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/AppBase/BaseConfigurationManager.cs

@@ -87,6 +87,12 @@ namespace Emby.Server.Implementations.AppBase
         /// <value>The application paths.</value>
         public IApplicationPaths CommonApplicationPaths { get; private set; }
 
+        /// <summary>
+        /// Gets or sets the bus announcing configuration writes to the other instances sharing this
+        /// configuration directory. Defaults to a no-op, which is the single-instance behaviour.
+        /// </summary>
+        public IConfigurationInvalidationBus InvalidationBus { get; set; } = NullConfigurationInvalidationBus.Instance;
+
         /// <summary>
         /// Gets or sets the system configuration.
         /// </summary>
@@ -169,6 +175,8 @@ namespace Emby.Server.Implementations.AppBase
             }
 
             OnConfigurationUpdated();
+
+            InvalidationBus.Publish(ConfigurationInvalidationScope.SystemConfiguration, null);
         }
 
         /// <summary>
@@ -350,6 +358,29 @@ namespace Emby.Server.Implementations.AppBase
             }
 
             OnNamedConfigurationUpdated(key, configuration);
+
+            InvalidationBus.Publish(ConfigurationInvalidationScope.NamedConfiguration, key);
+        }
+
+        /// <inheritdoc />
+        public void InvalidateCachedConfiguration(string? key)
+        {
+            if (string.IsNullOrEmpty(key))
+            {
+                lock (_configurationSyncLock)
+                {
+                    _configuration = null;
+                }
+
+                // Reloads the system configuration off the shared file as a side effect of re-deriving
+                // the cache path from it, then tells the in-process consumers to re-read it.
+                OnConfigurationUpdated();
+                return;
+            }
+
+            _configurations.TryRemove(key, out _);
+
+            OnNamedConfigurationUpdated(key, GetConfiguration(key));
         }
 
         /// <summary>