[api] add API for Promise status and result.

src/api.cc

Index: src/api.cc
diff --git a/src/api.cc b/src/api.cc
index 04e7c894d4584e3d4cce1ad50b73b22885acfff4..20108439e985d26e6f2c9d1484f49916198a4fd6 100644
--- a/src/api.cc
+++ b/src/api.cc
@@ -7289,6 +7289,32 @@ bool Promise::HasHandler() {
   return false;
 }
 
+MaybeLocal<Value> Promise::Result() {
+  i::Handle<i::JSReceiver> promise = Utils::OpenHandle(this);
+  i::Isolate* isolate = promise->GetIsolate();
+  LOG_API(isolate, Promise, Result);
+  ENTER_V8(isolate);

[jochen (gone - plz use gerrit), 2016/12/20 07:49:52]

this method (and Status) don't execute script, so we don't need to enter here

[Yang, 2016/12/20 11:38:27]

Done.

+  Utils::ApiCheck(promise->IsJSPromise(), "v8_Promise_Result", "Not a Promise");

[jochen (gone - plz use gerrit), 2016/12/20 07:49:52]

that check should never trigger, so I'd suggest to remove it

[Yang, 2016/12/20 11:38:27]

Done.

+  i::Handle<i::JSPromise> js_promise = i::Handle<i::JSPromise>::cast(promise);
+  if (js_promise->status() != i::kPromisePending) {
+    i::Handle<i::Object> result(js_promise->result(), isolate);
+    return Utils::ToLocal(result);
+  }
+  return MaybeLocal<Value>();

[jochen (gone - plz use gerrit), 2016/12/20 07:49:52]

APIs should return empty handles iff an exception was thrown.

[Yang, 2016/12/20 11:38:27]

Done. Now requiring status not to be pending guarded by ApiCheck.

+}
+
+Promise::PromiseStatus Promise::Status() {
+  i::Handle<i::JSReceiver> promise = Utils::OpenHandle(this);
+  i::Isolate* isolate = promise->GetIsolate();
+  LOG_API(isolate, Promise, Status);
+  ENTER_V8(isolate);
+  Utils::ApiCheck(promise->IsJSPromise(), "v8_Promise_Status", "Not a Promise");
+  i::Handle<i::JSPromise> js_promise = i::Handle<i::JSPromise>::cast(promise);
+  STATIC_ASSERT(static_cast<int>(i::kPromisePending) == kPending);

[jochen (gone - plz use gerrit), 2016/12/20 07:49:52]

an alternative approach was to use the external enum internally

[Yang, 2016/12/20 11:38:27]

Done.

+  STATIC_ASSERT(static_cast<int>(i::kPromiseFulfilled) == kFulfilled);
+  STATIC_ASSERT(static_cast<int>(i::kPromiseRejected) == kRejected);
+  return static_cast<PromiseStatus>(js_promise->status());
+}
 
 Local<Object> Proxy::GetTarget() {
   i::Handle<i::JSProxy> self = Utils::OpenHandle(this);