinds = const [];
+  List<RelayTrackSpec> tracks = const [];
  final _known = Completer<void>();
  var _ids = 0;
  RelayMedia? _media;
@@ -51,12 +61,43 @@ class ActiveRelay {
  int _nextId() => ++_ids;
-  void _setTracks(List<String> kinds) {
+  void _setTracks(List<RelayTrackSpec> tracks) {
-    this.kinds = kinds.length > 1 ? List.unmodifiable(kinds) : const [];
+    this.tracks = MediaRelay.validTracks(tracks) ? List.unmodifiable(tracks) : const [];
-    buffers = [for (var i = 0; i < max(1, kinds.length); i++) RelayBuffer(nextId: _nextId)];
+    buffers = this.tracks.isEmpty
        ? [RelayBuffer(nextId: _nextId)]
        : [
            for (final t in this.tracks)
              t.pieces ? PieceBuffer(nextId: _nextId, rate: _piecesRate(t.kind)) : RelayBuffer(nextId: _nextId),
          ];
    if (!_known.isCompleted) _known.complete();
  }
  /// The info of every track matches [tracks] (bytes for a byte track,
  /// pieces for a pieces one).
  bool _matches(List<_TrackInfo> infos) {
    if (infos.length != buffers.length) return false;
    for (var i = 0; i < infos.length; i++) {
      if ((infos[i].pieces != null) != (buffers[i] is PieceBuffer)) return false;
    }
    return true;
  }
  /// Attaches every buffer to its feed.
  void _attachAll(List<RelayFeed> feeds, List<_TrackInfo> infos) {
    _budget([
      for (var i = 0; i < infos.length; i++)
        infos[i].size ?? (_piecesRate(tracks[i].kind) * infos[i].pieces!.duration).round(),
    ]);
    for (var i = 0; i < buffers.length; i++) {
      final b = buffers[i];
      if (b is PieceBuffer) {
        b.attach(feeds[i], infos[i].pieces!);
      } else if (b is RelayBuffer) {
        b.attach(feeds[i], infos[i].size!);
      }
    }
  }
  void _budget(List<int> sizes) {
    final b = relayBudgets(sizes);
    for (var i = 0; i < buffers.length; i++) {
@@ -69,12 +110,12 @@ class ActiveRelay {
  /// Bytes from member [sourceToken] over its relay channel, as the
  /// [tracks] its swap named (empty: one file).
  factory ActiveRelay.fromMember(int mediaId, String name, String sourceToken,
-          {List<String> tracks = const [], required void Function() onLiveChanged}) =>
+          {List<RelayTrackSpec> tracks = const [], required void Function() onLiveChanged}) =>
      ActiveRelay._(mediaId, name, sourceToken, onLiveChanged).._setTracks(tracks);
  /// The server reads [url] itself: a media file as is, a web page
-  /// through [ytDlp] (one file, or video + audio tracks). [onFail] gets
+  /// through [ytDlp] (one file, video + audio tracks, or segmented
-  /// the reason when it cannot.
+  /// playlists). [onFail] gets the reason when it cannot.
  static ActiveRelay fetch(int mediaId, String name, Uri url,
      {YtDlp? ytDlp, required void Function() onLiveChanged, void Function(String reason)? onFail}) {
    final r = ActiveRelay._(mediaId, name, null, onLiveChanged);
@@ -82,14 +123,14 @@ class ActiveRelay {
    unawaited(() async {
      try {
        final tracks = await media.open();
-        final sizes = [for (final t in tracks) await t.bytes.size()];
+        final infos = <_TrackInfo>[
          for (final t in tracks)
            t.pieces != null ? (size: null, pieces: await t.pieces!.list()) : (size: await t.bytes!.size(), pieces: null),
        ];
        if (r._disposed) return;
-        r._setTracks([for (final t in tracks) t.kind]);
+        r._setTracks(relayTrackSpecs(tracks));
-        r._budget(sizes);
+        r._local = [for (var i = 0; i < tracks.length; i++) LocalRelayFeed(tracks[i], r.buffers[i])];
-        r._local = [for (var i = 0; i < tracks.length; i++) LocalRelayFeed(tracks[i].bytes, r.buffers[i])];
+        r._attachAll(r._local, infos);
        for (var i = 0; i < tracks.length; i++) {
          r.buffers[i].attach(r._local[i], sizes[i]);
        }
        onLiveChanged();
      } catch (e) {
        _log.warning('relay fetch: cannot read the link: $e');
@@ -113,10 +154,10 @@ class ActiveRelay {
    return known;
  }
-  /// The buffer of [track]; null (the single-file URL) only for a media of
+  /// The buffer of [track]; null (the single-file URL) only for one file.
