import quincy beta 17.1.0

[ceph.git] / ceph / src / seastar / include / seastar / core / when_any.hh

diff --git a/ceph/src/seastar/include/seastar/core/when_any.hh b/ceph/src/seastar/include/seastar/core/when_any.hh
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+/*
+ * This file is open source software, licensed to you under the terms
+ * of the Apache License, Version 2.0 (the "License").  See the NOTICE file
+ * distributed with this work for additional information regarding copyright
+ * ownership.  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.
+ */
+
+/*
+ * author: Niek J Bouman
+ * reviewer: Avi Kivity
+ * November 2021
+ */ 
+
+#pragma once
+
+#include <iterator>
+#include <cstddef>
+#include <type_traits>
+#include <vector>
+#include <seastar/core/future.hh>
+#include <seastar/core/shared_ptr.hh>
+
+namespace seastar {
+
+template <class Sequence>
+struct when_any_result {
+    std::size_t index;
+    Sequence futures;
+};
+
+namespace internal {
+class waiter {
+    bool _done = false;
+    promise<std::size_t> _promise;
+
+public:
+    void done(std::size_t index) {
+        if (!_done) {
+            _done = true;
+            _promise.set_value(index);
+        }
+    }
+    auto get_future() { return _promise.get_future(); }
+};
+
+} // namespace internal
+
+/// Wait for the first of multiple futures to complete (iterator version).
+///
+/// Given a range of futures as input, wait for the first of them
+/// to resolve (either successfully or with an exception), and return
+/// all of them in a \c when_any_result (following the concurrency TS from 
+/// the standard library), containing a std::vector to all futures
+/// and the index (into the vector) of the future that resolved.
+///
+/// \param begin an \c InputIterator designating the beginning of the range of futures
+/// \param end an \c InputIterator designating the end of the range of futures
+/// \return a \c when_any_result of all the futures in the input; when
+///         ready, at least one of the contained futures (the one indicated by index) will be ready.
+template <class FutureIterator>
+SEASTAR_CONCEPT( requires requires (FutureIterator i) { { *i++ }; requires is_future<std::remove_reference_t<decltype(*i)>>::value; } )
+auto when_any(FutureIterator begin, FutureIterator end) noexcept
+  -> future<when_any_result<std::vector<std::decay_t<typename std::iterator_traits<FutureIterator>::value_type>>>>
+{
+    using ReturnType = when_any_result<std::vector<typename std::iterator_traits<FutureIterator>::value_type>>;
+    if (begin == end) {
+        return make_ready_future<ReturnType>();
+    }
+    ReturnType result;
+    result.futures.reserve(std::distance(begin, end));
+    auto waiter_obj = make_lw_shared<internal::waiter>();
+    std::size_t index{0};
+    for (auto it = begin; it != end; ++it) {
+        if (it->available()) {
+            result.futures.push_back(std::move(*it));
+            waiter_obj->done(index);
+        } else {
+            result.futures.push_back(it->finally([waiter_obj, index] {
+                waiter_obj->done(index);
+            }));
+        }
+        index++;
+    }
+    return waiter_obj->get_future().then(
+        [result = std::move(result)](std::size_t index) mutable {
+            result.index = index;
+            return std::move(result);
+        }
+    );
+}
+} // namespace seastar