cluster: don't send messages if no IPC channel

Avoid sending messages if the IPC channel is already disconnected. It
avoids undesired errors when calling `process.disconnect` when there are
still pending IPC messages.

PR-URL: #7132
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>

lib/cluster.js

@@ -740,6 +740,9 @@ function workerInit() {
 var seq = 0;
 var callbacks = {};
 function sendHelper(proc, message, handle, cb) {
+  if (!proc.connected)
+    return false;
+
   // Mark message as internal. See INTERNAL_PREFIX in lib/child_process.js
   message = util._extend({ cmd: 'NODE_CLUSTER' }, message);
   if (cb) callbacks[seq] = cb;

test/parallel/test-cluster-process-disconnect.js

@@ -0,0 +1,18 @@
+'use strict';
+const common = require('../common');
+const assert = require('assert');
+const cluster = require('cluster');
+
+if (cluster.isMaster) {
+  const worker = cluster.fork();
+  worker.on('exit', common.mustCall((code, signal) => {
+    assert.strictEqual(code, 0, 'worker did not exit normally');
+    assert.strictEqual(signal, null, 'worker did not exit normally');
+  }));
+} else {
+  const net = require('net');
+  const server = net.createServer();
+  server.listen(common.PORT, common.mustCall(() => {
+    process.disconnect();
+  }));
+}