propagate shared-config and library-option changes between instances

Add a Redis pub/sub invalidation bus, publish from the configuration and library-option write paths, and drop the matching local cache entry on the instances that did not write. No-op without a Redis connection string, and fails open when it is unreachable.

Emby.Server.Implementations/Configuration/RedisConfigurationInvalidationBus.cs

@@ -0,0 +1,96 @@
+using System;
+using System.Text.Json;
+using Jellyfin.Extensions.Json;
+using MediaBrowser.Common.Configuration;
+using Microsoft.Extensions.Logging;
+using StackExchange.Redis;
+
+namespace Emby.Server.Implementations.Configuration
+{
+    /// <summary>
+    /// A Redis pub/sub <see cref="IConfigurationInvalidationBus"/>. Notices are broadcast on one channel
+    /// and every instance but the publisher applies them.
+    /// </summary>
+    public sealed class RedisConfigurationInvalidationBus : IConfigurationInvalidationBus
+    {
+        private const string ChannelName = "jellyfin:configinvalidation";
+
+        private static readonly JsonSerializerOptions _jsonOptions = JsonDefaults.Options;
+
+        private readonly ISubscriber _subscriber;
+        private readonly ILogger<RedisConfigurationInvalidationBus> _logger;
+        private readonly string _originId;
+
+        /// <summary>
+        /// Initializes a new instance of the <see cref="RedisConfigurationInvalidationBus"/> class.
+        /// </summary>
+        /// <param name="redis">The Redis connection multiplexer.</param>
+        /// <param name="logger">The logger.</param>
+        public RedisConfigurationInvalidationBus(IConnectionMultiplexer redis, ILogger<RedisConfigurationInvalidationBus> logger)
+        {
+            ArgumentNullException.ThrowIfNull(redis);
+
+            _subscriber = redis.GetSubscriber();
+            _logger = logger;
+            _originId = Environment.GetEnvironmentVariable("JELLYFIN_INSTANCE_ID") ?? Environment.MachineName;
+        }
+
+        /// <inheritdoc />
+        public void Publish(ConfigurationInvalidationScope scope, string? target)
+        {
+            var invalidation = new ConfigurationInvalidation
+            {
+                Scope = scope,
+                Target = target,
+                OriginId = _originId
+            };
+
+            try
+            {
+                // Fire and forget: an admin saving configuration must never wait on, or fail because of,
+                // the bus. The write has already reached the shared directory by this point.
+                _subscriber.Publish(
+                    RedisChannel.Literal(ChannelName),
+                    JsonSerializer.Serialize(invalidation, _jsonOptions),
+                    CommandFlags.FireAndForget);
+            }
+            catch (Exception ex)
+            {
+                _logger.LogWarning(ex, "Failed to publish {Scope} invalidation for {Target}; other instances keep their cached copy until they restart.", scope, target);
+            }
+        }
+
+        /// <inheritdoc />
+        public void Subscribe(Action<ConfigurationInvalidation> handler)
+        {
+            ArgumentNullException.ThrowIfNull(handler);
+
+            try
+            {
+                _subscriber.Subscribe(RedisChannel.Literal(ChannelName), (_, value) => Dispatch(handler, value));
+            }
+            catch (Exception ex)
+            {
+                _logger.LogWarning(ex, "Failed to subscribe to configuration invalidations; this instance keeps its cached configuration until it restarts.");
+            }
+        }
+
+        private void Dispatch(Action<ConfigurationInvalidation> handler, RedisValue value)
+        {
+            try
+            {
+                var invalidation = JsonSerializer.Deserialize<ConfigurationInvalidation>(value.ToString(), _jsonOptions);
+                if (invalidation is null || string.Equals(invalidation.OriginId, _originId, StringComparison.Ordinal))
+                {
+                    return;
+                }
+
+                handler(invalidation);
+            }
+            catch (Exception ex)
+            {
+                _logger.LogWarning(ex, "Failed to apply a configuration invalidation.");
+            }
+        }
+    }
+}