worker: prevent event loop starvation through MessagePorts

Limit the number of messages processed without interruption on a
given `MessagePort` to prevent event loop starvation, but still
make sure that all messages are emitted that were already in the
queue when emitting began.

This aligns the behaviour better with the web.

Refs: nodejs#28030

Author: addaleax

src/node_messaging.cc

@@ -609,11 +609,25 @@ void MessagePort::OnMessage() {
   HandleScope handle_scope(env()->isolate());
   Local<Context> context = object(env()->isolate())->CreationContext();
 
+  ssize_t processing_limit;
+  {
+    Mutex::ScopedLock(data_->mutex_);
+    processing_limit = data_->incoming_messages_.size();
+  }
+
   // data_ can only ever be modified by the owner thread, so no need to lock.
   // However, the message port may be transferred while it is processing
   // messages, so we need to check that this handle still owns its `data_` field
   // on every iteration.
   while (data_) {
+    if (--processing_limit < 0) {
+      // Prevent event loop starvation by only processing those messages without
+      // interruption that were already present when the OnMessage() call was
+      // first triggered.
+      TriggerAsync();
+      return;
+    }
+
     HandleScope handle_scope(env()->isolate());
     Context::Scope context_scope(context);
 

test/parallel/test-worker-message-port-close-while-receiving.js

@@ -0,0 +1,15 @@
+'use strict';
+const common = require('../common');
+
+const { MessageChannel } = require('worker_threads');
+
+// Make sure that closing a message port while receiving messages on it does
+// not stop messages that are already in the queue from being emitted.
+
+const { port1, port2 } = new MessageChannel();
+
+port1.on('message', common.mustCall(() => {
+  port1.close();
+}, 2));
+port2.postMessage('foo');
+port2.postMessage('bar');

test/parallel/test-worker-message-port-infinite-message-loop.js

@@ -0,0 +1,27 @@
+'use strict';
+const common = require('../common');
+const assert = require('assert');
+
+const { MessageChannel } = require('worker_threads');
+
+// Make sure that an infinite asynchronous .on('message')/postMessage loop
+// does not lead to a stack overflow and does not starve the event loop.
+// We schedule timeouts both from before the the .on('message') handler and
+// inside of it, which both should run.
+
+const { port1, port2 } = new MessageChannel();
+let count = 0;
+port1.on('message', () => {
+  if (count === 0) {
+    setTimeout(common.mustCall(() => {
+      port1.close();
+    }), 0);
+  }
+
+  port2.postMessage(0);
+  assert(count++ < 10000, `hit ${count} loop iterations`);
+});
+
+port2.postMessage(0);
+
+setTimeout(common.mustCall(), 0);