FutureSplitter.h

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/*
 * Copyright 2017-present Facebook, Inc.
 *
 * 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.
 */

#pragma once

#include <folly/futures/Future.h>
#include <folly/futures/SharedPromise.h>
#include <folly/lang/Exception.h>

namespace folly {

class FOLLY_EXPORT FutureSplitterInvalid : public FutureException {
 public:
  FutureSplitterInvalid()
      : FutureException("No Future in this FutureSplitter") {}
};

/*
 * FutureSplitter provides a `getFuture()' method which can be called multiple
 * times, returning a new Future each time. These futures are completed when the
 * original Future passed to the FutureSplitter constructor is completed, and
 * are completed on the same executor (if any) and at the same priority as the
 * original Future. Calls to `getFuture()' after that time return a completed
 * Future.
 */
template <class T>
class FutureSplitter {
 public:
  FutureSplitter() = default;

  explicit FutureSplitter(Future<T>&& future)
      : promise_(std::make_shared<SharedPromise<T>>()),
        e_(getExecutorFrom(future)),
        priority_(future.getPriority()) {
    std::move(future).thenTry([promise = promise_](Try<T>&& theTry) {
      promise->setTry(std::move(theTry));
    });
  }

  Future<T> getFuture() {
    if (promise_ == nullptr) {
      throw_exception<FutureSplitterInvalid>();
    }
    return promise_->getSemiFuture().via(e_, priority_);
  }

  SemiFuture<T> getSemiFuture() {
    if (promise_ == nullptr) {
      throw_exception<FutureSplitterInvalid>();
    }
    return promise_->getSemiFuture();
  }

 private:
  std::shared_ptr<SharedPromise<T>> promise_;
  Executor* e_ = nullptr;
  int8_t priority_{-1};

  static Executor* getExecutorFrom(Future<T>& f) {
    // If the passed future had a null executor, use an inline executor
    // to ensure that .via is safe
    auto* e = f.getExecutor();
    return e ? e : &InlineExecutor::instance();
  }
};

template <class T>
FutureSplitter<T> splitFuture(Future<T>&& future) {
  return FutureSplitter<T>{std::move(future)};
}
} // namespace folly