worker: add hasRef() to the handle object

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>

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 +1,33 @@
+'use strict';
+const common = require('../common');
+
+const { Worker } = require('worker_threads');
+const { createHook } = require('async_hooks');
+const { strictEqual } = require('assert');
+
+let handle;
+
+createHook({
+  init(asyncId, type, triggerAsyncId, resource) {
+    if (type === 'WORKER') {
+      handle = resource;
+      this.disable();
+    }
+  }
+}).enable();
+
+const w = new Worker('', { eval: true });
+
+strictEqual(handle.hasRef(), true);
+w.unref();
+strictEqual(handle.hasRef(), false);
+w.ref();
+strictEqual(handle.hasRef(), true);
+
+w.on('exit', common.mustCall((exitCode) => {
+  strictEqual(exitCode, 0);
+  strictEqual(handle.hasRef(), true);
+  setTimeout(common.mustCall(() => {
+    strictEqual(handle.hasRef(), undefined);
+  }), 0);
+}));