feat(session): share the session directory between instances

Publish every session to valkey with the instance holding it, and route a
command for a non-local session to its owner over a per-instance pub/sub
channel. Entries expire, so a dead instance leaves the directory.

Emby.Server.Implementations/Session/RemoteSessionController.cs

@@ -0,0 +1,65 @@
+using System;
+using System.Text.Json;
+using System.Threading;
+using System.Threading.Tasks;
+using Jellyfin.Extensions.Json;
+using MediaBrowser.Controller.Session;
+using MediaBrowser.Model.Session;
+
+namespace Emby.Server.Implementations.Session;
+
+/// <summary>
+/// Stands in for the websocket of a session another instance holds: messages are forwarded to that
+/// instance, which writes them to the connection it owns.
+/// </summary>
+public sealed class RemoteSessionController : ISessionController
+{
+    private readonly IPodMessageBus _bus;
+    private readonly string _ownerPod;
+    private readonly string _sessionId;
+
+    /// <summary>
+    /// Initializes a new instance of the <see cref="RemoteSessionController"/> class.
+    /// </summary>
+    /// <param name="bus">The cross-instance bus.</param>
+    /// <param name="ownerPod">The instance holding the connection.</param>
+    /// <param name="sessionId">The session identifier.</param>
+    /// <param name="isSessionActive">Whether the owner reported the session as active.</param>
+    /// <param name="supportsMediaControl">Whether the owner reported the session as controllable.</param>
+    public RemoteSessionController(IPodMessageBus bus, string ownerPod, string sessionId, bool isSessionActive, bool supportsMediaControl)
+    {
+        _bus = bus;
+        _ownerPod = ownerPod;
+        _sessionId = sessionId;
+        IsSessionActive = isSessionActive;
+        SupportsMediaControl = supportsMediaControl;
+    }
+
+    /// <inheritdoc />
+    public bool IsSessionActive { get; }
+
+    /// <inheritdoc />
+    public bool SupportsMediaControl { get; }
+
+    /// <inheritdoc />
+    public Task SendMessage<T>(SessionMessageType name, Guid messageId, T data, CancellationToken cancellationToken)
+    {
+        var routed = new RoutedSessionMessage
+        {
+            SessionId = _sessionId,
+            MessageType = name,
+            MessageId = messageId,
+            Data = JsonSerializer.Serialize(data, JsonDefaults.Options)
+        };
+
+        _bus.Publish(
+            _ownerPod,
+            new PodMessage
+            {
+                Kind = RoutedSessionMessage.Kind,
+                Payload = JsonSerializer.Serialize(routed, JsonDefaults.Options)
+            });
+
+        return Task.CompletedTask;
+    }
+}