Add onMasterConfigFieldChanged hook

Add hook for master plugin code to react to the master config changing.

packages/lib/plugin.js

@@ -355,6 +355,19 @@ class BaseMasterPlugin {
 	 */
 	async onInstanceStatusChanged(instance, prev) { }
 
+	/**
+	 * Called when the value of a master config field changed.
+	 *
+	 * Invoked after the value of the config field given by `field` has
+	 * changed on the master.
+	 *
+	 * @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 onMasterConfigFieldChanged(group, field, prev) { }
+
 	/**
 	 * Called when the value of an instance config field changed.
 	 *

packages/master/src/Master.js

@@ -162,6 +162,9 @@ class Master {
 		this.userManager = new UserManager(this.config);
 		await this.userManager.load(path.join(databaseDirectory, "users.json"));
 
+		this.config.on("fieldChanged", (group, field, prev) => {
+			libPlugin.invokeHook(this.plugins, "onMasterConfigFieldChanged", group, field, prev);
+		});
 		for (let instance of this.instances.values()) {
 			this.addInstanceHooks(instance);
 		}

test/lib/plugin.js

@@ -62,6 +62,8 @@ describe("lib/plugin", function() {
 		});
 		it("should define defaults for hooks", async function() {
 			await masterPlugin.onInstanceStatusChanged({}, "running", "initialized");
+			await masterPlugin.onMasterConfigFieldChanged({}, "foo", null);
+			await masterPlugin.onInstanceConfigFieldChanged({}, {}, "foo", null);
 			await masterPlugin.onMetrics();
 			await masterPlugin.onShutdown();
 			masterPlugin.onSlaveConnectionEvent({}, "connect");