promise.h

// Copyright (c) 2018 GitHub, Inc.
// Use of this source code is governed by the MIT license that can be
// found in the LICENSE file.

#ifndef SHELL_COMMON_GIN_HELPER_PROMISE_H_
#define SHELL_COMMON_GIN_HELPER_PROMISE_H_

#include <string>
#include <tuple>
#include <type_traits>
#include <utility>

#include "base/strings/string_piece.h"
#include "base/task/post_task.h"
#include "content/public/browser/browser_task_traits.h"
#include "content/public/browser/browser_thread.h"
#include "shell/common/gin_converters/std_converter.h"
#include "shell/common/gin_helper/locker.h"
#include "shell/common/gin_helper/microtasks_scope.h"

namespace gin_helper {

// A wrapper around the v8::Promise.
//
// This is the non-template base class to share code between templates
// instances.
//
// This is a move-only type that should always be `std::move`d when passed to
// callbacks, and it should be destroyed on the same thread of creation.
class PromiseBase {
 public:
  explicit PromiseBase(v8::Isolate* isolate);
  PromiseBase(v8::Isolate* isolate, v8::Local<v8::Promise::Resolver> handle);
  ~PromiseBase();

  // Support moving.
  PromiseBase(PromiseBase&&);
  PromiseBase& operator=(PromiseBase&&);

  // Helper for rejecting promise with error message.
  //
  // Note: The parameter type is PromiseBase&& so it can take the instances of
  // Promise<T> type.
  static void RejectPromise(PromiseBase&& promise, base::StringPiece errmsg) {
    if (gin_helper::Locker::IsBrowserProcess() &&
        !content::BrowserThread::CurrentlyOn(content::BrowserThread::UI)) {
      base::PostTask(
          FROM_HERE, {content::BrowserThread::UI},
          base::BindOnce(
              // Note that this callback can not take StringPiece,
              // as StringPiece only references string internally and
              // will blow when a temporary string is passed.
              [](PromiseBase&& promise, std::string str) {
                promise.RejectWithErrorMessage(str);
              },
              std::move(promise), std::string(errmsg.data(), errmsg.size())));
    } else {
      promise.RejectWithErrorMessage(errmsg);
    }
  }

  v8::Maybe<bool> Reject();
  v8::Maybe<bool> Reject(v8::Local<v8::Value> except);
  v8::Maybe<bool> RejectWithErrorMessage(base::StringPiece message);

  v8::Local<v8::Context> GetContext() const;
  v8::Local<v8::Promise> GetHandle() const;

  v8::Isolate* isolate() const { return isolate_; }

 protected:
  v8::Local<v8::Promise::Resolver> GetInner() const;

 private:
  v8::Isolate* isolate_;
  v8::Global<v8::Context> context_;
  v8::Global<v8::Promise::Resolver> resolver_;

  DISALLOW_COPY_AND_ASSIGN(PromiseBase);
};

// Template implementation that returns values.
template <typename RT>
class Promise : public PromiseBase {
 public:
  using PromiseBase::PromiseBase;

  // Helper for resolving the promise with |result|.
  static void ResolvePromise(Promise<RT> promise, RT result) {
    if (gin_helper::Locker::IsBrowserProcess() &&
        !content::BrowserThread::CurrentlyOn(content::BrowserThread::UI)) {
      base::PostTask(FROM_HERE, {content::BrowserThread::UI},
                     base::BindOnce([](Promise<RT> promise,
                                       RT result) { promise.Resolve(result); },
                                    std::move(promise), std::move(result)));
    } else {
      promise.Resolve(result);
    }
  }

  // Returns an already-resolved promise.
  static v8::Local<v8::Promise> ResolvedPromise(v8::Isolate* isolate,
                                                RT result) {
    Promise<RT> resolved(isolate);
    resolved.Resolve(result);
    return resolved.GetHandle();
  }

  // Promise resolution is a microtask
  // We use the MicrotasksRunner to trigger the running of pending microtasks
  v8::Maybe<bool> Resolve(const RT& value) {
    gin_helper::Locker locker(isolate());
    v8::HandleScope handle_scope(isolate());
    gin_helper::MicrotasksScope microtasks_scope(isolate());
    v8::Context::Scope context_scope(GetContext());

    return GetInner()->Resolve(GetContext(),
                               gin::ConvertToV8(isolate(), value));
  }

  template <typename... ResolveType>
  v8::MaybeLocal<v8::Promise> Then(
      base::OnceCallback<void(ResolveType...)> cb) {
    static_assert(sizeof...(ResolveType) <= 1,
                  "A promise's 'Then' callback should only receive at most one "
                  "parameter");
    static_assert(
        std::is_same<RT, std::tuple_element_t<0, std::tuple<ResolveType...>>>(),
        "A promises's 'Then' callback must handle the same type as the "
        "promises resolve type");
    gin_helper::Locker locker(isolate());
    v8::HandleScope handle_scope(isolate());
    v8::Context::Scope context_scope(GetContext());

    v8::Local<v8::Value> value = gin::ConvertToV8(isolate(), std::move(cb));
    v8::Local<v8::Function> handler = v8::Local<v8::Function>::Cast(value);

    return GetHandle()->Then(GetContext(), handler);
  }
};

// Template implementation that returns nothing.
template <>
class Promise<void> : public PromiseBase {
 public:
  using PromiseBase::PromiseBase;

  // Helper for resolving the empty promise.
  static void ResolvePromise(Promise<void> promise);

  // Returns an already-resolved promise.
  static v8::Local<v8::Promise> ResolvedPromise(v8::Isolate* isolate);

  v8::Maybe<bool> Resolve();
};

}  // namespace gin_helper

namespace gin {

template <typename T>
struct Converter<gin_helper::Promise<T>> {
  static v8::Local<v8::Value> ToV8(v8::Isolate* isolate,
                                   const gin_helper::Promise<T>& val) {
    return val.GetHandle();
  }
  // TODO(MarshallOfSound): Implement FromV8 to allow promise chaining
  //                        in native land
  // static bool FromV8(v8::Isolate* isolate,
  //                    v8::Local<v8::Value> val,
  //                    Promise* out);
};

}  // namespace gin

#endif  // SHELL_COMMON_GIN_HELPER_PROMISE_H_