-  /// one track. Null when there is no such track.
+  /// Null when there is no such track.
-  RelayBuffer? bufferAt(int? track) {
+  TrackBuffer? bufferAt(int? track) {
-    if (track == null) return buffers.length == 1 ? buffers.single : null;
+    if (track == null) return tracks.isEmpty && buffers.length == 1 ? buffers.single : null;
    return track >= 0 && track < buffers.length ? buffers[track] : null;
  }
@@ -139,18 +180,15 @@ class ActiveRelay {
    if (ch != null) unawaited(_closeNotSource(ch));
  }
-  void _attachChannel(RelicWebSocket ws, List<int> sizes) {
+  void _attachChannel(RelicWebSocket ws, List<_TrackInfo> infos) {
    final old = _channel;
    if (old != null && !identical(old, ws)) unawaited(old.tryClose());
    _channel = ws;
-    final multi = buffers.length > 1;
+    final named = tracks.isNotEmpty;
    final feeds = _channelFeeds = [
-      for (var i = 0; i < buffers.length; i++) ChannelRelayFeed((t) => ws.trySendText(t), track: multi ? i : null),
+      for (var i = 0; i < buffers.length; i++) ChannelRelayFeed((t) => ws.trySendText(t), track: named ? i : null),
    ];
-    _budget(sizes);
+    _attachAll(feeds, infos);
    for (var i = 0; i < buffers.length; i++) {
      buffers[i].attach(feeds[i], sizes[i]);
    }
    onLiveChanged();
  }
@@ -166,17 +204,25 @@ class ActiveRelay {
  }
 }
-/// The track sizes of a channel's `info` frame (`size`, or `tracks[].size`),
+/// The tracks of a channel's `info` frame (`size`, or `tracks[]` each
-/// or null when malformed.
+/// with a `size` or, `"as":"pieces"`, a piece list), or null when
-List<int>? _infoSizes(Map j) {
+/// malformed.
 List<_TrackInfo>? _infoTracks(Map j) {
  final size = j['size'], tracks = j['tracks'];
-  if (size is int) return size > 0 ? [size] : null;
+  if (size is int) return size > 0 ? [(size: size, pieces: null)] : null;
-  if (tracks is! List || tracks.isEmpty) return null;
+  if (tracks is! List || tracks.isEmpty || tracks.length > MediaRelay.maxTracks) return null;
-  final out = <int>[];
+  final out = <_TrackInfo>[];
  for (final t in tracks) {
-    final n = t is Map ? t['size'] : null;
+    if (t is! Map) return null;
    if (t['as'] == 'pieces') {
      final list = RelayPieceList.fromJson(t);
      if (list == null) return null;
      out.add((size: null, pieces: list));
    } else {
      final n = t['size'];
      if (n is! int || n <= 0) return null;
-    out.add(n);
+      out.add((size: n, pieces: null));
    }
  }
  return out;
 }
@@ -192,7 +238,9 @@ Future<void> _closeNotSource(RelicWebSocket ws) async {
 /// `GET /r/<sid>/<token>/source` (WebSocket): the source member's relay
 /// channel. `GET /r/<sid>/<token>/<mediaId>/<name>`: a viewer reads the
 /// relayed file, Range as for `/m`; `.../<mediaId>/<track>/<name>` one
-/// track of a media relayed as several. [relay] gives the current relay.
+/// track of a media with tracks (a pieces track's media playlist), and
 /// `.../<mediaId>/<track>/p/<i>` piece `i` of a pieces track. [relay]
 /// gives the current relay.
 void mountRelayRoutes(
  RelicApp app,
  SessionMap sessions,
@@ -230,16 +278,17 @@ void mountRelayRoutes(
            switch (j['t']) {
              case 'info':
                if (attached) continue;
-                final sizes = _infoSizes(j);
+                final infos = _infoTracks(j);
-                if (sizes == null || sizes.length != r.buffers.length) {
+                if (infos == null || !r._matches(infos)) {
                  // Not the media this swap named: the source stops.
