child_process: fix handle passing w large payloads

Fix situations in which the handle passed along with a message
that has a large payload and can’t be read entirely by a single
`recvmsg()` call isn’t associated with the message to which it
belongs.

Fixes: #13778

lib/internal/child_process.js

@@ -455,10 +455,14 @@ function setupChannel(target, channel) {
 
   var decoder = new StringDecoder('utf8');
   var jsonBuffer = '';
+  var pendingHandle = null;
   channel.buffering = false;
   channel.onread = function(nread, pool, recvHandle) {
     // TODO(bnoordhuis) Check that nread > 0.
     if (pool) {
+      if (recvHandle)
+        pendingHandle = recvHandle;
+
       // Linebreak is used as a message end sign
       var chunks = decoder.write(pool).split('\n');
       var numCompleteChunks = chunks.length - 1;
@@ -478,10 +482,12 @@ function setupChannel(target, channel) {
         // read because SCM_RIGHTS messages don't get coalesced. Make sure
         // that we deliver the handle with the right message however.
         if (isInternal(message)) {
-          if (message.cmd === 'NODE_HANDLE')
-            handleMessage(message, recvHandle, true);
-          else
+          if (message.cmd === 'NODE_HANDLE') {
+            handleMessage(message, pendingHandle, true);
+            pendingHandle = null;
+          } else {
             handleMessage(message, undefined, true);
+          }
         } else {
           handleMessage(message, undefined, false);
         }

test/parallel/test-cluster-send-handle-large-payload.js

@@ -0,0 +1,37 @@
+'use strict';
+const common = require('../common');
+const assert = require('assert');
+const cluster = require('cluster');
+const net = require('net');
+
+const payload = 'a'.repeat(800004);
+
+if (cluster.isMaster) {
+  const server = net.createServer();
+
+  server.on('connection', common.mustCall((socket) => socket.unref()));
+
+  const worker = cluster.fork();
+  worker.on('message', common.mustCall(({ payload: received }, handle) => {
+    assert.strictEqual(payload, received);
+    assert(handle instanceof net.Socket);
+    server.close();
+    worker.kill();
+    handle.destroy();
+  }));
+
+  server.listen(0, common.mustCall(() => {
+    const port = server.address().port;
+    const socket = new net.Socket();
+    socket.connect(port, (err) => {
+      assert.ifError(err);
+      worker.send({ payload }, socket);
+    });
+  }));
+} else {
+  process.on('message', common.mustCall(({ payload: received }, handle) => {
+    assert.strictEqual(payload, received);
+    assert(handle instanceof net.Socket);
+    process.send({ payload }, handle);
+  }));
+}