Implement CallbackScope class

This is a wrapper class to support the following N-APIs.
  - napi_open_callback_scope()
  - napi_close_callback_scope()

Refs: https://nodejs.org/api/n-api.html#n_api_napi_open_callback_scope

doc/async_operations.md

@@ -27,3 +27,5 @@ other asynchronous mechanism, the following API is necessary to ensure an
 asynchronous operation is properly tracked by the runtime:
 
 - **[AsyncContext](async_context.md)**
+
+- **[CallbackScope](callback_scope.md)**

doc/callback_scope.md

@@ -0,0 +1,54 @@
+# CallbackScope
+
+There are cases (for example, resolving promises) where it is necessary to have
+the equivalent of the scope associated with a callback in place when making
+certain N-API calls.
+
+## Methods
+
+### Constructor
+
+Creates a new callback scope on the stack.
+
+```cpp
+Napi::CallbackScope::CallbackScope(napi_env env, napi_callback_scope scope);
+```
+
+- `[in] env`: The environment in which to create the `Napi::CallbackScope`.
+- `[in] scope`: pre-existing `napi_callback_scope` or `Napi::CallbackScope`.
+ 
+### Constructor
+
+Creates a new callback scope on the stack.
+
+```cpp
+Napi::CallbackScope::CallbackScope(napi_env env, napi_async_context context);
+```
+
+- `[in] env`: The environment in which to create the `Napi::CallbackScope`.
+- `[in] async_context`: pre-existing `napi_async_context` or `Napi::AsyncContext`.
+
+### Destructor
+
+The `Napi::CallbackScope` to be destroyed.
+
+```cpp
+virtual Napi::CallbackScope::~CallbackScope();
+```
+
+### Env
+
+```cpp
+Napi::Env Napi::CallbackScope::Env() const;
+```
+
+Returns the `Napi::Env` associated with the `Napi::CallbackScope`.
+
+## Operator
+
+```cpp
+Napi::CallbackScope::operator napi_callback_scope() const;
+```
+
+Returns the N-API `napi_callback_scope` wrapped by the `Napi::CallbackScope`
+object. This can be used to mix usage of the C N-API and node-addon-api.

napi-inl.h

@@ -3404,6 +3404,34 @@ inline Value EscapableHandleScope::Escape(napi_value escapee) {
   return Value(_env, result);
 }
 
+////////////////////////////////////////////////////////////////////////////////
+// CallbackScope class
+////////////////////////////////////////////////////////////////////////////////
+
+inline CallbackScope::CallbackScope(
+  napi_env env, napi_callback_scope scope) : _env(env), _scope(scope) {
+}
+
+inline CallbackScope::CallbackScope(napi_env env, napi_async_context context)
+    : _env(env),
+      _async_context(context) {
+  napi_status status = napi_open_callback_scope(
+      _env, Object::New(env), context, &_scope);
+  NAPI_THROW_IF_FAILED_VOID(_env, status);
+}
+
+inline CallbackScope::~CallbackScope() {
+  napi_close_callback_scope(_env, _scope);
+}
+
+inline CallbackScope::operator napi_callback_scope() const {
+  return _scope;
+}
+
+inline Napi::Env CallbackScope::Env() const {
+  return Napi::Env(_env);
+}
+
 ////////////////////////////////////////////////////////////////////////////////
 // AsyncContext class
 ////////////////////////////////////////////////////////////////////////////////

napi.h

@@ -1671,6 +1671,22 @@ namespace Napi {
     napi_escapable_handle_scope _scope;
   };
 
+  class CallbackScope {
+  public:
+    CallbackScope(napi_env env, napi_callback_scope scope);
+    CallbackScope(napi_env env, napi_async_context context);
+    virtual ~CallbackScope();
+
+    operator napi_callback_scope() const;
+
+    Napi::Env Env() const;
+
+  private:
+    napi_env _env;
+    napi_async_context _async_context;
+    napi_callback_scope _scope;
+  };
+
   class AsyncContext {
   public:
     explicit AsyncContext(napi_env env, const char* resource_name);

test/binding.cc

@@ -15,6 +15,7 @@ Object InitBasicTypesValue(Env env);
 Object InitBigInt(Env env);
 #endif
 Object InitBuffer(Env env);
+Object InitCallbackScope(Env env);
 Object InitDataView(Env env);
 Object InitDataViewReadWrite(Env env);
 Object InitError(Env env);
@@ -47,6 +48,7 @@ Object Init(Env env, Object exports) {
   exports.Set("bigint", InitBigInt(env));
 #endif
   exports.Set("buffer", InitBuffer(env));
+  exports.Set("callbackscope", InitCallbackScope(env));
   exports.Set("dataview", InitDataView(env));
   exports.Set("dataview_read_write", InitDataView(env));
   exports.Set("dataview_read_write", InitDataViewReadWrite(env));

test/binding.gyp

@@ -14,6 +14,7 @@
         'bigint.cc',
         'binding.cc',
         'buffer.cc',
+        'callbackscope.cc',
         'dataview/dataview.cc',
         'dataview/dataview_read_write.cc',
         'error.cc',

test/callbackscope.cc

@@ -0,0 +1,20 @@
+#include "napi.h"
+
+using namespace Napi;
+
+namespace {
+
+static void RunInCallbackScope(const CallbackInfo& info) {
+  Function callback = info[0].As<Function>();
+  AsyncContext context(info.Env(), "callback_scope_test");
+  CallbackScope scope(info.Env(), context);
+  callback.Call({});
+}
+
+} // end anonymous namespace
+
+Object InitCallbackScope(Env env) {
+  Object exports = Object::New(env);
+  exports["runInCallbackScope"] = Function::New(env, RunInCallbackScope);
+  return exports;
+}

test/callbackscope.js

@@ -0,0 +1,48 @@
+'use strict';
+const buildType = process.config.target_defaults.default_configuration;
+const assert = require('assert');
+const common = require('./common');
+
+// we only check async hooks on 8.x an higher were
+// they are closer to working properly
+const nodeVersion = process.versions.node.split('.')[0]
+let async_hooks = undefined;
+function checkAsyncHooks() {
+  if (nodeVersion >= 8) {
+    if (async_hooks == undefined) {
+      async_hooks = require('async_hooks');
+    }
+    return true;
+  }
+  return false;
+}
+
+test(require(`./build/${buildType}/binding.node`));
+test(require(`./build/${buildType}/binding_noexcept.node`));
+
+function test(binding) {
+  if (!checkAsyncHooks())
+    return;
+
+  let id;
+  let insideHook = false;
+  async_hooks.createHook({
+    init(asyncId, type, triggerAsyncId, resource) {
+      if (id === undefined && type === 'callback_scope_test') {
+        id = asyncId;
+      }
+    },
+    before(asyncId) {
+      if (asyncId === id)
+        insideHook = true;
+    },
+    after(asyncId) {
+      if (asyncId === id)
+        insideHook = false;
+    }
+  }).enable();
+
+  binding.callbackscope.runInCallbackScope(function() {
+    assert(insideHook);
+  });
+}

test/index.js

@@ -16,6 +16,7 @@ let testModules = [
   'basic_types/value',
   'bigint',
   'buffer',
+  'callbackscope',
   'dataview/dataview',
   'dataview/dataview_read_write',
   'error',