faux-folly/folly/futures/helpers.h

/*
 * Copyright 2015 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/Portability.h>

namespace folly {

/// This namespace is for utility functions that would usually be static
/// members of Future, except they don't make sense there because they don't
/// depend on the template type (rather, on the type of their arguments in
/// some cases). This is the least-bad naming scheme we could think of. Some
/// of the functions herein have really-likely-to-collide names, like "map"
/// and "sleep".
namespace futures {
  /// Returns a Future that will complete after the specified duration. The
  /// Duration typedef of a `std::chrono` duration type indicates the
  /// resolution you can expect to be meaningful (milliseconds at the time of
  /// writing). Normally you wouldn't need to specify a Timekeeper, we will
  /// use the global futures timekeeper (we run a thread whose job it is to
  /// keep time for futures timeouts) but we provide the option for power
  /// users.
  ///
  /// The Timekeeper thread will be lazily created the first time it is
  /// needed. If your program never uses any timeouts or other time-based
  /// Futures you will pay no Timekeeper thread overhead.
  Future<Unit> sleep(Duration, Timekeeper* = nullptr);

  /**
   * Set func as the callback for each input Future and return a vector of
   * Futures containing the results in the input order.
   */
  template <class It, class F,
            class ItT = typename std::iterator_traits<It>::value_type,
            class Result
      = typename decltype(std::declval<ItT>().then(std::declval<F>()))::value_type>
  std::vector<Future<Result>> map(It first, It last, F func);

  // Sugar for the most common case
  template <class Collection, class F>
  auto map(Collection&& c, F&& func)
      -> decltype(map(c.begin(), c.end(), func)) {
    return map(c.begin(), c.end(), std::forward<F>(func));
  }

} // namespace futures

/**
  Make a completed Future by moving in a value. e.g.

    string foo = "foo";
    auto f = makeFuture(std::move(foo));

  or

    auto f = makeFuture<string>("foo");
*/
template <class T>
Future<typename std::decay<T>::type> makeFuture(T&& t);

/** Make a completed void Future. */
Future<Unit> makeFuture();

/** Make a completed Future by executing a function. If the function throws
  we capture the exception, otherwise we capture the result. */
template <class F>
auto makeFutureWith(F&& func)
    -> Future<typename Unit::Lift<decltype(func())>::type>;

/// Make a failed Future from an exception_ptr.
/// Because the Future's type cannot be inferred you have to specify it, e.g.
///
///   auto f = makeFuture<string>(std::current_exception());
template <class T>
FOLLY_DEPRECATED("use makeFuture(exception_wrapper)")
Future<T> makeFuture(std::exception_ptr const& e);

/// Make a failed Future from an exception_wrapper.
template <class T>
Future<T> makeFuture(exception_wrapper ew);

/** Make a Future from an exception type E that can be passed to
  std::make_exception_ptr(). */
template <class T, class E>
typename std::enable_if<std::is_base_of<std::exception, E>::value,
                        Future<T>>::type
makeFuture(E const& e);

/** Make a Future out of a Try */
template <class T>
Future<T> makeFuture(Try<T>&& t);

/*
 * Return a new Future that will call back on the given Executor.
 * This is just syntactic sugar for makeFuture().via(executor)
 *
 * @param executor the Executor to call back on
 * @param priority optionally, the priority to add with. Defaults to 0 which
 * represents medium priority.
 *
 * @returns a void Future that will call back on the given executor
 */
inline Future<Unit> via(
    Executor* executor,
    int8_t priority = Executor::MID_PRI);

/// Execute a function via the given executor and return a future.
/// This is semantically equivalent to via(executor).then(func), but
/// easier to read and slightly more efficient.
template <class Func>
auto via(Executor*, Func func)
  -> Future<typename isFuture<decltype(func())>::Inner>;

/** When all the input Futures complete, the returned Future will complete.
  Errors do not cause early termination; this Future will always succeed
  after all its Futures have finished (whether successfully or with an
  error).

