async_hooks: use resource objects for Promises

Use `PromiseWrap` resource objects whose lifetimes are tied to the `Promise` instances themselves to track promises, and have a `.promise` getter that points to the `Promise` and a `.parent` property that points to the parent Promise’s resource object, if there is any. The properties are implemented as getters for internal fields rather than normal properties in the hope that it helps keep performance for the common case that async_hooks users will often not inspect them.
nodejs · addaleax · Jun 4, 2017 · Jun 4, 2017 · Jun 4, 2017 · Jun 4, 2017
commit a187daad7fc19f1526e865416da8b30ff206fb8d
diff --git a/src/async-wrap.cc b/src/async-wrap.cc
@@ -36,6 +36,7 @@ using v8::Context;
 using v8::Float64Array;
 using v8::Function;
 using v8::FunctionCallbackInfo;
+using v8::FunctionTemplate;
 using v8::HandleScope;
 using v8::HeapProfiler;
 using v8::Integer;
@@ -44,8 +45,10 @@ using v8::Local;
 using v8::MaybeLocal;
 using v8::Number;
 using v8::Object;
+using v8::ObjectTemplate;
 using v8::Promise;
 using v8::PromiseHookType;
+using v8::PropertyCallbackInfo;
 using v8::RetainedObjectInfo;
 using v8::String;
 using v8::Symbol;
@@ -282,37 +285,91 @@ bool AsyncWrap::EmitAfter(Environment* env, double async_id) {
 class PromiseWrap : public AsyncWrap {
  public:
   PromiseWrap(Environment* env, Local<Object> object, bool silent)
-    : AsyncWrap(env, object, PROVIDER_PROMISE, silent) {}
+    : AsyncWrap(env, object, PROVIDER_PROMISE, silent) {
+    MakeWeak(this);
+  }
   size_t self_size() const override { return sizeof(*this); }
+
+  static constexpr int kPromiseField = 1;
+  static constexpr int kParentField = 2;
+  static constexpr int kInternalFieldCount = 3;
+
+  static PromiseWrap* New(Environment* env,
+                          Local<Promise> promise,
+                          PromiseWrap* parent_wrap,
+                          bool silent);
+  static void GetPromise(Local<String> property,
+                         const PropertyCallbackInfo<Value>& info);
+  static void GetParent(Local<String> property,
+                        const PropertyCallbackInfo<Value>& info);
 };
 
+PromiseWrap* PromiseWrap::New(Environment* env,
+                              Local<Promise> promise,
+                              PromiseWrap* parent_wrap,
+                              bool silent) {
+  Local<Object> object = env->promise_wrap_template()
+                            ->NewInstance(env->context()).ToLocalChecked();
+  object->SetInternalField(PromiseWrap::kPromiseField, promise);
+  object->SetAlignedPointerInInternalField(
+      PromiseWrap::kParentField,
+      static_cast<void*>(parent_wrap));
+  CHECK_EQ(promise->GetAlignedPointerFromInternalField(0), nullptr);
+  promise->SetInternalField(0, object);
+  return new PromiseWrap(env, object, silent);
+}
+
+void PromiseWrap::GetPromise(Local<String> property,
+                             const PropertyCallbackInfo<Value>& info) {
+  info.GetReturnValue().Set(info.Holder()->GetInternalField(kPromiseField));
+}
+
+void PromiseWrap::GetParent(Local<String> property,
+                            const PropertyCallbackInfo<Value>& info) {
+  Isolate* isolate = info.GetIsolate();
+  PromiseWrap* parent = static_cast<PromiseWrap*>(
+      info.Holder()->GetAlignedPointerFromInternalField(kParentField));
+  if (parent == nullptr)
+    info.GetReturnValue().Set(Undefined(isolate));
+  else
+    info.GetReturnValue().Set(parent->object());
+}
 
 static void PromiseHook(PromiseHookType type, Local<Promise> promise,
                         Local<Value> parent, void* arg) {
   Local<Context> context = promise->CreationContext();
   Environment* env = Environment::GetCurrent(context);
-  PromiseWrap* wrap = Unwrap<PromiseWrap>(promise);
+  Local<Value> resource_object_value = promise->GetInternalField(0);
+  Local<Object> resource_object;
+  PromiseWrap* wrap = nullptr;
+  if (resource_object_value->IsObject()) {
+    resource_object = resource_object_value.As<Object>();
+    wrap = Unwrap<PromiseWrap>(resource_object);
+  }
   if (type == PromiseHookType::kInit || wrap == nullptr) {
     bool silent = type != PromiseHookType::kInit;
+    PromiseWrap* parent_wrap = nullptr;
+
     // set parent promise's async Id as this promise's triggerId
     if (parent->IsPromise()) {
       // parent promise exists, current promise
       // is a chained promise, so we set parent promise's id as
       // current promise's triggerId
       Local<Promise> parent_promise = parent.As<Promise>();
-      auto parent_wrap = Unwrap<PromiseWrap>(parent_promise);
+      Local<Value> parent_resource = parent_promise->GetInternalField(0);
+      if (parent_resource->IsObject()) {
+        parent_wrap = Unwrap<PromiseWrap>(parent_resource.As<Object>());
+      }
 
