Sync fixes: clean seeks, clock offset, end of file, 2.5 min read-ahead (proto 6)

- User seeks are intercepted at the player (SyncPlayer), previewed
  locally, sent once 300 ms after the drag ends; own echoes are matched
  by seekSeq and never re-applied; the landed check can't reapply a
  stale target. 1 s position poll removed.
- ping/pong clock offset (min-RTT ClockSync); positions use server time.
- Reports carry duration; buffer gate is min(4 s, remaining - 2 s);
  new finished transport instead of a HELD stall at the end.
- cache/readahead 150 s, 256 MiB demuxer cache, pins read back.
- Resumed reports put a stale ready member back in the set; hello.resume
  takes over the old member after an unclean drop (no ghost).
- Docs: roadmap rendering findings (HW path works; shaders verified).

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

packages/core/lib/src/clock_sync.dart

@@ -0,0 +1,45 @@
+/// Estimates how far the server's clock is from ours (NTP style).
+///
+/// Each ping/pong round trip gives one sample: the client sent at `t0`, the
+/// server stamped `s`, the reply arrived at `t3` (client clock). Assuming the
+/// two legs take equally long, `offset = s - (t0 + t3) / 2` with an error of
+/// at most `rtt / 2`. The sample with the smallest round trip among the last
+/// [window] is the most trustworthy, so that one is used.
+class ClockSync {
+  final int window;
+  final List<({int rttMs, double offsetMs})> _samples = [];
+
+  ClockSync({this.window = 16});
+
+  /// Server clock minus local clock, in ms; 0 until the first sample.
+  double get offsetMs {
+    if (_samples.isEmpty) return 0;
+    var best = _samples.first;
+    for (final s in _samples) {
+      if (s.rttMs < best.rttMs) best = s;
+    }
+    return best.offsetMs;
+  }
+
+  /// Round trip of the sample in use, null without samples.
+  int? get rttMs {
+    if (_samples.isEmpty) return null;
+    return _samples.map((s) => s.rttMs).reduce((a, b) => a < b ? a : b);
+  }
+
+  bool get hasSample => _samples.isNotEmpty;
+
+  /// [sentMs] and [receivedMs] are local epoch ms, [serverMs] the server's.
+  void add({required int sentMs, required int serverMs, required int receivedMs}) {
+    final rtt = receivedMs - sentMs;
+    if (rtt < 0) return;
+    _samples.add((rttMs: rtt, offsetMs: serverMs - (sentMs + receivedMs) / 2));
+    if (_samples.length > window) _samples.removeAt(0);
+  }
+
+  void reset() => _samples.clear();
+
+  /// The server's clock now, given the local clock [local].
+  DateTime serverNow(DateTime local) =>
+      local.add(Duration(microseconds: (offsetMs * 1000).round()));
+}

packages/core/lib/src/constants.dart

@@ -1,5 +1,5 @@
 abstract class RelayConstants {
-  static const int protoVersion = 5;
+  static const int protoVersion = 6;
 
   static const int chunkBytes = 256 * 1024;
   static const int creditWindowChunks = 8;
@@ -10,6 +10,10 @@ abstract class RelayConstants {
   static const double slowDownThresholdSecs = 1.5;
   static const double slowDownRate = 0.95;
   static const double minBufferSecs = 4.0;
+  /// Near the end the buffer requirement is what is left minus this, so a
+  /// client slightly ahead of the clock (or whose last packet ends a bit
+  /// early) still passes.
+  static const double endSlackSecs = 2.0;
   static const double resumeHysteresisSecs = 4.0;
 
   static const int stateHeartbeatSecs = 1;

packages/core/lib/src/protocol.dart

@@ -70,12 +70,17 @@ class HelloMessage {
   /// The hosting app's own client proves it is the host with this; empty
   /// for everyone else.
   final String hostKey;
+  /// Token from this client's previous `welcome`, sent on reconnect: if the
+  /// server still has that member (an unclean drop it has not noticed), the
+  /// connection takes it over instead of leaving a ghost.
+  final String resume;
   const HelloMessage(
       {required this.proto,
       required this.secret,
       required this.user,
       required this.media,
-      this.hostKey = ''});
+      this.hostKey = '',
+      this.resume = ''});
 
