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.

PR-URL: #17109
Reviewed-By: Ben Noordhuis <info@bnoordhuis.nl>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>

doc/api/n-api.md

@@ -3688,6 +3688,23 @@ 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

@@ -18,7 +18,7 @@
 #include "node_api.h"
 #include "node_internals.h"
 
-#define NAPI_VERSION  1
+#define NAPI_VERSION  2
 
 static
 napi_status napi_set_last_error(napi_env env, napi_status error_code,
@@ -28,8 +28,10 @@ static
 napi_status napi_clear_last_error(napi_env env);
 
 struct napi_env__ {
-  explicit napi_env__(v8::Isolate* _isolate): isolate(_isolate),
-      last_error() {}
+  explicit napi_env__(v8::Isolate* _isolate, uv_loop_t* _loop)
+      : isolate(_isolate),
+        last_error(),
+        loop(_loop) {}
   ~napi_env__() {
     last_exception.Reset();
     wrap_template.Reset();
@@ -43,6 +45,7 @@ struct napi_env__ {
   v8::Persistent<v8::ObjectTemplate> accessor_data_template;
   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) \
@@ -771,7 +774,7 @@ 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
@@ -3402,15 +3405,22 @@ 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_

test/addons-napi/test_general/test.js

@@ -34,7 +34,7 @@ assert.ok(test_general.testGetPrototype(baseObject) !==
 
 // test version management functions
 // 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],

test/addons-napi/test_uv_loop/binding.gyp

@@ -0,0 +1,8 @@
+{
+  "targets": [
+    {
+      "target_name": "test_uv_loop",
+      "sources": [ "test_uv_loop.cc" ]
+    }
+  ]
+}

test/addons-napi/test_uv_loop/test.js

@@ -0,0 +1,5 @@
+'use strict';
+const common = require('../../common');
+const { SetImmediate } = require(`./build/${common.buildType}/test_uv_loop`);
+
+SetImmediate(common.mustCall());

test/addons-napi/test_uv_loop/test_uv_loop.cc

@@ -0,0 +1,78 @@
+#include <node_api.h>
+#include <uv.h>
+#include <utility>
+#include <memory>
+#include <assert.h>
+#include "../common.h"
+
+template <typename T>
+void* SetImmediate(napi_env env, T&& cb) {
+  T* ptr = new T(std::move(cb));
+  uv_loop_t* loop = nullptr;
+  uv_check_t* check = new uv_check_t;
+  check->data = ptr;
+  NAPI_ASSERT(env,
+              napi_get_uv_event_loop(env, &loop) == napi_ok,
+              "can get event loop");
+  uv_check_init(loop, check);
+  uv_check_start(check, [](uv_check_t* check) {
+    std::unique_ptr<T> ptr {static_cast<T*>(check->data)};
+    T cb = std::move(*ptr);
+    uv_close(reinterpret_cast<uv_handle_t*>(check), [](uv_handle_t* handle) {
+      delete reinterpret_cast<uv_check_t*>(handle);
+    });
+
+    assert(cb() != nullptr);
+  });
+  return nullptr;
+}
+
+static char dummy;
+
+napi_value SetImmediateBinding(napi_env env, napi_callback_info info) {
+  size_t argc = 1;
+  napi_value argv[1];
+  napi_value _this;
+  void* data;
+  NAPI_CALL(env,
+    napi_get_cb_info(env, info, &argc, argv, &_this, &data));
+  NAPI_ASSERT(env, argc >= 1, "Not enough arguments, expected 1.");
+
+  napi_valuetype t;
+  NAPI_CALL(env, napi_typeof(env, argv[0], &t));
+  NAPI_ASSERT(env, t == napi_function,
+      "Wrong first argument, function expected.");
+
+  napi_ref cbref;
+  NAPI_CALL(env,
+    napi_create_reference(env, argv[0], 1, &cbref));
+
+  SetImmediate(env, [=]() -> char* {
+    napi_value undefined;
+    napi_value callback;
+    napi_handle_scope scope;
+    NAPI_CALL(env, napi_open_handle_scope(env, &scope));
+    NAPI_CALL(env, napi_get_undefined(env, &undefined));
+    NAPI_CALL(env, napi_get_reference_value(env, cbref, &callback));
+    NAPI_CALL(env, napi_delete_reference(env, cbref));
+    NAPI_CALL(env,
+        napi_call_function(env, undefined, callback, 0, nullptr, nullptr));
+    NAPI_CALL(env, napi_close_handle_scope(env, scope));
+    return &dummy;
+  });
+
+  return nullptr;
+}
+
+napi_value Init(napi_env env, napi_value exports) {
+  napi_property_descriptor properties[] = {
+    DECLARE_NAPI_PROPERTY("SetImmediate", SetImmediateBinding)
+  };
+
+  NAPI_CALL(env, napi_define_properties(
+      env, exports, sizeof(properties) / sizeof(*properties), properties));
+
+  return exports;
+}
+
+NAPI_MODULE(NODE_GYP_MODULE_NAME, Init)