worker: add hasRef()

This should help projects like
https://github.com/mafintosh/why-is-node-running and
https://github.com/facebook/jest to detect if Worker instances are
keeping the event loop active correctly.

Fixes: nodejs#42091
Refs: mafintosh/why-is-node-running#59
Signed-off-by: Darshan Sen <raisinten@gmail.com>

doc/api/worker_threads.md

@@ -1144,6 +1144,16 @@ Send a message to the worker that is received via
 [`require('worker_threads').parentPort.on('message')`][].
 See [`port.postMessage()`][] for more details.
 
+### `worker.hasRef()`
+
+<!-- YAML
+added: REPLACEME
+-->
+
+> Stability: 1 - Experimental
+
+If true, the `Worker` object will keep the Node.js event loop active.
+
 ### `worker.ref()`
 
 <!-- YAML

lib/internal/worker.js

@@ -367,6 +367,12 @@ class Worker extends EventEmitter {
     });
   }
 
+  hasRef() {
+    if (this[kHandle] === null) return false;
+
+    return this[kHandle].hasRef();
+  }
+
   ref() {
     if (this[kHandle] === null) return;
 

src/node_worker.cc

@@ -665,6 +665,12 @@ void Worker::Ref(const FunctionCallbackInfo<Value>& args) {
   }
 }
 
+void Worker::HasRef(const FunctionCallbackInfo<Value>& args) {
+  Worker* w;
+  ASSIGN_OR_RETURN_UNWRAP(&w, args.This());
+  args.GetReturnValue().Set(w->has_ref_);
+}
+
 void Worker::Unref(const FunctionCallbackInfo<Value>& args) {
   Worker* w;
   ASSIGN_OR_RETURN_UNWRAP(&w, args.This());
@@ -827,6 +833,7 @@ void InitWorker(Local<Object> target,
 
     env->SetProtoMethod(w, "startThread", Worker::StartThread);
     env->SetProtoMethod(w, "stopThread", Worker::StopThread);
+    env->SetProtoMethod(w, "hasRef", Worker::HasRef);
     env->SetProtoMethod(w, "ref", Worker::Ref);
     env->SetProtoMethod(w, "unref", Worker::Unref);
     env->SetProtoMethod(w, "getResourceLimits", Worker::GetResourceLimits);
@@ -890,6 +897,7 @@ void RegisterExternalReferences(ExternalReferenceRegistry* registry) {
   registry->Register(Worker::New);
   registry->Register(Worker::StartThread);
   registry->Register(Worker::StopThread);
+  registry->Register(Worker::HasRef);
   registry->Register(Worker::Ref);
   registry->Register(Worker::Unref);
   registry->Register(Worker::GetResourceLimits);

src/node_worker.h

@@ -61,6 +61,7 @@ class Worker : public AsyncWrap {
   static void SetEnvVars(const v8::FunctionCallbackInfo<v8::Value>& args);
   static void StartThread(const v8::FunctionCallbackInfo<v8::Value>& args);
   static void StopThread(const v8::FunctionCallbackInfo<v8::Value>& args);
+  static void HasRef(const v8::FunctionCallbackInfo<v8::Value>& args);
   static void Ref(const v8::FunctionCallbackInfo<v8::Value>& args);
   static void Unref(const v8::FunctionCallbackInfo<v8::Value>& args);
   static void GetResourceLimits(

test/parallel/test-worker-hasref.js

@@ -0,0 +1,12 @@
+'use strict';
+require('../common');
+
+const { Worker } = require('worker_threads');
+const { strictEqual } = require('assert');
+
+const w = new Worker('', { eval: true });
+strictEqual(w.hasRef(), true);
+w.unref();
+strictEqual(w.hasRef(), false);
+w.ref();
+strictEqual(w.hasRef(), true);