/**
 *    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/base/status.h"
#include "mongo/util/functional.h"

namespace mongo {

/**
 * Provides the minimal api for a simple out of line executor that can run non-cancellable
 * callbacks.
 *
 * 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.
 *
 * If you manage the lifetime of your executor using a shared_ptr, you can begin a chain of
 * execution like this:
 *      ExecutorFuture(myExec)
 *          .then([] { return doThing1(); })
 *          .then([] { return doThing2(); })
 *          ...
 */
class OutOfLineExecutor {
public:
    using Task = unique_function<void(Status)>;

public:
    /**
     * Delegates invocation of the Task to this executor
     *
     * Execution of the Task can happen in one of three contexts:
     * * By default, on an execution context maintained by the OutOfLineExecutor (i.e. a thread).
     * * During shutdown, on the execution context of shutdown/join/dtor for the OutOfLineExecutor.
     * * Post-shutdown, on the execution context of the calling code.
     *
     * The Task will be passed a Status schedStatus that is either:
     * * schedStatus.isOK() if the function is run in an out-of-line context
     * * isCancelationError(schedStatus.code()) if the function is run in an inline context
     *
     * All of this is to say: CHECK YOUR STATUS.
     */
    virtual void schedule(Task func) = 0;

    virtual ~OutOfLineExecutor() = default;
};

using ExecutorPtr = std::shared_ptr<OutOfLineExecutor>;

}  // namespace mongo