worker: add hasRef() to MessagePort

doc/api/worker_threads.md

@@ -763,6 +763,18 @@ port2.postMessage(new URL('https://example.org'));
 // Prints: { }
 ```
 
+### `port.hasRef()`
+
+<!-- YAML
+added: REPLACEME
+-->
+
+> Stability: 1 - Experimental
+
+* Returns: {boolean}
+
+If true, the `MessagePort` object will keep the Node.js event loop active.
+
 ### `port.ref()`
 
 <!-- YAML

lib/internal/worker/io.js

@@ -91,7 +91,7 @@ const messageTypes = {
 // We have to mess with the MessagePort prototype a bit, so that a) we can make
 // it inherit from NodeEventTarget, even though it is a C++ class, and b) we do
 // not provide methods that are not present in the Browser and not documented
-// on our side (e.g. hasRef).
+// on our side (e.g. stopMessagePort).
 // Save a copy of the original set of methods as a shallow clone.
 const MessagePortPrototype = ObjectCreate(
   ObjectGetPrototypeOf(MessagePort.prototype),
@@ -103,6 +103,9 @@ ObjectSetPrototypeOf(MessagePort.prototype, NodeEventTarget.prototype);
 // changing the prototype of MessagePort.prototype implicitly removed them.
 MessagePort.prototype.ref = MessagePortPrototype.ref;
 MessagePort.prototype.unref = MessagePortPrototype.unref;
+MessagePort.prototype.hasRef = function() {
+  return !!FunctionPrototypeCall(MessagePortPrototype.hasRef, this);
+};
 
 function validateMessagePort(port, name) {
   if (!checkMessagePort(port))

test/parallel/test-messageport-hasref.js

@@ -0,0 +1,57 @@
+'use strict';
+const common = require('../common');
+
+const { MessageChannel } = require('worker_threads');
+const { createHook } = require('async_hooks');
+const { strictEqual } = require('assert');
+
+const handles = [];
+
+createHook({
+  init(asyncId, type, triggerAsyncId, resource) {
+    if (type === 'MESSAGEPORT') {
+      handles.push(resource);
+    }
+  }
+}).enable();
+
+const { port1, port2 } = new MessageChannel();
+strictEqual(handles[0], port1);
+strictEqual(handles[1], port2);
+
+strictEqual(handles[0].hasRef(), false);
+strictEqual(handles[1].hasRef(), false);
+
+port1.unref();
+strictEqual(handles[0].hasRef(), false);
+
+port1.ref();
+strictEqual(handles[0].hasRef(), true);
+
+port1.unref();
+strictEqual(handles[0].hasRef(), false);
+
+port1.on('message', () => {});
+strictEqual(handles[0].hasRef(), true);
+
+port2.unref();
+strictEqual(handles[1].hasRef(), false);
+
+port2.ref();
+strictEqual(handles[1].hasRef(), true);
+
+port2.unref();
+strictEqual(handles[1].hasRef(), false);
+
+port2.on('message', () => {});
+strictEqual(handles[0].hasRef(), true);
+
+port1.on('close', common.mustCall(() => {
+  strictEqual(handles[0].hasRef(), false);
+  strictEqual(handles[1].hasRef(), false);
+}));
+
+port2.close();
+
+strictEqual(handles[0].hasRef(), true);
+strictEqual(handles[1].hasRef(), true);

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

@@ -179,7 +179,7 @@ const { MessageChannel, MessagePort } = require('worker_threads');
   assert.deepStrictEqual(
     Object.getOwnPropertyNames(MessagePort.prototype).sort(),
     [
-      'close', 'constructor', 'onmessage', 'onmessageerror', 'postMessage',
-      'ref', 'start', 'unref',
+      'close', 'constructor', 'hasRef', 'onmessage', 'onmessageerror',
+      'postMessage', 'ref', 'start', 'unref',
     ]);
 }

test/parallel/test-worker-messageport-hasref.js

@@ -0,0 +1,45 @@
+'use strict';
+const common = require('../common');
+
+const { Worker } = require('worker_threads');
+const { createHook } = require('async_hooks');
+const { deepStrictEqual, strictEqual } = require('assert');
+
+const m = new Map();
+createHook({
+  init(asyncId, type, triggerAsyncId, resource) {
+    if (['WORKER', 'MESSAGEPORT'].includes(type)) {
+      m.set(asyncId, { type, resource });
+    }
+  },
+  destroy(asyncId) {
+    m.delete(asyncId);
+  }
+}).enable();
+
+function getActiveWorkerAndMessagePortTypes() {
+  const activeWorkerAndMessagePortTypes = [];
+  for (const asyncId of m.keys()) {
+    const { type, resource } = m.get(asyncId);
+    // Same logic as https://github.com/mafintosh/why-is-node-running/blob/24fb4c878753390a05d00959e6173d0d3c31fddd/index.js#L31-L32.
+    if (typeof resource.hasRef !== 'function' || resource.hasRef() === true) {
+      activeWorkerAndMessagePortTypes.push(type);
+    }
+  }
+  return activeWorkerAndMessagePortTypes;
+}
+
+const w = new Worker('', { eval: true });
+deepStrictEqual(getActiveWorkerAndMessagePortTypes(), ['WORKER']);
+w.unref();
+deepStrictEqual(getActiveWorkerAndMessagePortTypes(), []);
+w.ref();
+deepStrictEqual(getActiveWorkerAndMessagePortTypes(), ['WORKER', 'MESSAGEPORT']);
+
+w.on('exit', common.mustCall((exitCode) => {
+  strictEqual(exitCode, 0);
+  deepStrictEqual(getActiveWorkerAndMessagePortTypes(), ['WORKER']);
+  setTimeout(common.mustCall(() => {
+    deepStrictEqual(getActiveWorkerAndMessagePortTypes(), []);
+  }), 0);
+}));