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.

MediaBrowser.Controller/Session/IPodMessageBus.cs

@@ -0,0 +1,29 @@
+using System;
+using System.Threading.Tasks;
+
+namespace MediaBrowser.Controller.Session;
+
+/// <summary>
+/// Point-to-point delivery between instances: every instance listens on a channel of its own, so a
+/// message can be addressed to the one instance holding a given connection.
+/// </summary>
+public interface IPodMessageBus
+{
+    /// <summary>
+    /// Gets the identity of this instance.
+    /// </summary>
+    string PodId { get; }
+
+    /// <summary>
+    /// Sends a message to one instance. Delivery is best effort and never throws.
+    /// </summary>
+    /// <param name="targetPod">The instance to deliver to.</param>
+    /// <param name="message">The message.</param>
+    void Publish(string targetPod, PodMessage message);
+
+    /// <summary>
+    /// Registers a handler for the messages addressed to this instance.
+    /// </summary>
+    /// <param name="handler">The handler.</param>
+    void Subscribe(Func<PodMessage, Task> handler);
+}

MediaBrowser.Controller/Session/ISessionDirectory.cs

@@ -0,0 +1,35 @@
+using System.Collections.Generic;
+using System.Threading;
+using System.Threading.Tasks;
+
+namespace MediaBrowser.Controller.Session;
+
+/// <summary>
+/// The shared record of which instance holds which session. Entries expire, so an instance that stops
+/// refreshing them drops out of every other instance's view instead of lingering.
+/// </summary>
+public interface ISessionDirectory
+{
+    /// <summary>
+    /// Publishes an entry and restarts its expiry.
+    /// </summary>
+    /// <param name="entry">The entry.</param>
+    /// <param name="cancellationToken">The cancellation token.</param>
+    /// <returns>A task representing the operation.</returns>
+    Task PublishAsync(SessionDirectoryEntry entry, CancellationToken cancellationToken = default);
+
+    /// <summary>
+    /// Removes an entry.
+    /// </summary>
+    /// <param name="sessionId">The session identifier.</param>
+    /// <param name="cancellationToken">The cancellation token.</param>
+    /// <returns>A task representing the operation.</returns>
+    Task RemoveAsync(string sessionId, CancellationToken cancellationToken = default);
+
+    /// <summary>
+    /// Gets every entry that has not expired.
+    /// </summary>
+    /// <param name="cancellationToken">The cancellation token.</param>
+    /// <returns>The entries.</returns>
+    Task<IReadOnlyList<SessionDirectoryEntry>> GetAllAsync(CancellationToken cancellationToken = default);
+}

MediaBrowser.Controller/Session/ISessionManager.cs

@@ -306,8 +306,9 @@ namespace MediaBrowser.Controller.Session
         /// <param name="activeWithinSeconds">Active within session limit.</param>
         /// <param name="controllableUserToCheck">Filter for sessions remote controllable for this user.</param>
         /// <param name="isApiKey">Is the request authenticated with API key.</param>
-        /// <returns>IReadOnlyList{SessionInfoDto}.</returns>
-        IReadOnlyList<SessionInfoDto> GetSessions(Guid userId, string deviceId, int? activeWithinSeconds, Guid? controllableUserToCheck, bool isApiKey);
+        /// <param name="cancellationToken">The cancellation token.</param>
+        /// <returns>IReadOnlyList{SessionInfoDto}, including the sessions held by the other instances.</returns>
+        Task<IReadOnlyList<SessionInfoDto>> GetSessions(Guid userId, string deviceId, int? activeWithinSeconds, Guid? controllableUserToCheck, bool isApiKey, CancellationToken cancellationToken);
 
         /// <summary>
         /// Gets the session by authentication token.

MediaBrowser.Controller/Session/NullPodMessageBus.cs

@@ -0,0 +1,28 @@
+using System;
+using System.Threading.Tasks;
+
+namespace MediaBrowser.Controller.Session;
+
+/// <summary>
+/// The single-instance <see cref="IPodMessageBus"/>: there is no other instance to reach.
+/// </summary>
+public sealed class NullPodMessageBus : IPodMessageBus
+{
+    /// <summary>
+    /// Gets the shared instance.
+    /// </summary>
+    public static NullPodMessageBus Instance { get; } = new NullPodMessageBus();
+
+    /// <inheritdoc />
+    public string PodId => PodIdentity.Current;
+
+    /// <inheritdoc />
+    public void Publish(string targetPod, PodMessage message)
+    {
+    }
+
+    /// <inheritdoc />
+    public void Subscribe(Func<PodMessage, Task> handler)
+    {
+    }
+}