                  _log.warning('relay channel: info does not match the media (${r.name})');
                  await _closeNotSource(ws);
                  return;
                }
                attached = true;
-                _log.info('relay channel up (${r.name}, ${sizes.join(' + ')} bytes)');
+                _log.info('relay channel up (${r.name}, '
-                r._attachChannel(ws, sizes);
+                    '${infos.map((t) => t.size != null ? '${t.size} bytes' : '${t.pieces!.length} pieces').join(' + ')})');
                r._attachChannel(ws, infos);
              case 'end':
                if (id is int) {
                  for (final b in r.buffers) {
@@ -272,7 +321,7 @@ void mountRelayRoutes(
    });
  });
-  Future<Response> viewer(Request req, {required bool tracked}) async {
+  Future<Response> viewer(Request req, {required bool tracked, bool piece = false}) async {
    final stopwatch = Stopwatch()..start();
    final sid = req.pathParameters.raw[#sid] ?? '';
    final token = req.pathParameters.raw[#token] ?? '';
@@ -294,15 +343,39 @@ void mountRelayRoutes(
        headers: Headers.build((h) => h['retry-after'] = ['2']));
    if (!await r.whenKnown()) return notConnected;
    final buffer = r.bufferAt(track);
-    if (buffer == null) return _plain(404);
+    if (buffer == null || (piece && buffer is! PieceBuffer)) return _plain(404);
    if (!await buffer.ready()) return notConnected;
    final size = buffer.size;
    final typed = req.headers.range;
    String? rangeHeader;
    if (typed != null && typed.ranges.isNotEmpty) {
      final x = typed.ranges.first;
      rangeHeader = '${typed.unit}=${x.start?.toString() ?? ''}-${x.end?.toString() ?? ''}';
    }
    if (buffer is PieceBuffer) {
      if (!piece) {
        buffer.opened(token);
        return Response(200,
            body: Body.fromString(buffer.playlist ?? '', mimeType: MimeType('application', 'vnd.apple.mpegurl')),
            headers: Headers.build((h) => h['cache-control'] = ['no-store']));
      }
      final index = int.tryParse(req.pathParameters.raw[#piece] ?? '');
      if (index == null || index < 0 || index >= buffer.list!.length) return _plain(404);
      final data = await buffer.piece(token, index);
      if (data == null) return notConnected;
      final res = resolveMediaRequest(rangeHeader: rangeHeader, size: data.length, filename: 'piece');
      final headers = Headers.build((h) => res.headers.forEach((k, v) => h[k] = [v]));
      if (res.status != 200 && res.status != 206) {
        return Response(res.status, body: Body.fromString('status ${res.status}'), headers: headers);
      }
      final end = res.end ?? data.length - 1;
      final n = end - res.start + 1;
      buffer.bytesToViewers += n;
      session.addBytes(token, n);
      return Response(res.status,
          body: Body.fromData(Uint8List.sublistView(data, res.start, end + 1), mimeType: MimeType.octetStream),
          headers: headers);
    }
    final size = (buffer as RelayBuffer).size;
    final res = resolveMediaRequest(rangeHeader: rangeHeader, size: size, filename: r.name);
    final headers = Headers.build((h) => res.headers.forEach((k, v) => h[k] = [v]));
    if (res.status != 200 && res.status != 206) {
@@ -337,6 +410,7 @@ void mountRelayRoutes(
  // wants one parameter name per level).
  app.get('/r/:sid/:token/:mediaId/:seg', (final req) => viewer(req, tracked: false));
  app.get('/r/:sid/:token/:mediaId/:seg/:name', (final req) => viewer(req, tracked: true));
  app.get('/r/:sid/:token/:mediaId/:seg/p/:piece', (final req) => viewer(req, tracked: true, piece: true));
 }
 Response _plain(int status) => Response(status, body: Body.fromString('status $status'));
@@ -58,8 +58,8 @@ class ServerHub {
    final m = engine.media;
    if (!identical(relay, r) || m?.relay == null) return;
    final was = m!.relay!;
-    if (was.live == r.live && was.tracks.join(',') == r.kinds.join(',')) return;
+    if (was.live == r.live && was.tracks.join(',') == r.tracks.join(',')) return;
-    engine.media = m.copyWith(relay: was.copyWith(live: r.live, tracks: r.kinds));
+    engine.media = m.copyWith(relay: was.copyWith(live: r.live, tracks: r.tracks));
    changed();
  }
@@ -244,7 +244,8 @@ void mountSyncRoute(
  String Function(String sid, String token, SessionMedia? media, String host)
      urlBuilder, {
  String Function(String sid, String token, String itemId, String host)? relayUrlBuilder,
-  String Function(String sid, String token, int mediaId, String name, int tracks, String host)? relayMediaUrlBuilder,
+  String Function(String sid, String token, int mediaId, String name, List<RelayTrackSpec> tracks, String host)?
      relayMediaUrlBuilder,
 }) {
  app.get('/sync', (final req) {
    // Host the client dialled (Host header), so file URLs work for remote joiners.