  The Futures are moved in, so your copies are invalid. If you need to
  chain further from these Futures, use the variant with an output iterator.

  This function is thread-safe for Futures running on different threads. But
  if you are doing anything non-trivial after, you will probably want to
  follow with `via(executor)` because it will complete in whichever thread the
  last Future completes in.

  The return type for Future<T> input is a Future<std::vector<Try<T>>>
  */
template <class InputIterator>
Future<std::vector<Try<
  typename std::iterator_traits<InputIterator>::value_type::value_type>>>
collectAll(InputIterator first, InputIterator last);

/// Sugar for the most common case
template <class Collection>
auto collectAll(Collection&& c) -> decltype(collectAll(c.begin(), c.end())) {
  return collectAll(c.begin(), c.end());
}

/// This version takes a varying number of Futures instead of an iterator.
/// The return type for (Future<T1>, Future<T2>, ...) input
/// is a Future<std::tuple<Try<T1>, Try<T2>, ...>>.
/// The Futures are moved in, so your copies are invalid.
template <typename... Fs>
typename detail::CollectAllVariadicContext<
  typename std::decay<Fs>::type::value_type...>::type
collectAll(Fs&&... fs);

/// Like collectAll, but will short circuit on the first exception. Thus, the
/// type of the returned Future is std::vector<T> instead of
/// std::vector<Try<T>>
template <class InputIterator>
Future<typename detail::CollectContext<
  typename std::iterator_traits<InputIterator>::value_type::value_type
>::result_type>
collect(InputIterator first, InputIterator last);

/// Sugar for the most common case
template <class Collection>
auto collect(Collection&& c) -> decltype(collect(c.begin(), c.end())) {
  return collect(c.begin(), c.end());
}

/// Like collectAll, but will short circuit on the first exception. Thus, the
/// type of the returned Future is std::tuple<T1, T2, ...> instead of
/// std::tuple<Try<T1>, Try<T2>, ...>
template <typename... Fs>
typename detail::CollectVariadicContext<
  typename std::decay<Fs>::type::value_type...>::type
collect(Fs&&... fs);

/** The result is a pair of the index of the first Future to complete and
  the Try. If multiple Futures complete at the same time (or are already
  complete when passed in), the "winner" is chosen non-deterministically.

  This function is thread-safe for Futures running on different threads.
  */
template <class InputIterator>
Future<std::pair<
  size_t,
  Try<typename std::iterator_traits<InputIterator>::value_type::value_type>>>
collectAny(InputIterator first, InputIterator last);

/// Sugar for the most common case
template <class Collection>
auto collectAny(Collection&& c) -> decltype(collectAny(c.begin(), c.end())) {
  return collectAny(c.begin(), c.end());
}

/** when n Futures have completed, the Future completes with a vector of
  the index and Try of those n Futures (the indices refer to the original
  order, but the result vector will be in an arbitrary order)

  Not thread safe.
  */
template <class InputIterator>
Future<std::vector<std::pair<
  size_t,
  Try<typename std::iterator_traits<InputIterator>::value_type::value_type>>>>
collectN(InputIterator first, InputIterator last, size_t n);

/// Sugar for the most common case
template <class Collection>
auto collectN(Collection&& c, size_t n)
    -> decltype(collectN(c.begin(), c.end(), n)) {
  return collectN(c.begin(), c.end(), n);
}

/** window creates up to n Futures using the values
    in the collection, and then another Future for each Future
    that completes

    this is basically a sliding window of Futures of size n

    func must return a Future for each value in input
  */
template <class Collection, class F,
          class ItT = typename std::iterator_traits<
            typename Collection::iterator>::value_type,
          class Result = typename detail::resultOf<F, ItT&&>::value_type>
std::vector<Future<Result>>
window(Collection input, F func, size_t n);

template <typename F, typename T, typename ItT>
using MaybeTryArg = typename std::conditional<
  detail::callableWith<F, T&&, Try<ItT>&&>::value, Try<ItT>, ItT>::type;

template<typename F, typename T, typename Arg>
using isFutureResult = isFuture<typename std::result_of<F(T&&, Arg&&)>::type>;

/** repeatedly calls func on every result, e.g.
    reduce(reduce(reduce(T initial, result of first), result of second), ...)