       if (parent_wrap == nullptr) {
-        // create a new PromiseWrap for parent promise with silent parameter
-        parent_wrap = new PromiseWrap(env, parent_promise, true);
-        parent_wrap->MakeWeak(parent_wrap);
+        parent_wrap = PromiseWrap::New(env, parent_promise, nullptr, true);
       }
       // get id from parentWrap
       double trigger_id = parent_wrap->get_id();
       env->set_init_trigger_id(trigger_id);
     }
-    wrap = new PromiseWrap(env, promise, silent);
-    wrap->MakeWeak(wrap);
+
+    wrap = PromiseWrap::New(env, promise, parent_wrap, silent);
   } else if (type == PromiseHookType::kResolve) {
     // TODO(matthewloring): need to expose this through the async hooks api.
   }
@@ -351,6 +408,23 @@ static void SetupHooks(const FunctionCallbackInfo<Value>& args) {
   SET_HOOK_FN(destroy);
   env->AddPromiseHook(PromiseHook, nullptr);
 #undef SET_HOOK_FN
+
+  {
+    Local<FunctionTemplate> ctor =
+        FunctionTemplate::New(env->isolate());
+    ctor->SetClassName(FIXED_ONE_BYTE_STRING(env->isolate(), "PromiseWrap"));
+    Local<ObjectTemplate> promise_wrap_template = ctor->InstanceTemplate();
+    promise_wrap_template->SetInternalFieldCount(
+        PromiseWrap::kInternalFieldCount);
+    promise_wrap_template->SetAccessor(
+        FIXED_ONE_BYTE_STRING(env->isolate(), "promise"),
+        PromiseWrap::GetPromise);
+    promise_wrap_template->SetAccessor(
+        FIXED_ONE_BYTE_STRING(env->isolate(), "parent"),
+        PromiseWrap::GetParent);
+    env->SetProtoMethod(ctor, "getAsyncId", AsyncWrap::GetAsyncId);
+    env->set_promise_wrap_template(promise_wrap_template);
+  }
 }
 
 

diff --git a/src/env.h b/src/env.h
@@ -268,6 +268,7 @@ namespace node {
   V(pipe_constructor_template, v8::FunctionTemplate)                          \
   V(process_object, v8::Object)                                               \
   V(promise_reject_function, v8::Function)                                    \
+  V(promise_wrap_template, v8::ObjectTemplate)                                \
   V(push_values_to_array_function, v8::Function)                              \
   V(randombytes_constructor_template, v8::ObjectTemplate)                     \
   V(script_context_constructor_template, v8::FunctionTemplate)                \

diff --git a/test/parallel/test-async-hooks-promise.js b/test/parallel/test-async-hooks-promise.js
@@ -0,0 +1,25 @@
+'use strict';
+const common = require('../common');
+const assert = require('assert');
+const async_hooks = require('async_hooks');
+
+const initCalls = [];
+
+async_hooks.createHook({
+  init: common.mustCall((id, type, triggerId, resource) => {
+    assert.strictEqual(type, 'PROMISE');
+    initCalls.push({id, triggerId, resource});
+  }, 2)
+}).enable();
+
+const a = Promise.resolve(42);
+const b = a.then(common.mustCall());
+
+assert.strictEqual(initCalls[0].triggerId, 1);
+assert.strictEqual(initCalls[0].resource.parent, undefined);
+assert.strictEqual(initCalls[0].resource.promise, a);
+assert.strictEqual(initCalls[0].resource.getAsyncId(), initCalls[0].id);
+assert.strictEqual(initCalls[1].triggerId, initCalls[0].id);
+assert.strictEqual(initCalls[1].resource.parent, initCalls[0].resource);
+assert.strictEqual(initCalls[1].resource.promise, b);
+assert.strictEqual(initCalls[1].resource.getAsyncId(), initCalls[1].id);