src/core/lib/promise/try_join.h

// Copyright 2021 gRPC authors.
//
// Licensed under the Apache License, Version 2.0 (the "License");
// you may not use this file except in compliance with the License.
// You may obtain a copy of the License at
//
//     http://www.apache.org/licenses/LICENSE-2.0
//
// Unless required by applicable law or agreed to in writing, software
// distributed under the License is distributed on an "AS IS" BASIS,
// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
// See the License for the specific language governing permissions and
// limitations under the License.

#ifndef GRPC_CORE_LIB_PROMISE_TRY_JOIN_H
#define GRPC_CORE_LIB_PROMISE_TRY_JOIN_H

#include <grpc/support/port_platform.h>

#include <type_traits>

#include "absl/meta/type_traits.h"
#include "absl/status/status.h"
#include "absl/status/statusor.h"

#include "src/core/lib/promise/detail/basic_join.h"
#include "src/core/lib/promise/detail/status.h"
#include "src/core/lib/promise/poll.h"

namespace grpc_core {

namespace promise_detail {

// Extract the T from a StatusOr<T>
template <typename T>
T IntoResult(absl::StatusOr<T>* status) {
  return std::move(**status);
}

// TryJoin returns a StatusOr<tuple<A,B,C>> for f()->Poll<StatusOr<A>>,
// g()->Poll<StatusOr<B>>, h()->Poll<StatusOr<C>>. If one of those should be a
// Status instead, we need a placeholder type to return, and this is it.
struct Empty {};
inline Empty IntoResult(absl::Status*) { return Empty{}; }

// Traits object to pass to BasicJoin
struct TryJoinTraits {
  template <typename T>
  using ResultType =
      decltype(IntoResult(std::declval<absl::remove_reference_t<T>*>()));
  template <typename T, typename F>
  static auto OnResult(T result, F kontinue)
      -> decltype(kontinue(IntoResult(&result))) {
    using Result =
        typename PollTraits<decltype(kontinue(IntoResult(&result)))>::Type;
    if (!result.ok()) {
      return Result(IntoStatus(&result));
    }
    return kontinue(IntoResult(&result));
  }
  template <typename T>
  static absl::StatusOr<T> Wrap(T x) {
    return absl::StatusOr<T>(std::move(x));
  }
};

// Implementation of TryJoin combinator.
template <typename... Promises>
using TryJoin = BasicJoin<TryJoinTraits, Promises...>;

}  // namespace promise_detail

// Run all promises.
// If any fail, cancel the rest and return the failure.
// If all succeed, return Ok(tuple-of-results).
template <typename... Promises>
promise_detail::TryJoin<Promises...> TryJoin(Promises... promises) {
  return promise_detail::TryJoin<Promises...>(std::move(promises)...);
}

}  // namespace grpc_core

#endif  // GRPC_CORE_LIB_PROMISE_TRY_JOIN_H