   Map<String, Object?> toJson() => {
         't': 'hello',
@@ -84,6 +89,7 @@ class HelloMessage {
         'user': user.toJson(),
         'media': media.toJson(),
         if (hostKey.isNotEmpty) 'hostKey': hostKey,
+        if (resume.isNotEmpty) 'resume': resume,
       };
 
   factory HelloMessage.fromJson(Map<String, Object?> json) {
@@ -97,6 +103,7 @@ class HelloMessage {
       user: HelloUser.fromJson(Map<String, Object?>.from(user)),
       media: HelloMedia.fromJson(Map<String, Object?>.from(media)),
       hostKey: (json['hostKey'] as String?) ?? '',
+      resume: (json['resume'] as String?) ?? '',
     );
   }
 }
@@ -212,13 +219,19 @@ class ForceIntent {
 
 class SeekIntent {
   final double position;
-  const SeekIntent({required this.position});
+  /// Client-chosen, increasing per connection; echoed in `state.seekSeq` so
+  /// the sender recognises its own seek.
+  final int seq;
+  const SeekIntent({required this.position, this.seq = 0});
 
-  Map<String, Object?> toJson() => {'t': 'seek', 'position': position};
+  Map<String, Object?> toJson() =>
+      {'t': 'seek', 'position': position, if (seq != 0) 'seq': seq};
 
   factory SeekIntent.fromJson(Map<String, Object?> json) {
     if (json['t'] != 'seek') throw const ProtocolException('not a seek intent');
-    return SeekIntent(position: finiteDouble(json['position'], 'position'));
+    return SeekIntent(
+        position: finiteDouble(json['position'], 'position'),
+        seq: finiteInt(json['seq'], 'seq'));
   }
 }
 
@@ -240,10 +253,25 @@ class ReportMessage {
   final double position;
   final double buffered;
   final int eventId;
-  const ReportMessage({required this.position, required this.buffered, required this.eventId});
+  /// Length of the open media in seconds, 0 while unknown.
+  final double duration;
+  /// The `mediaId` the client has open; [duration] belongs to it.
+  final int mediaId;
+  const ReportMessage(
+      {required this.position,
+      required this.buffered,
+      required this.eventId,
+      this.duration = 0,
+      this.mediaId = 0});
 
-  Map<String, Object?> toJson() =>
-      {'t': 'report', 'position': position, 'buffered': buffered, 'eventId': eventId};
+  Map<String, Object?> toJson() => {
+        't': 'report',
+        'position': position,
+        'buffered': buffered,
+        'eventId': eventId,
+        if (duration > 0) 'duration': duration,
+        if (mediaId != 0) 'mediaId': mediaId,
+      };
 
   factory ReportMessage.fromJson(Map<String, Object?> json) {
     if (json['t'] != 'report') throw const ProtocolException('not a report');
@@ -251,10 +279,40 @@ class ReportMessage {
       position: finiteDouble(json['position'], 'position'),
       buffered: finiteDouble(json['buffered'], 'buffered'),
       eventId: finiteInt(json['eventId'], 'eventId', fallback: 0),
+      duration: finiteDouble(json['duration'], 'duration'),
+      mediaId: finiteInt(json['mediaId'], 'mediaId'),
     );
   }
 }
 
+/// Clock probe: the client sends its own clock in [clientMs]; the server
+/// answers with a [PongMessage] carrying it back plus its clock.
+class PingMessage {
+  final int clientMs;
+  const PingMessage({required this.clientMs});
+
+  Map<String, Object?> toJson() => {'t': 'ping', 'c': clientMs};
+
+  factory PingMessage.fromJson(Map<String, Object?> json) {
+    if (json['t'] != 'ping') throw const ProtocolException('not a ping');
+    return PingMessage(clientMs: finiteInt(json['c'], 'c'));
+  }
+}
+
+class PongMessage {
+  final int clientMs;
+  final int serverMs;
+  const PongMessage({required this.clientMs, required this.serverMs});
+
+  Map<String, Object?> toJson() => {'t': 'pong', 'c': clientMs, 's': serverMs};
+
+  factory PongMessage.fromJson(Map<String, Object?> json) {
+    if (json['t'] != 'pong') throw const ProtocolException('not a pong');
+    return PongMessage(
+        clientMs: finiteInt(json['c'], 'c'), serverMs: finiteInt(json['s'], 's'));
+  }
+}
+
 class ChatMessage {
   final String text;
   const ChatMessage({required this.text});
@@ -314,6 +372,10 @@ class StateBroadcast {
   final int mediaId;
   final Map<String, Object?>? media;
   final List<MemberView> members;
+  /// Member id and seq of the seek that made [eventId]; null when the
+  /// current event is not a seek.
+  final String? seekBy;
+  final int? seekSeq;
   const StateBroadcast({
     required this.eventId,
     required this.transport,
@@ -322,6 +384,8 @@ class StateBroadcast {
     required this.mediaId,
     this.media,
     required this.members,
+    this.seekBy,
+    this.seekSeq,
   });
 