MediaBrowser.Controller/Session/NullSessionDirectory.cs

@@ -0,0 +1,30 @@
+using System;
+using System.Collections.Generic;
+using System.Threading;
+using System.Threading.Tasks;
+
+namespace MediaBrowser.Controller.Session;
+
+/// <summary>
+/// The single-instance <see cref="ISessionDirectory"/>: nothing is published and no session is held
+/// anywhere but here, which is exactly the behaviour of a deployment without a shared store.
+/// </summary>
+public sealed class NullSessionDirectory : ISessionDirectory
+{
+    /// <summary>
+    /// Gets the shared instance.
+    /// </summary>
+    public static NullSessionDirectory Instance { get; } = new NullSessionDirectory();
+
+    /// <inheritdoc />
+    public Task PublishAsync(SessionDirectoryEntry entry, CancellationToken cancellationToken = default)
+        => Task.CompletedTask;
+
+    /// <inheritdoc />
+    public Task RemoveAsync(string sessionId, CancellationToken cancellationToken = default)
+        => Task.CompletedTask;
+
+    /// <inheritdoc />
+    public Task<IReadOnlyList<SessionDirectoryEntry>> GetAllAsync(CancellationToken cancellationToken = default)
+        => Task.FromResult<IReadOnlyList<SessionDirectoryEntry>>(Array.Empty<SessionDirectoryEntry>());
+}

MediaBrowser.Controller/Session/PodIdentity.cs

@@ -0,0 +1,14 @@
+using System;
+
+namespace MediaBrowser.Controller.Session;
+
+/// <summary>
+/// The identity of this instance among the replicas sharing a deployment.
+/// </summary>
+public static class PodIdentity
+{
+    /// <summary>
+    /// Gets the identity of this instance.
+    /// </summary>
+    public static string Current => Environment.GetEnvironmentVariable("JELLYFIN_INSTANCE_ID") ?? Environment.MachineName;
+}

MediaBrowser.Controller/Session/PodMessage.cs

@@ -0,0 +1,23 @@
+namespace MediaBrowser.Controller.Session;
+
+/// <summary>
+/// An envelope addressed to one instance. <see cref="Kind"/> names the payload so that features other
+/// than session routing can share the same channel.
+/// </summary>
+public sealed class PodMessage
+{
+    /// <summary>
+    /// Gets or sets the payload discriminator.
+    /// </summary>
+    public string Kind { get; set; } = string.Empty;
+
+    /// <summary>
+    /// Gets or sets the identity of the sending instance.
+    /// </summary>
+    public string OriginPod { get; set; } = string.Empty;
+
+    /// <summary>
+    /// Gets or sets the serialized payload.
+    /// </summary>
+    public string Payload { get; set; } = string.Empty;
+}

MediaBrowser.Controller/Session/RoutedSessionMessage.cs

@@ -0,0 +1,35 @@
+using System;
+using MediaBrowser.Model.Session;
+
+namespace MediaBrowser.Controller.Session;
+
+/// <summary>
+/// A websocket message for a session held by another instance, carried as a <see cref="PodMessage"/>.
+/// </summary>
+public sealed class RoutedSessionMessage
+{
+    /// <summary>
+    /// The <see cref="PodMessage.Kind"/> this payload travels under.
+    /// </summary>
+    public const string Kind = "SessionMessage";
+
+    /// <summary>
+    /// Gets or sets the session the message is addressed to.
+    /// </summary>
+    public string SessionId { get; set; } = string.Empty;
+
+    /// <summary>
+    /// Gets or sets the message type.
+    /// </summary>
+    public SessionMessageType MessageType { get; set; }
+
+    /// <summary>
+    /// Gets or sets the message identifier.
+    /// </summary>
+    public Guid MessageId { get; set; }
+
+    /// <summary>
+    /// Gets or sets the message data, serialized as JSON.
+    /// </summary>
+    public string Data { get; set; } = "null";
+}

MediaBrowser.Controller/Session/SessionDirectoryEntry.cs

@@ -0,0 +1,19 @@
+using MediaBrowser.Model.Dto;
+
+namespace MediaBrowser.Controller.Session;
+
+/// <summary>
+/// A session held by one instance, as the other instances see it.
+/// </summary>
+public sealed class SessionDirectoryEntry
+{
+    /// <summary>
+    /// Gets or sets the identity of the instance holding the connection.
+    /// </summary>
+    public string OwnerPod { get; set; } = string.Empty;
+
+    /// <summary>
+    /// Gets or sets the session as its owner last rendered it.
+    /// </summary>
+    public SessionInfoDto? Session { get; set; }
+}

MediaBrowser.Controller/Session/SessionDirectoryOptions.cs

@@ -0,0 +1,23 @@
+namespace MediaBrowser.Controller.Session;
+
+/// <summary>
+/// Configuration options for the session directory and the cross-instance bus that goes with it.
+/// </summary>
+public sealed class SessionDirectoryOptions
+{
+    /// <summary>
+    /// The configuration section these options bind from.
+    /// </summary>
+    public const string ConfigurationSection = "Jellyfin:SessionDirectory";
+
+    /// <summary>
+    /// Gets or sets how long in seconds a published entry survives without being refreshed. An instance
+    /// that dies stops refreshing, so its sessions leave the directory after this long.
+    /// </summary>
+    public int EntryTtlSeconds { get; set; } = 60;
+
+    /// <summary>
+    /// Gets or sets how often in seconds an instance republishes the sessions it holds.
+    /// </summary>
+    public int RefreshIntervalSeconds { get; set; } = 20;
+}