Implement broadcast for events

Add broadcastTo property to events that broadcasts them to all entities
of the specified target that's downstream to the connection it was sent
to.

client.js

@@ -300,6 +300,17 @@ class InstanceConnection extends link.Link {
 	async forwardEventToMaster(message, event) {
 		event.send(this.slave, message.data);
 	}
+
+	async broadcastEventToInstance(message, event) {
+		for (let instanceConnection of this.slave.instanceConnections.values()) {
+			if (instanceConnection === this) {
+				continue; // Do not broadcast back to the source
+			}
+
+			console.log("Broadcasting event");
+			event.send(instanceConnection, message.data);
+		}
+	}
 }
 
 class SlaveConnector extends link.SocketIOClientConnector {
@@ -377,6 +388,12 @@ class Slave extends link.Link {
 		return await request.send(instanceConnection, message.data);
 	}
 
+	async broadcastEventToInstance(message, event) {
+		for (let instanceConnection of this.instanceConnections.values()) {
+			event.send(instanceConnection, message.data);
+		}
+	}
+
 	async createInstanceRequestHandler(message) {
 		let { id, options } = message.data;
 		if (this.instanceInfos.has(id)) {

lib/link/messages.js

@@ -313,19 +313,30 @@ class Event {
 	 * @param {?string} forwardTo -
 	 *     Optional target to forward this request to.  Only 'master' is
 	 *     currently supported.
+	 * @param {?string} broadcastTo -
+	 *     Optional target to broadcast this event to.  Only 'instance' is
+	 *     currently supported.  An event will only be broadcast towards
+	 *     links that are downstream, so sending an event set to instance
+	 *     broadcast that's sent to a slave will only be broadcast to that
+	 *     slave's instances.
 	 * @param {Object<string, Object>} eventProperties -
 	 *     Mapping of property values to JSON schema specifications for the
 	 *     properties that are valid in the data payload of this event.
 	 */
-	constructor({ type, links, forwardTo = null, eventProperties = {} }) {
+	constructor({ type, links, forwardTo = null, broadcastTo = null, eventProperties = {} }) {
 		this.type = type;
 		this.links = links;
 		this.forwardTo = forwardTo;
+		this.broadcastTo = broadcastTo;
 
 		if (forwardTo !== 'master' && forwardTo !== null) {
 			throw new Error(`Invalid forwardTo value ${forwardTo}`);
 		}
 
+		if (broadcastTo !== 'instance' && broadcastTo !== null) {
+			throw new Error(`Invalid broadcastTo value ${broadcastTo}`);
+		}
+
 		this.eventType = type + '_event';
 		this._eventValidator = schema.compile({
 			$schema: "http://json-schema.org/draft-07/schema#",
@@ -349,6 +360,9 @@ class Event {
 	 * If forwardTo was set to 'master' the `forwardEventToMaster` method on
 	 * the link is used as the default handler.
 	 *
+	 * If broadcastTo was set to 'instance' the `broadcastEventToInstance`
+	 * method on the link is invoked before the handler.  BUT HOW?
+	 *
 	 * Does nothing if the link is not a target for this event.
 	 *
 	 * @param {Link} link - The link to attach to.
@@ -367,6 +381,23 @@ class Event {
 				}
 			}
 
+			if (
+				this.broadcastTo === 'instance'
+				&& [
+					'instance-slave', 'slave-master', 'control-master', 'master-slave'
+				].includes(`${link.target}-${link.source}`)
+			) {
+				if (!handler) {
+					handler = link.broadcastEventToInstance;
+				} else {
+					let originalHandler = handler;
+					handler = async (message, event) => {
+						await link.broadcastEventToInstance(message, event);
+						await originalHandler.call(link, message, event);
+					}
+				}
+			}
+
 			if (!handler) {
 				throw new Error(`Missing handler for ${this.eventType} on ${link.source}-${link.target} link`);
 			}

master.js

@@ -267,6 +267,16 @@ class BaseConnection extends link.Link {
 
 		return await request.send(connection, message.data);
 	}
+
+	async broadcastEventToInstance(message, event) {
+		for (let slaveConnection of slaveConnections.values()) {
+			if (slaveConnection === this) {
+				continue; // Do not broadcast back to the source
+			}
+
+			event.send(slaveConnection, message.data);
+		}
+	}
 }
 
 let controlConnections = new Array();

test/lib/link/messages.js

@@ -115,6 +115,12 @@ describe("lib/link/messages", function() {
 					new Error("Invalid forwardTo value invalid")
 				);
 			});
+			it("should throw on invalid broadcastTo", function() {
+				assert.throws(
+					() => new link.Event({ broadcastTo: 'invalid'}),
+					new Error("Invalid broadcastTo value invalid")
+				);
+			});
 		});
 
 		describe(".attach()", function() {