   Map<String, Object?> toJson() => {
@@ -333,6 +397,8 @@ class StateBroadcast {
         'mediaId': mediaId,
         if (media != null) 'media': media,
         'members': members.map((m) => m.toJson()).toList(),
+        if (seekBy != null) 'seekBy': seekBy,
+        if (seekSeq != null) 'seekSeq': seekSeq,
       };
 
   factory StateBroadcast.fromJson(Map<String, Object?> json) {
@@ -352,6 +418,8 @@ class StateBroadcast {
               .map((m) => MemberView.fromJson(Map<String, Object?>.from(m)))
               .toList()
           : const [],
+      seekBy: json['seekBy'] as String?,
+      seekSeq: json['seekSeq'] == null ? null : finiteInt(json['seekSeq'], 'seekSeq'),
     );
   }
 }

packages/core/lib/src/sync_engine.dart

@@ -3,7 +3,7 @@ import 'protocol.dart';
 
 typedef SyncClock = DateTime Function();
 
-enum Transport { playing, held }
+enum Transport { playing, held, finished }
 
 enum Correction { none, slowDown, seek }
 
@@ -66,6 +66,10 @@ class SyncEngine {
   int mediaId = 1;
   HelloMedia? media;
   int _nextId = 1;
+  // Who sent the seek that produced [_seekEventId], for the echo check.
+  String? _seekBy;
+  int _seekSeq = 0;
+  int _seekEventId = -1;
 
   SyncEngine({SyncClock? clock, Duration? staleAfter, Duration? dropAfter})
       : clock = clock ?? (() => DateTime.now().toUtc()),
@@ -84,6 +88,36 @@ class SyncEngine {
   bool isStale(EngineMember member, [DateTime? at]) =>
       (at ?? now).difference(member.lastSeen) > staleAfter;
 
+  /// Media length in seconds, null while nobody has reported it.
+  double? get duration {
+    final d = media?.duration ?? 0;
+    return d > 0 ? d : null;
+  }
+
+  /// Buffer every in-set member needs: [RelayConstants.minBufferSecs], or
+  /// less near the end where that much is not left to buffer.
+  double requiredBufferSecs([DateTime? at]) {
+    final d = duration;
+    const full = RelayConstants.minBufferSecs;
+    if (d == null) return full;
+    final left = d - positionAt(at) - RelayConstants.endSlackSecs;
+    return left.clamp(0.0, full).toDouble();
+  }
+
+  /// Any frame from a member: fresh again, and a ready member the stale
+  /// sweep took out of the set goes back in (ready && !inSet only arises
+  /// from that sweep).
+  void _seen(EngineMember member) {
+    member.lastSeen = now;
+    if (member.ready && !member.inSet) member.inSet = true;
+  }
+
+  /// Heartbeat: stale sweep, end of media.
+  void tick() {
+    sweepStale();
+    evaluate();
+  }
+
   String applyJoin(String token, String name, String avatar) {
     final id = 'm${_nextId++}';
     members[token] =
@@ -92,6 +126,14 @@ class SyncEngine {
     return id;
   }
 
+  /// A reconnect took over [token]'s member (see `hello.resume`).
+  void applyResume(String token) {
+    final member = members[token];
+    if (member == null) return;
+    _seen(member);
+    evaluate();
+  }
+
   String? applyLeave(String token) {
     final removed = members.remove(token);
     if (removed == null) return null;
@@ -110,7 +152,7 @@ class SyncEngine {
   void applyReady(String token, bool ready, {String source = 'manual'}) {
     final member = members[token];
     if (member == null) return;
-    member.lastSeen = now;
+    _seen(member);
     if (member.ready == ready && member.inSet) {
       evaluate();
       return;
@@ -153,15 +195,23 @@ class SyncEngine {
     evaluate();
   }
 
