Implement config notification event

Add fieldChanged event to config objects that is triggered when the
value of a field in a group belonging to it is changed.

lib/config/classes.js

@@ -1,6 +1,9 @@
 // Configuration classes
 "use strict";
 
+const events = require("events");
+const util = require("util");
+
 const { basicType } = require("lib/helpers");
 
 
@@ -55,10 +58,20 @@ function splitOn(separator, string) {
  * Represents a collection of ConfigGroup instances that can be
  * managed as one.
  *
+ * This is an EventEmitter with the following events:
+ * - fieldChanged -
+ *     invoked after a field has been updated in one of the config groups
+ *     belonging to this config.  Takes three parameters: group, field,
+ *     prev.
+ *
+ * @extends events.EventEmitter
+ *
  * @memberof module:lib/config
  */
-class Config {
+class Config extends events.EventEmitter {
 	constructor() {
+		super();
+
 		if (!this.constructor._finalized) {
 			throw new Error(`Cannot instantiate incomplete Config class ${this.constructor.name}`)
 		}
@@ -71,7 +84,7 @@ class Config {
 		for (let [name, GroupClass] of this.constructor._groups) {
 			if (!this._groups.has(name)) {
 				let group = new GroupClass();
-				await group.init();
+				await group.init(this);
 				this._groups.set(name, group);
 			}
 		}
@@ -98,7 +111,7 @@ class Config {
 			}
 
 			let group = new GroupClass();
-			await group.load(serializedGroup);
+			await group.load(this, serializedGroup);
 			this._groups.set(GroupClass.groupName, group);
 		}
 
@@ -123,12 +136,12 @@ class Config {
 	 * Updates all groups in the config with groups in the serialized
 	 * config passed.
 	 */
-	update(serializedConfig) {
+	update(serializedConfig, notify) {
 		this._validate(serializedConfig);
 		for (let serializedGroup of serializedConfig.groups) {
 			let group = this._groups.get(serializedGroup.name);
 			if (group) {
-				group.update(serializedGroup);
+				group.update(serializedGroup, notify);
 			} else {
 				this._unknownGroups.set(serializedGroup.name, serializedGroup);
 			}
@@ -226,6 +239,11 @@ class Config {
 /**
  * Collection of related config entries
  *
+ * Type checks and stores a collection of related config entries.  Attempt
+ * to set invalid values for fields will result in an error thrown while
+ * loading a config with invalid values will result in those being replaced
+ * by the innitial value for those fields.
+ *
  * @memberof module:lib/config
  */
 class ConfigGroup {
@@ -242,6 +260,7 @@ class ConfigGroup {
 
 		this._fields = new Map();
 		this._unknownFields = new Map();
+		this._config = null;
 	}
 
 	/**
@@ -250,7 +269,9 @@ class ConfigGroup {
 	 * Computes and assigns the initial values for all fields of the
 	 * config group.
 	 */
-	async init() {
+	async init(config) {
+		this._config = config;
+
 		for (let [name, def] of this.constructor._definitions) {
 			if (this._fields.has(name)) {
 				continue;
@@ -275,9 +296,9 @@ class ConfigGroup {
 	 * group and then initializes any possible missing fields from it to
 	 * their default values.
 	 */
-	async load(serializedGroup) {
-		this.update(serializedGroup);
-		await this.init();
+	async load(config, serializedGroup) {
+		this.update(serializedGroup, false);
+		await this.init(config);
 	}
 
 	/**
@@ -345,7 +366,11 @@ class ConfigGroup {
 			}
 		}
 
+		let prev = this._fields.get(name);
 		this._fields.set(name, value);
+		if (this._config && !util.isDeepStrictEqual(value, prev)) {
+			this._config.emit("fieldChanged", this, name, prev);
+		}
 	}
 
 	/**
@@ -354,7 +379,7 @@ class ConfigGroup {
 	 * Overwrites the values of all the fields in this config group that
 	 * has a valid value in the passed serialized group with that value.
 	 */
-	update(serializedGroup) {
+	update(serializedGroup, notify) {
 		if (basicType(serializedGroup) !== "object") {
 			throw new Error(`Expected object, not ${basicType(serializedGroup)} for ConfigGroup`);
 		}
@@ -388,7 +413,11 @@ class ConfigGroup {
 				continue;
 			}
 
+			let prev = this._fields.get(name);
 			this._fields.set(name, value);
+			if (notify && this._config && !util.isDeepStrictEqual(value, prev)) {
+				this._config.emit("fieldChanged", this, name, prev);
+			}
 		}
 	}
 

lib/plugin.js

@@ -42,6 +42,19 @@ class BaseInstancePlugin {
 	 */
 	async init() { }
 
+	/**
+	 * Called when the value of a config field changed.
+	 *
+	 * Invoked after the value of the config field given by `field` has
+	 * changed.
+	 *
+	 * @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(group, field, prev) { }
+
 	/**
 	 * Called before collecting Prometheus metrics
 	 *