[v14.x backport] child_process: queue pending messages

lib/internal/child_process.js

@@ -2,8 +2,10 @@
 
 const {
   ArrayIsArray,
+  ArrayPrototypePush,
   ObjectDefineProperty,
   ObjectSetPrototypeOf,
+  ReflectApply,
   Symbol,
   Uint8Array,
 } = primordials;
@@ -73,6 +75,7 @@ let HTTPParser;
 const MAX_HANDLE_RETRANSMISSIONS = 3;
 const kChannelHandle = Symbol('kChannelHandle');
 const kIsUsedAsStdio = Symbol('kIsUsedAsStdio');
+const kPendingMessages = Symbol('kPendingMessages');
 
 // This object contain function to convert TCP objects to native handle objects
 // and back again.
@@ -520,6 +523,7 @@ class Control extends EventEmitter {
   constructor(channel) {
     super();
     this.#channel = channel;
+    this[kPendingMessages] = [];
   }
 
   // The methods keeping track of the counter are being used to track the
@@ -693,6 +697,24 @@ function setupChannel(target, channel, serializationMode) {
     });
   });
 
+  target.on('newListener', function() {
+
+    process.nextTick(() => {
+      if (!target.channel || !target.listenerCount('message'))
+        return;
+
+      const messages = target.channel[kPendingMessages];
+      const { length } = messages;
+      if (!length) return;
+
+      for (let i = 0; i < length; i++) {
+        ReflectApply(target.emit, target, messages[i]);
+      }
+
+      target.channel[kPendingMessages] = [];
+    });
+  });
+
   target.send = function(message, handle, options, callback) {
     if (typeof handle === 'function') {
       callback = handle;
@@ -909,7 +931,15 @@ function setupChannel(target, channel, serializationMode) {
   };
 
   function emit(event, message, handle) {
-    target.emit(event, message, handle);
+    if ('internalMessage' === event || target.listenerCount('message')) {
+      target.emit(event, message, handle);
+      return;
+    }
+
+    ArrayPrototypePush(
+      target.channel[kPendingMessages],
+      [event, message, handle]
+    );
   }
 
   function handleMessage(message, handle, internal) {

test/es-module/test-esm-child-process-fork-main.mjs

@@ -0,0 +1,20 @@
+import '../common/index.mjs';
+import assert from 'assert';
+import { fork } from 'child_process';
+import { once } from 'events';
+import { fileURLToPath } from 'url';
+
+if (process.argv[2] !== 'child') {
+  const filename = fileURLToPath(import.meta.url);
+  const cp = fork(filename, ['child']);
+  const message = 'Hello World';
+  cp.send(message);
+
+  const [received] = await once(cp, 'message');
+  assert.deepStrictEqual(received, message);
+
+  cp.disconnect();
+  await once(cp, 'exit');
+} else {
+  process.on('message', (msg) => process.send(msg));
+}