-  void applySeek(String token, double seekPosition) {
+  void applySeek(String token, double seekPosition, {int seq = 0}) {
     final member = members[token];
     if (member == null) return;
     if (!seekPosition.isFinite || seekPosition < 0) return;
-    member.lastSeen = now;
+    _seen(member);
     position = seekPosition;
     serverTime = now;
     eventId++;
+    _seekBy = member.id;
+    _seekSeq = seq;
+    _seekEventId = eventId;
     feed.add(FeedMessage(actor: member.name, action: 'seek', at: now));
+    // Seeking back from the end: held, and resumes once the gate passes.
+    if (transport == Transport.finished) {
+      transport = Transport.held;
+      evaluate();
+    }
   }
 
   /// Returns false (no state change) for media with non-finite numbers.
@@ -183,12 +233,19 @@ class SyncEngine {
     return true;
   }
 
-  void applyReport(String token, double reportPosition, double buffered, int reportEventId) {
+  void applyReport(String token, double reportPosition, double buffered, int reportEventId,
+      {double duration = 0, int mediaId = 0}) {
     final member = members[token];
     if (member == null) return;
     if (!reportPosition.isFinite) return;
-    member.lastSeen = now;
+    _seen(member);
     member.buffered = !buffered.isFinite || buffered < 0 ? 0 : buffered;
+    final m = media;
+    // First report of the length wins; only for the media the client has open.
+    if (m != null && m.duration <= 0 && mediaId == this.mediaId &&
+        duration.isFinite && duration > 0) {
+      media = HelloMedia(name: m.name, size: m.size, duration: duration, url: m.url);
+    }
     if (reportEventId < eventId) return;
     evaluate();
   }
@@ -221,10 +278,21 @@ class SyncEngine {
 
   void evaluate() {
     final t = now;
+    if (transport == Transport.finished) return;
+    final need = requiredBufferSecs(t);
     if (transport == Transport.playing) {
+      final d = duration;
+      if (d != null && positionAt(t) >= d) {
+        position = d;
+        serverTime = t;
+        eventId++;
+        transport = Transport.finished;
+        feed.add(FeedMessage(actor: 'server', action: 'finished', at: t));
+        return;
+      }
       final blocked = members.values
           .where((m) => m.inSet)
-          .any((m) => !m.ready || m.buffered < RelayConstants.minBufferSecs || isStale(m, t));
+          .any((m) => !m.ready || m.buffered < need || isStale(m, t));
       if (blocked) {
         position = positionAt(t);
         serverTime = t;
@@ -240,7 +308,7 @@ class SyncEngine {
         members.values.where((m) => !isStale(m, t)).length;
     final quorum = inSet.length >= 2 || freshCount <= 1;
     final allGood = inSet.every(
-        (m) => m.ready && m.buffered >= RelayConstants.minBufferSecs && !isStale(m, t));
+        (m) => m.ready && m.buffered >= need && !isStale(m, t));
     if (allGood && quorum) {
       serverTime = t;
       eventId++;
@@ -249,13 +317,18 @@ class SyncEngine {
     }
   }
 
-  StateBroadcast broadcast() => StateBroadcast(
-        eventId: eventId,
-        transport: transport == Transport.playing ? 'playing' : 'held',
-        position: position,
-        serverTime: serverTime,
-        mediaId: mediaId,
-        media: media?.toJson(),
-        members: members.values.map((m) => m.view(stale: isStale(m))).toList(),
-      );
+  StateBroadcast broadcast() {
+    final bySeek = _seekEventId == eventId;
+    return StateBroadcast(
+      eventId: eventId,
+      transport: transport.name,
+      position: position,
+      serverTime: serverTime,
+      mediaId: mediaId,
+      media: media?.toJson(),
+      members: members.values.map((m) => m.view(stale: isStale(m))).toList(),
+      seekBy: bySeek ? _seekBy : null,
+      seekSeq: bySeek ? _seekSeq : null,
+    );
+  }
 }