Relay: a member's media through the server's buffer (proto 8)

Viewers who cannot reach the media get it through the server: a file on
any member's computer, a link passed on by a member ("Through me") or
fetched by the server ("Server fetches it", password-protected servers
only), and Jellyfin items for guests without an account, sourced by
whoever put them up with their own sign-in (or a pasted link's key),
which never leaves their machine. Fixes guests on a dedicated server
without a Jellyfin login being blocked.

The source dials /r/.../source (works behind NAT) and answers pulls with
an adaptive in-flight window (2 x rate x min RTT, 256 KiB..8 MiB). The
server keeps a memory-only buffer of 1 MiB slices sized like a viewer's
mpv buffer (pinned head/tail, 64 MiB back, 256 MiB ahead), fills it with
one sequential pull at a time (blocked viewers first, newest request
only), fetches nothing while nobody watches through it, and drops it on
the next swap or stop. Measured with mpv through shaped links
(tool/relay_e2e.dart): 400 Mbit/s seeks ~0.03 s; 40 Mbit/s no stalls,
3 viewers cost the source ~1x the bytes.

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

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

@@ -37,6 +37,14 @@ class MediaPickerController extends ChangeNotifier {
   final TextEditingController file;
   MediaKind _source;
   String? _notice;
+  LinkRelay _linkRelay = LinkRelay.direct;
+
+  LinkRelay get linkRelay => _linkRelay;
+  set linkRelay(LinkRelay v) {
+    if (v == _linkRelay) return;
+    _linkRelay = v;
+    notifyListeners();
+  }
   JellyfinMedia? _jellyfin;
 
   /// The Jellyfin item picked in the browser (kept while another source
@@ -71,7 +79,7 @@ class MediaPickerController extends ChangeNotifier {
     switch (_source) {
       case MediaKind.link:
         final url = link.text.trim();
-        return url.isEmpty ? null : LinkMedia(url);
+        return url.isEmpty ? null : LinkMedia(url, relay: _linkRelay);
       case MediaKind.file:
         final path = file.text.trim();
         return path.isEmpty ? null : FileMedia(path);
@@ -117,15 +125,21 @@ class MediaPicker extends StatefulWidget {
   final bool expanded;
   /// Where the Jellyfin browser opens instead of its remembered location.
   final BrowseStart? jellyfinStart;
-  /// False hides the File source (only the host can serve a file).
+  /// False hides the File source.
   final bool allowFile;
   /// Shown under the Link field instead of the default help.
   final String? linkHelp;
+  /// Shown under the File field instead of the default help.
+  final String? fileHelp;
+  /// Offer the [LinkRelay] choice under the Link field (in a session).
+  final bool linkRelayChoice;
   const MediaPicker({
     super.key,
     this.jellyfinStart,
     this.allowFile = true,
     this.linkHelp,
+    this.fileHelp,
+    this.linkRelayChoice = false,
     required this.controller,
     this.onChanged,
     this.onSubmit,
@@ -247,10 +261,25 @@ class _MediaPickerState extends State<MediaPicker> {
             icon: const Icon(Icons.content_paste, size: 18),
             label: const Text('Paste'),
           ),
-          help: widget.linkHelp ??
-              'A direct http(s) link to a video file; everyone streams it from there. '
-                  'Jellyfin links are shared as the item, without your sign-in.',
+          help: widget.linkRelayChoice
+              ? _linkRelayHelp(_c.linkRelay)
+              : widget.linkHelp ??
+                  'A direct http(s) link to a video file; everyone streams it from there. '
+                      'Jellyfin links are shared as the item, without your sign-in.',
           notice: _c.notice,
+          extra: widget.linkRelayChoice
+              ? SegmentedButton<LinkRelay>(
+                  key: const ValueKey('link-relay'),
+                  showSelectedIcon: false,
+                  segments: const [
+                    ButtonSegment(value: LinkRelay.direct, label: Text('Everyone fetches it')),
+                    ButtonSegment(value: LinkRelay.server, label: Text('Server fetches it')),
+                    ButtonSegment(value: LinkRelay.me, label: Text('Through me')),
+                  ],
+                  selected: {_c.linkRelay},
+                  onSelectionChanged: (s) => _c.linkRelay = s.first,
+                )
+              : null,
         ),
       MediaKind.file => _FieldRow(
           key: const ValueKey('media-file'),
@@ -264,7 +293,8 @@ class _MediaPickerState extends State<MediaPicker> {
             icon: const Icon(Icons.folder_open, size: 18),
             label: const Text('Browse'),
           ),
-          help: 'Friends stream it from you while you watch. Optional — you can host without media.',
+          help: widget.fileHelp ??
+              'Friends stream it from you while you watch. Optional — you can host without media.',
           notice: _c.notice,
         ),
     };
@@ -325,8 +355,10 @@ class _FieldRow extends StatelessWidget {
   final Widget button;
   final String help;
   final String? notice;
+  /// Under the field, above the help (e.g. the link relay choice).
+  final Widget? extra;
   const _FieldRow(
-      {super.key, required this.field, required this.button, required this.help, this.notice});
+      {super.key, required this.field, required this.button, required this.help, this.notice, this.extra});
 
