ceph/src/seastar/include/seastar/core/when_any.hh

   1 /*
   2  * This file is open source software, licensed to you under the terms
   3  * of the Apache License, Version 2.0 (the "License").  See the NOTICE file
   4  * distributed with this work for additional information regarding copyright
   5  * ownership.  You may not use this file except in compliance with the License.
   6  *
   7  * You may obtain a copy of the License at
   8  *
   9  *   http://www.apache.org/licenses/LICENSE-2.0
  10  *
  11  * Unless required by applicable law or agreed to in writing,
  12  * software distributed under the License is distributed on an
  13  * "AS IS" BASIS, WITHOUT WARRANTIES OR CONDITIONS OF ANY
  14  * KIND, either express or implied.  See the License for the
  15  * specific language governing permissions and limitations
  16  * under the License.
  17  */
  18
  19 /*
  20  * author: Niek J Bouman
  21  * reviewer: Avi Kivity
  22  * November 2021
  23  */
  24
  25 #pragma once
  26
  27 #include <iterator>
  28 #include <cstddef>
  29 #include <type_traits>
  30 #include <vector>
  31 #include <seastar/core/future.hh>
  32 #include <seastar/core/shared_ptr.hh>
  33
  34 namespace seastar {
  35
  36 template <class Sequence>
  37 struct when_any_result {
  38     std::size_t index;
  39     Sequence futures;
  40 };
  41
  42 namespace internal {
  43 class waiter {
  44     bool _done = false;
  45     promise<std::size_t> _promise;
  46
  47 public:
  48     void done(std::size_t index) {
  49         if (!_done) {
  50             _done = true;
  51             _promise.set_value(index);
  52         }
  53     }
  54     auto get_future() { return _promise.get_future(); }
  55 };
  56
  57 } // namespace internal
  58
  59 /// Wait for the first of multiple futures to complete (iterator version).
  60 ///
  61 /// Given a range of futures as input, wait for the first of them
  62 /// to resolve (either successfully or with an exception), and return
  63 /// all of them in a \c when_any_result (following the concurrency TS from
  64 /// the standard library), containing a std::vector to all futures
  65 /// and the index (into the vector) of the future that resolved.
  66 ///
  67 /// \param begin an \c InputIterator designating the beginning of the range of futures
  68 /// \param end an \c InputIterator designating the end of the range of futures
  69 /// \return a \c when_any_result of all the futures in the input; when
  70 ///         ready, at least one of the contained futures (the one indicated by index) will be ready.
  71 template <class FutureIterator>
  72 SEASTAR_CONCEPT( requires requires (FutureIterator i) { { *i++ }; requires is_future<std::remove_reference_t<decltype(*i)>>::value; } )
  73 auto when_any(FutureIterator begin, FutureIterator end) noexcept
  74   -> future<when_any_result<std::vector<std::decay_t<typename std::iterator_traits<FutureIterator>::value_type>>>>
  75 {
  76     using ReturnType = when_any_result<std::vector<typename std::iterator_traits<FutureIterator>::value_type>>;
  77     if (begin == end) {
  78         return make_ready_future<ReturnType>();
  79     }
  80     ReturnType result;
  81     result.futures.reserve(std::distance(begin, end));
  82     auto waiter_obj = make_lw_shared<internal::waiter>();
  83     std::size_t index{0};
  84     for (auto it = begin; it != end; ++it) {
  85         if (it->available()) {
  86             result.futures.push_back(std::move(*it));
  87             waiter_obj->done(index);
  88         } else {
  89             result.futures.push_back(it->finally([waiter_obj, index] {
  90                 waiter_obj->done(index);
  91             }));
  92         }
  93         index++;
  94     }
  95     return waiter_obj->get_future().then(
  96         [result = std::move(result)](std::size_t index) mutable {
  97             result.index = index;
  98             return std::move(result);
  99         }
 100     );
 101 }
 102 } // namespace seastar