Add instance config event for master plugins

Add onInstanceConfigFieldChanged hook for master plugin code which
triggers when instance configs are changed on the master server.

packages/lib/plugin.js

@@ -286,6 +286,20 @@ class BaseMasterPlugin {
 	 */
 	async onInstanceStatusChanged(instance, prev) { }
 
+	/**
+	 * Called when the value of an instance config field changed.
+	 *
+	 * Invoked after the value of the config field given by `field` has
+	 * changed on an instance.
+	 *
+	 * @param {Object} instance - The instance the config changed on.
+	 * @param {module:lib/config.ConfigGroup} group -
+	 *     The group who's field got changed on.
+	 * @param {string} field - Name of the field that changed.
+	 * @param {*} prev - The previous value of the field.
+	 */
+	async onInstanceConfigFieldChanged(instance, group, field, prev) { }
+
 	/**
 	 * Called before collecting Prometheus metrics
 	 *

packages/master/src/ControlConnection.js

@@ -174,6 +174,7 @@ class ControlConnection extends BaseConnection {
 		let instance = { config: instanceConfig, status: "unassigned" };
 		this._master.instances.set(instanceId, instance);
 		await libPlugin.invokeHook(this._master.plugins, "onInstanceStatusChanged", instance, null);
+		this._master.addInstancePluginHooks(instance);
 	}
 
 	async deleteInstanceRequestHandler(message, request) {

packages/master/src/Master.js

@@ -151,6 +151,10 @@ class Master {
 		this.userManager = new UserManager(this.config);
 		await this.userManager.load(path.join(databaseDirectory, "users.json"));
 
+		for (let instance of this.instances.values()) {
+			this.addInstancePluginHooks(instance);
+		}
+
 		// Make sure we're actually going to listen on a port
 		let httpPort = this.config.get("master.http_port");
 		let httpsPort = this.config.get("master.https_port");
@@ -305,7 +309,8 @@ class Master {
 				let instanceConfig = new libConfig.InstanceConfig("master");
 				await instanceConfig.load(serializedConfig);
 				let status = instanceConfig.get("instance.assigned_slave") === null ? "unassigned" : "unknown";
-				instances.set(instanceConfig.get("instance.id"), { config: instanceConfig, status });
+				let instance = { config: instanceConfig, status };
+				instances.set(instanceConfig.get("instance.id"), instance);
 			}
 
 		} catch (err) {
@@ -356,6 +361,12 @@ class Master {
 		}
 	}
 
+	addInstancePluginHooks(instance) {
+		instance.config.on("fieldChanged", (group, field, prev) => {
+			libPlugin.invokeHook(this.plugins, "onInstanceConfigFieldChanged", instance, group, field, prev);
+		});
+	}
+
 	async loadPlugins() {
 		for (let pluginInfo of this.pluginInfos) {
 			if (!this.config.group(pluginInfo.name).get("load_plugin")) {

packages/master/src/SlaveConnection.js

@@ -98,6 +98,7 @@ class SlaveConnection extends BaseConnection {
 			instanceConfig.set("instance.assigned_slave", this._id);
 			let newInstance = { config: instanceConfig, status: instanceData.status };
 			this._master.instances.set(instanceConfig.get("instance.id"), newInstance);
+			this._master.addInstancePluginHooks(newInstance);
 			await libLink.messages.assignInstance.send(this, {
 				instance_id: instanceConfig.get("instance.id"),
 				serialized_config: instanceConfig.serialize("slave"),