    The type of the final result is a Future of the type of the initial value.

    Func can either return a T, or a Future<T>

    func is called in order of the input, see unorderedReduce if that is not
    a requirement
  */
template <class It, class T, class F>
Future<T> reduce(It first, It last, T&& initial, F&& func);

/// Sugar for the most common case
template <class Collection, class T, class F>
auto reduce(Collection&& c, T&& initial, F&& func)
    -> decltype(reduce(c.begin(), c.end(), std::forward<T>(initial),
                std::forward<F>(func))) {
  return reduce(
      c.begin(),
      c.end(),
      std::forward<T>(initial),
      std::forward<F>(func));
}

/** like reduce, but calls func on finished futures as they complete
    does NOT keep the order of the input
  */
template <class It, class T, class F,
          class ItT = typename std::iterator_traits<It>::value_type::value_type,
          class Arg = MaybeTryArg<F, T, ItT>>
Future<T> unorderedReduce(It first, It last, T initial, F func);

/// Sugar for the most common case
template <class Collection, class T, class F>
auto unorderedReduce(Collection&& c, T&& initial, F&& func)
    -> decltype(unorderedReduce(c.begin(), c.end(), std::forward<T>(initial),
                std::forward<F>(func))) {
  return unorderedReduce(
      c.begin(),
      c.end(),
      std::forward<T>(initial),
      std::forward<F>(func));
}

namespace futures {

/**
 *  retrying
 *
 *  Given a policy and a future-factory, creates futures according to the
 *  policy.
 *
 *  The policy must be moveable - retrying will move it a lot - and callable of
 *  either of the two forms:
 *  - Future<bool>(size_t, exception_wrapper)
 *  - bool(size_t, exception_wrapper)
 *  Internally, the latter is transformed into the former in the obvious way.
 *  The first parameter is the attempt number of the next prospective attempt;
 *  the second parameter is the most recent exception. The policy returns a
 *  Future<bool> which, when completed with true, indicates that a retry is
 *  desired.
 *
 *  We provide a few generic policies:
 *  - Basic
 *  - CappedJitteredexponentialBackoff
 *
 *  Custom policies may use the most recent try number and exception to decide
 *  whether to retry and optionally to do something interesting like delay
 *  before the retry. Users may pass inline lambda expressions as policies, or
 *  may define their own data types meeting the above requirements. Users are
 *  responsible for managing the lifetimes of anything pointed to or referred to
 *  from inside the policy.
 *
 *  For example, one custom policy may try up to k times, but only if the most
 *  recent exception is one of a few types or has one of a few error codes
 *  indicating that the failure was transitory.
 *
 *  Cancellation is not supported.
 */
template <class Policy, class FF>
typename std::result_of<FF(size_t)>::type
retrying(Policy&& p, FF&& ff);

/**
 *  generic retrying policies
 */

inline
std::function<bool(size_t, const exception_wrapper&)>
retryingPolicyBasic(
    size_t max_tries);

template <class Policy, class URNG>
std::function<Future<bool>(size_t, const exception_wrapper&)>
retryingPolicyCappedJitteredExponentialBackoff(
    size_t max_tries,
    Duration backoff_min,
    Duration backoff_max,
    double jitter_param,
    URNG rng,
    Policy&& p);

inline
std::function<Future<bool>(size_t, const exception_wrapper&)>
retryingPolicyCappedJitteredExponentialBackoff(
    size_t max_tries,
    Duration backoff_min,
    Duration backoff_max,
    double jitter_param);

}

} // namespace