n-api: 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.

While the libuv API is not directly part of N-API, it
provides a quite stable C API as well, and is tightly
integrated with Node itself.

As a particular use case, without access to the event loop
it is hard to do something interesting from inside a N-API
finalizer function, since calls into JS and therefore virtually
all other N-API functions are not allowed.

Backport-PR-URL: https://github.com/nodejs/node/pull/19447
PR-URL: https://github.com/nodejs/node/pull/17109
Reviewed-By: Ben Noordhuis <info@bnoordhuis.nl>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>

doc/api/n-api.md

@@ -3684,6 +3684,23 @@ NAPI_EXTERN napi_status napi_run_script(napi_env env,
 - `[in] script`: A JavaScript string containing the script to execute.
 - `[out] result`: The value resulting from having executed the script.
 
+## libuv event loop
+
+N-API provides a function for getting the current event loop associated with
+a specific `napi_env`.
+
+### napi_get_uv_event_loop
+<!-- YAML
+added: REPLACEME
+-->
+```C
+NAPI_EXTERN napi_status napi_get_uv_event_loop(napi_env env,
+                                               uv_loop_t** loop);
+```
+
+- `[in] env`: The environment that the API is invoked under.
+- `[out] loop`: The current libuv loop instance.
+
 [Promises]: #n_api_promises
 [Simple Asynchronous Operations]: #n_api_simple_asynchronous_operations
 [Custom Asynchronous Operations]: #n_api_custom_asynchronous_operations

src/node_api.cc

@@ -22,7 +22,7 @@
 #include "node_api_backport.h"
 #include "util.h"
 
-#define NAPI_VERSION  1
+#define NAPI_VERSION  2
 
 static
 napi_status napi_set_last_error(napi_env env, napi_status error_code,
@@ -31,8 +31,11 @@ napi_status napi_set_last_error(napi_env env, napi_status error_code,
 static napi_status napi_clear_last_error(napi_env env);
 
 struct napi_env__ {
-  explicit napi_env__(v8::Isolate* _isolate): isolate(_isolate),
-      has_instance_available(true), last_error() {}
+  explicit napi_env__(v8::Isolate* _isolate, uv_loop_t *_loop):
+      isolate(_isolate),
+      has_instance_available(true),
+      last_error(),
+      loop(_loop) {}
   ~napi_env__() {
     last_exception.Reset();
     has_instance.Reset();
@@ -49,6 +52,7 @@ struct napi_env__ {
   bool has_instance_available;
   napi_extended_error_info last_error;
   int open_handle_scopes = 0;
+  uv_loop_t* loop = nullptr;
 };
 
 #define ENV_OBJECT_TEMPLATE(env, prefix, destination, field_count) \
@@ -780,7 +784,7 @@ napi_env GetEnv(v8::Local<v8::Context> context) {
   if (value->IsExternal()) {
     result = static_cast<napi_env>(value.As<v8::External>()->Value());
   } else {
-    result = new napi_env__(isolate);
+    result = new napi_env__(isolate, node::GetCurrentEventLoop(isolate));
     auto external = v8::External::New(isolate, result);
 
     // We must also stop hard if the result of assigning the env to the global
@@ -3494,15 +3498,22 @@ napi_status napi_delete_async_work(napi_env env, napi_async_work work) {
   return napi_clear_last_error(env);
 }
 
+napi_status napi_get_uv_event_loop(napi_env env, uv_loop_t** loop) {
+  CHECK_ENV(env);
+  CHECK_ARG(env, loop);
+  *loop = env->loop;
+  return napi_clear_last_error(env);
+}
+
 napi_status napi_queue_async_work(napi_env env, napi_async_work work) {
   CHECK_ENV(env);
   CHECK_ARG(env, work);
 
-  // Consider: Encapsulate the uv_loop_t into an opaque pointer parameter.
-  // Currently the environment event loop is the same as the UV default loop.
-  // Someday (if node ever supports multiple isolates), it may be better to get
-  // the loop from node::Environment::GetCurrent(env->isolate)->event_loop();
-  uv_loop_t* event_loop = uv_default_loop();
+  napi_status status;
+  uv_loop_t* event_loop = nullptr;
+  status = napi_get_uv_event_loop(env, &event_loop);
+  if (status != napi_ok)
+    return napi_set_last_error(env, status);
 
   uvimpl::Work* w = reinterpret_cast<uvimpl::Work*>(work);
 

src/node_api.h

@@ -14,6 +14,8 @@
 #include <stdbool.h>
 #include "node_api_types.h"
 
+struct uv_loop_s;  // Forward declaration.
+
 #ifdef _WIN32
   #ifdef BUILDING_NODE_EXTENSION
     #ifdef EXTERNAL_NAPI
@@ -581,6 +583,10 @@ NAPI_EXTERN napi_status napi_run_script(napi_env env,
                                         napi_value script,
                                         napi_value* result);
 
+// Return the current libuv event loop for a given environment
+NAPI_EXTERN napi_status napi_get_uv_event_loop(napi_env env,
+                                               struct uv_loop_s** loop);
+
 EXTERN_C_END
 
 #endif  // SRC_NODE_API_H_

src/node_api_backport.cc

@@ -71,6 +71,14 @@ CallbackScope::~CallbackScope() {
   env->tick_callback_function()->Call(env->process_object(), 0, nullptr);
 }
 
+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();
+}
+
 AsyncResource::AsyncResource(v8::Isolate* _isolate,
                              v8::Local<v8::Object> _object,
                              char* name) : isolate(_isolate) {

src/node_api_backport.h

@@ -31,6 +31,8 @@ class CallbackScope {
   Environment::AsyncCallbackScope callback_scope;
 };
 
+uv_loop_t *GetCurrentEventLoop(v8::Isolate *isolate);
+
 NODE_EXTERN async_context EmitAsyncInit(v8::Isolate* isolate,
                                         v8::Local<v8::Object> resource,
                                         v8::Local<v8::String> name,

test/addons-napi/test_general/test.js

@@ -34,7 +34,7 @@ assert.ok(test_general.testGetPrototype(baseObject) !==
 
 // test version management funcitons
 // expected version is currently 1
-assert.strictEqual(test_general.testGetVersion(), 1);
+assert.strictEqual(test_general.testGetVersion(), 2);
 
 const [ major, minor, patch, release ] = test_general.testGetNodeVersion();
 assert.strictEqual(process.version.split('-')[0],