   @override
   Widget build(BuildContext context) {
@@ -346,6 +378,7 @@ class _FieldRow extends StatelessWidget {
                 button,
               ],
             ),
+            if (extra != null) ...[const SizedBox(height: 10), extra!],
             const SizedBox(height: 8),
             Text(notice ?? help, style: small),
           ],
@@ -354,3 +387,12 @@ class _FieldRow extends StatelessWidget {
     );
   }
 }
+
+String _linkRelayHelp(LinkRelay r) => switch (r) {
+      LinkRelay.direct => 'Everyone streams the link themselves. Jellyfin links are shared as the item, '
+          'without your sign-in.',
+      LinkRelay.server => 'The server downloads it once and streams it to everyone: for links that are '
+          'slow or blocked for some. The server must be able to reach the link; it is not shown to anyone.',
+      LinkRelay.me => 'This computer downloads it and passes it on through the server: for links only you '
+          'can open. Keep the app open while it plays; the link is not shown to anyone.',
+    };

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

@@ -7,10 +7,16 @@ sealed class PickedMedia {
   String get label;
 }
 
-/// A direct http(s) link everyone streams from.
+/// How the others get a link: each fetches it ([direct]), the server
+/// fetches it once for everyone ([server]), or this computer does and
+/// passes it on through the server ([me]; for links only you can reach).
+enum LinkRelay { direct, server, me }
+
+/// An http(s) link.
 final class LinkMedia extends PickedMedia {
   final String url;
-  const LinkMedia(this.url);
+  final LinkRelay relay;
+  const LinkMedia(this.url, {this.relay = LinkRelay.direct});
 
   @override
   String get label {
@@ -25,14 +31,15 @@ final class LinkMedia extends PickedMedia {
   }
 
   @override
-  bool operator ==(Object other) => other is LinkMedia && other.url == url;
+  bool operator ==(Object other) => other is LinkMedia && other.url == url && other.relay == relay;
   @override
-  int get hashCode => url.hashCode;
+  int get hashCode => Object.hash(url, relay);
   @override
   String toString() => 'LinkMedia($url)';
 }
 
-/// A file on this computer; friends stream it from the host.
+/// A file on this computer; the others stream it from here (the host's
+/// app serves it itself, anyone else's goes through the server's relay).
 final class FileMedia extends PickedMedia {
   final String path;
   const FileMedia(this.path);

apps/desktop/lib/src/screens/session/jellyfin_widgets.dart

@@ -144,7 +144,9 @@ class JellyfinBlockedCard extends StatelessWidget {
               const SizedBox(height: 10),
               Text(blocked.message, textAlign: TextAlign.center, style: t.titleSmall),
               const SizedBox(height: 6),
-              Text('Link your own account in Integrations, or ask the host to relay it.',
+              Text(
+                  'Whoever put it up passes it on automatically while their app is open; '
+                  'if they left, ask them to put it up again. Or link your own account in Integrations.',
                   textAlign: TextAlign.center, style: t.bodySmall?.copyWith(color: Colors.grey)),
               const SizedBox(height: 14),
               Wrap(

apps/desktop/lib/src/screens/session/swap_panel.dart

@@ -6,8 +6,9 @@ import '../home/media_picker.dart';
 
 /// "Change video" in a session: the same [MediaPicker] as Home's Host tab
 /// (Jellyfin browser, Link, File) and a button that puts the pick up for
-/// everyone. File is offered to the host only (the file streams from the
-/// host's disk). [onSwap] returns an error text, or null when sent.
+/// everyone. A file streams from this computer: the host's app serves it,
+/// anyone else's goes through the server's relay. [onSwap] returns an
+/// error text, or null when sent.
 class SwapPanel extends StatefulWidget {
   final MediaPickerController controller;
   final JellyfinAccounts accounts;
@@ -35,13 +36,6 @@ class _SwapPanelState extends State<SwapPanel> {
   bool _busy = false;
   String? _error;
 
-  @override
-  void initState() {
-    super.initState();
-    final c = widget.controller;
-    if (!widget.isHost && c.source == MediaKind.file) c.source = MediaKind.link;
-  }
-
   Future<void> _swap() async {
     final media = widget.controller.selection;
     if (media == null || _busy) return;
@@ -91,7 +85,10 @@ class _SwapPanelState extends State<SwapPanel> {
                   controller: widget.controller,
                   jellyfin: widget.accounts,
                   enabled: !_busy,
-                  allowFile: widget.isHost,
+                  linkRelayChoice: true,
+                  fileHelp: widget.isHost
+                      ? 'Friends stream it from your computer.'
+                      : 'Streams from your computer through the server. Keep the app open while it plays.',
                   jellyfinStart: widget.jellyfinStart,
                   onOpenIntegrations: widget.onOpenIntegrations,
                   onSubmit: _swap,