barrier.h

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// Copyright (C) 2024 Jarek Kobus
// Copyright (C) 2024 The Qt Company Ltd.
// SPDX-License-Identifier: LicenseRef-Qt-Commercial OR LGPL-3.0-only OR GPL-2.0-only OR GPL-3.0-only
 
#ifndef TASKING_BARRIER_H
#define TASKING_BARRIER_H
 
//
//  W A R N I N G
//  -------------
//
// This file is not part of the Qt API. It exists purely as an
// implementation detail. This header file may change from version to
// version without notice, or even be removed.
//
// We mean it.
//
 
#include "tasking_global.h"
 
#include "tasktree.h"
 
QT_BEGIN_NAMESPACE
 
namespace Tasking {
 
class TASKING_EXPORT Barrier final : public QObject
{
    Q_OBJECT
 
public:
    void setLimit(int value);
    int limit() const { return m_limit; }
 
    void start();
    void advance(); // If limit reached, stops with true
    void stopWithResult(DoneResult result); // Ignores limit
 
    bool isRunning() const { return m_current >= 0; }
    int current() const { return m_current; }
    std::optional<DoneResult> result() const { return m_result; }
 
Q_SIGNALS:
    void done(DoneResult success);
 
private:
    std::optional<DoneResult> m_result = {};
    int m_limit = 1;
    int m_current = -1;
};
 
class TASKING_EXPORT BarrierTaskAdapter : public TaskAdapter<Barrier>
{
public:
    BarrierTaskAdapter() { connect(task(), &Barrier::done, this, &TaskInterface::done); }
    void start() final { task()->start(); }
};
 
using BarrierTask = CustomTask<BarrierTaskAdapter>;
 
template <int Limit = 1>
class SharedBarrier
{
public:
    static_assert(Limit > 0, "SharedBarrier's limit should be 1 or more.");
    SharedBarrier() : m_barrier(new Barrier) {
        m_barrier->setLimit(Limit);
        m_barrier->start();
    }
    Barrier *barrier() const { return m_barrier.get(); }
 
private:
    std::shared_ptr<Barrier> m_barrier;
};
 
template <int Limit = 1>
using MultiBarrier = Storage<SharedBarrier<Limit>>;
 
// Can't write: "MultiBarrier barrier;". Only "MultiBarrier<> barrier;" would work.
// Can't have one alias with default type in C++17, getting the following error:
// alias template deduction only available with C++20.
using SingleBarrier = MultiBarrier<1>;
 
template <int Limit>
GroupItem waitForBarrierTask(const MultiBarrier<Limit> &sharedBarrier)
{
    return BarrierTask([sharedBarrier](Barrier &barrier) {
        SharedBarrier<Limit> *activeBarrier = sharedBarrier.activeStorage();
        if (!activeBarrier) {
            qWarning("The barrier referenced from WaitForBarrier element "
                     "is not reachable in the running tree. "
                     "It is possible that no barrier was added to the tree, "
                     "or the storage is not reachable from where it is referenced. "
                     "The WaitForBarrier task finishes with an error. ");
            return SetupResult::StopWithError;
        }
        Barrier *activeSharedBarrier = activeBarrier->barrier();
        const std::optional<DoneResult> result = activeSharedBarrier->result();
        if (result.has_value()) {
            return result.value() == DoneResult::Success ? SetupResult::StopWithSuccess
                                                         : SetupResult::StopWithError;
        }
        QObject::connect(activeSharedBarrier, &Barrier::done, &barrier, &Barrier::stopWithResult);
        return SetupResult::Continue;
    });
}
 
} // namespace Tasking
 
QT_END_NAMESPACE
 
#endif // TASKING_BARRIER_H