Fix rare ERR_IPC_CHANNEL_CLOSED bug in cluster mode

CHANGELOG.md

@@ -11,6 +11,9 @@ project adheres to [Semantic Versioning](http://semver.org/).
 
 ### Changed
 
+- Check that cluster worker is still connected before attempting to query it for
+  metrics. (#244)
+
 ### Added
 
 ## [11.2.1]

lib/cluster.js

@@ -38,8 +38,6 @@ class AggregatorRegistry extends Registry {
 		const requestId = requestCtr++;
 
 		return new Promise((resolve, reject) => {
-			const nWorkers = Object.keys(cluster.workers).length;
-
 			function done(err, result) {
 				// Don't resolve/reject the promise if a callback is provided
 				if (typeof callback === 'function') {
@@ -50,13 +48,9 @@ class AggregatorRegistry extends Registry {
 				}
 			}
 
-			if (nWorkers === 0) {
-				return process.nextTick(() => done(null, ''));
-			}
-
 			const request = {
 				responses: [],
-				pending: nWorkers,
+				pending: 0,
 				done,
 				errorTimeout: setTimeout(() => {
 					request.failed = true;
@@ -71,7 +65,21 @@ class AggregatorRegistry extends Registry {
 				type: GET_METRICS_REQ,
 				requestId
 			};
-			for (const id in cluster.workers) cluster.workers[id].send(message);
+
+			for (const id in cluster.workers) {
+				// If the worker exits abruptly, it may still be in the workers
+				// list but not able to communicate.
+				if (cluster.workers[id].isConnected()) {
+					cluster.workers[id].send(message);
+					request.pending++;
+				}
+			}
+
+			if (request.pending === 0) {
+				// No workers were up
+				clearTimeout(request.errorTimeout);
+				process.nextTick(() => done(null, ''));
+			}
 		});
 	}
 
@@ -147,13 +155,7 @@ function addListeners() {
 
 	if (cluster.isMaster) {
 		// Listen for worker responses to requests for local metrics
-		cluster.on('message', function(worker, message) {
-			if (arguments.length === 2) {
-				// pre-Node.js v6.0
-				message = worker;
-				worker = undefined;
-			}
-
+		cluster.on('message', (worker, message) => {
 			if (message.type === GET_METRICS_RES) {
 				const request = requests.get(message.requestId);
 				message.metrics.forEach(registry => request.responses.push(registry));