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/**
* Copyright (C) 2018-present MongoDB, Inc.
*
* This program is free software: you can redistribute it and/or modify
* it under the terms of the Server Side Public License, version 1,
* as published by MongoDB, Inc.
*
* This program is distributed in the hope that it will be useful,
* but WITHOUT ANY WARRANTY; without even the implied warranty of
* MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
* Server Side Public License for more details.
*
* You should have received a copy of the Server Side Public License
* along with this program. If not, see
* <http://www.mongodb.com/licensing/server-side-public-license>.
*
* As a special exception, the copyright holders give permission to link the
* code of portions of this program with the OpenSSL library under certain
* conditions as described in each individual source file and distribute
* linked combinations including the program with the OpenSSL library. You
* must comply with the Server Side Public License in all respects for
* all of the code used other than as permitted herein. If you modify file(s)
* with this exception, you may extend this exception to your version of the
* file(s), but you are not obligated to do so. If you do not wish to do so,
* delete this exception statement from your version. If you delete this
* exception statement from all source files in the program, then also delete
* it in the license file.
*/
#pragma once
#include "mongo/stdx/functional.h"
#include "mongo/util/future.h"
namespace mongo {
/**
* Provides the minimal api for a simple out of line executor that can run non-cancellable
* callbacks.
*
* Adds in a minimal amount of support for futures.
*
* The contract for scheduling work on an executor is that it never blocks the caller. It doesn't
* necessarily need to offer forward progress guarantees, but actual calls to schedule() should not
* deadlock.
*
* As an explicit point of implementation: it will never invoke the passed callback from within the
* scheduling call.
*/
class OutOfLineExecutor {
public:
using Task = unique_function<void()>;
public:
/**
* Invokes the callback on the executor, as in schedule(), returning a future with its result.
* That future may be ready by the time the caller returns, which means that continuations
* chained on the returned future may be invoked on the caller of execute's stack.
*/
template <typename Callback>
Future<FutureContinuationResult<Callback>> execute(Callback&& cb) {
auto pf = makePromiseFuture<FutureContinuationResult<Callback>>();
schedule([ cb = std::forward<Callback>(cb), p = std::move(pf.promise) ]() mutable {
p.setWith(std::move(cb));
});
return std::move(pf.future);
}
/**
* Invokes the callback on the executor. This never happens immediately on the caller's stack.
*/
virtual void schedule(Task func) = 0;
protected:
~OutOfLineExecutor() noexcept {}
};
} // namespace mongo
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