src: add helper for addons to get the event loop

Add a utility functions for addons to use when they need
a reference to the current event loop.

Currently, `uv_default_loop()` works if the embedder is the
single-threaded default node executable, but even without
the presence of e.g. workers that might not really an API
guarantee for when Node is embedded.

PR-URL: https://github.com/nodejs/node/pull/17109
Reviewed-By: Ben Noordhuis <info@bnoordhuis.nl>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>

src/node.cc

@@ -4332,6 +4332,15 @@ void RunAtExit(Environment* env) {
 }
 
 
+uv_loop_t* GetCurrentEventLoop(v8::Isolate* isolate) {
+  HandleScope handle_scope(isolate);
+  auto context = isolate->GetCurrentContext();
+  if (context.IsEmpty())
+    return nullptr;
+  return Environment::GetCurrent(context)->event_loop();
+}
+
+
 static uv_key_t thread_local_env;
 
 

src/node.h

@@ -248,6 +248,10 @@ NODE_EXTERN void EmitBeforeExit(Environment* env);
 NODE_EXTERN int EmitExit(Environment* env);
 NODE_EXTERN void RunAtExit(Environment* env);
 
+// This may return nullptr if the current v8::Context is not associated
+// with a Node instance.
+NODE_EXTERN struct uv_loop_s* GetCurrentEventLoop(v8::Isolate* isolate);
+
 /* Converts a unixtime to V8 Date */
 #define NODE_UNIXTIME_V8(t) v8::Date::New(v8::Isolate::GetCurrent(),          \
     1000 * static_cast<double>(t))

test/addons/async-hello-world/binding.cc

@@ -77,7 +77,7 @@ void Method(const v8::FunctionCallbackInfo<v8::Value>& args) {
   v8::Local<v8::Function> callback = v8::Local<v8::Function>::Cast(args[1]);
   req->callback.Reset(isolate, callback);
 
-  uv_queue_work(uv_default_loop(),
+  uv_queue_work(node::GetCurrentEventLoop(isolate),
                 &req->req,
                 DoAsync,
                 (uv_after_work_cb)AfterAsync<use_makecallback>);

test/addons/callback-scope/binding.cc

@@ -52,7 +52,10 @@ static void TestResolveAsync(const v8::FunctionCallbackInfo<v8::Value>& args) {
 
     uv_work_t* req = new uv_work_t;
 
-    uv_queue_work(uv_default_loop(), req, [](uv_work_t*) {}, Callback);
+    uv_queue_work(node::GetCurrentEventLoop(isolate),
+                  req,
+                  [](uv_work_t*) {},
+                  Callback);
   }
 
   v8::Local<v8::Promise::Resolver> local =