worker: add hasRef() to MessagePort

Since we were removing the hasRef() method before exposing the
MessagePort object, the only way of knowing if the handle was keeping
the event loop active was to parse the string returned by
util.inspect(port), which is inconvenient and inconsistent with most of
the other async resources. So this change stops removing hasRef() from
the MessagePort prototype. The reason why this is also being documented
is that while reporting active resources, async_hooks returns the same
MessagePort object as the one that is accessible by users.

Refs: #42091 (comment)
Signed-off-by: Darshan Sen <raisinten@gmail.com>

PR-URL: #42849
Reviewed-By: Anna Henningsen <anna@addaleax.net>

doc/api/worker_threads.md

@@ -757,6 +757,18 @@ 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);
+}));