Fix socket.io not working properly in single-host clusters. (#6669)

This is because we don't use the cluster module, so the master nodejs process doesn't know about any of the clients through the socket API.

src/socket.io/single-host-cluster.js

@@ -0,0 +1,54 @@
+'use strict';
+
+var Client = {
+	sendMessage: function (channel, message) {
+		process.send({
+			action: 'socket.io',
+			channel: channel,
+			message: message,
+		});
+	},
+	trigger: function (channel, message) {
+		Client.message.concat(Client.pmessage).forEach(function (callback) {
+			setImmediate(function () {
+				callback.call(Client, channel, message);
+			});
+		});
+	},
+	publish: function (channel, message) {
+		Client.sendMessage(channel, message);
+	},
+	// we don't actually care about which channels we're subscribed to
+	subscribe: function () {},
+	psubscribe: function () {},
+	unsubscribe: function () {},
+	unpsubscribe: function () {},
+	message: [],
+	pmessage: [],
+	on: function (event, callback) {
+		if (event !== 'message' && event !== 'pmessage') {
+			return;
+		}
+		Client[event].push(callback);
+	},
+	off: function (event, callback) {
+		if (event !== 'message' && event !== 'pmessage') {
+			return;
+		}
+		if (callback) {
+			Client[event] = Client[event].filter(function (c) {
+				return c !== callback;
+			});
+		} else {
+			Client[event] = [];
+		}
+	},
+};
+
+process.on('message', function (message) {
+	if (message && typeof message === 'object' && message.action === 'socket.io') {
+		Client.trigger(message.channel, message.message);
+	}
+});
+
+module.exports = Client;