@@ -551,7 +552,7 @@ void mountSyncRoute(
                relayMediaUrlBuilder != null
            ? (relayed.known
                ? relayMediaUrlBuilder(
-                    'default', activeToken, relayed.mediaId, relayed.name, relayed.buffers.length, dialledHost)
+                    'default', activeToken, relayed.mediaId, relayed.name, relayed.tracks, dialledHost)
                : null)
            : media?.file != null
                ? urlBuilder('default', activeToken, media, dialledHost)
@@ -577,7 +578,7 @@ void mountSyncRoute(
  });
 }
-ActiveRelay _memberRelay(ServerHub hub, String token, String name, List<String> tracks) {
+ActiveRelay _memberRelay(ServerHub hub, String token, String name, List<RelayTrackSpec> tracks) {
  late final ActiveRelay r;
  r = ActiveRelay.fromMember(hub.engine.mediaId, name, token,
      tracks: tracks, onLiveChanged: () => hub.relayLiveChanged(r));
@@ -283,6 +283,46 @@ void main() {
      expect(carol.welcome.mediaUrl, mpvEdl(['$carolBase/0/$name', '$carolBase/1/$name']));
    });
    test('a pieces track and a bytes track: playlist, whole pieces, Range inside a piece', () async {
      final alice = await join('alice');
      final bob = await join('bob');
      await swapRelay(alice, relay: {
        'tracks': [
          {'kind': 'video', 'as': 'pieces'},
          {'kind': 'audio'},
        ],
      });
      final pieces = [for (var i = 0; i < 4; i++) Uint8List.fromList(List.generate(1000 + i, (j) => (i * 31 + j) % 256))];
      final shape = RelayPieceList([
        const RelayPiece.init(),
        for (var i = 1; i < 4; i++) const RelayPiece.segment(6, map: 0),
      ]);
      final reads = <int>[];
      await startSource(alice,
          from: _Tracks([
            RelayTrack.pieces('video', _MemPieces(shape, pieces, reads)),
            RelayTrack('audio', FileRelayBytes(file)),
          ]));
      final id = mediaIdOf(alice);
      final base = 'http://127.0.0.1:${server.boundPort}/r/default/${bob.welcome.token}/$id';
      final name = Uri.encodeComponent('ep 01.mkv');
      final (ps, ph, playlist) = await get('$base/0/$name.m3u8');
      expect(ps, 200);
      expect(ph['content-type'], startsWith('application/vnd.apple.mpegurl'));
      expect(utf8.decode(playlist), shape.playlist());
      expect((await get('$base/0/p/2')).$3, pieces[2]);
      final (rs, rh, part) = await get('$base/0/p/3', range: 'bytes=10-19');
      expect((rs, rh['content-range']), (206, 'bytes 10-19/1003'));
      expect(part, pieces[3].sublist(10, 20));
      expect((await get('$base/1/$name', range: 'bytes=5-')).$3, bytes.sublist(5));
      expect((await get('$base/0/p/4')).$1, 404);
      expect((await get('$base/1/p/0')).$1, 404);
      expect(reads, isNot(contains(-1)));
      final carol = await join('carol');
      final carolBase = base.replaceFirst(bob.welcome.token, carol.welcome.token);
      expect(carol.welcome.mediaUrl, mpvEdl(['$carolBase/0/$name.m3u8', '$carolBase/1/$name']));
    });
    test('Jellyfin swap can carry a relay', () async {
      final alice = await join('alice');
      final before = alice.raw.length;
@@ -412,7 +452,7 @@ void main() {
    setUp(() {
      buf = RelayBuffer(ahead: 2 * RelayBuffer.slice, back: RelayBuffer.slice, pin: RelayBuffer.slice);
      counted = _CountingBytes(FileRelayBytes(file));
-      feed = LocalRelayFeed(counted, buf);
+      feed = LocalRelayFeed(RelayTrack('', counted), buf);
      buf.attach(feed, bytes.length);
    });
    tearDown(() async {
@@ -470,6 +510,24 @@ class _Tracks implements RelayMedia {
  Future<void> close() async {}
 }
 /// A pieces track from memory; [reads] logs the pieces pulled.
 class _MemPieces implements RelayPieces {
  final RelayPieceList shape;
  final List<Uint8List> data;
  final List<int> reads;
  _MemPieces(this.shape, this.data, this.reads);
  @override
  Future<RelayPieceList> list() async => shape;
  @override
  Stream<List<int>> read(int index) async* {
    reads.add(index);
    yield data[index];
  }
  @override
  Future<void> close() async {}
 }
 /// yt-dlp that "resolves" any page to [tracks].
 class _FakeYtDlp extends YtDlp {
  final List<YtDlpTrack> tracks;