5 files changed, 179 insertions, 1 deletions
diff --git a/src/corelib/thread/qfuture_impl.h b/src/corelib/thread/qfuture_impl.h
index 82c89cfa385..883f8d4f092 100644
--- a/src/corelib/thread/qfuture_impl.h
+++ b/src/corelib/thread/qfuture_impl.h
@@ -66,6 +66,10 @@ enum class Launch { Sync, Async, Inherit };
 
 namespace QtPrivate {
 
+template<class T, class U>
+using EnableIfSameOrConvertible = std::enable_if_t<std::is_same_v<T, U>
+                                                   || std::is_convertible_v<T, U>>;
+
 template<class T>
 using EnableForVoid = std::enable_if_t<std::is_same_v<T, void>>;
 
diff --git a/src/corelib/thread/qfutureinterface.cpp b/src/corelib/thread/qfutureinterface.cpp
index 05482c40c21..1785815cf37 100644
--- a/src/corelib/thread/qfutureinterface.cpp
+++ b/src/corelib/thread/qfutureinterface.cpp
@@ -259,6 +259,37 @@ void QFutureInterfaceBase::waitForResume()
     d->pausedWaitCondition.wait(&d->m_mutex);
 }
 
+void QFutureInterfaceBase::suspendIfRequested()
+{
+    const auto canSuspend = [] (int state) {
+        // can suspend only if 1) in any suspend-related state; 2) not canceled
+        return (state & suspendingOrSuspended) && !(state & Canceled);
+    };
+
+    // return early if possible to avoid taking the mutex lock.
+    {
+        const int state = d->state.loadRelaxed();
+        if (!canSuspend(state))
+            return;
+    }
+
+    QMutexLocker lock(&d->m_mutex);
+    const int state = d->state.loadRelaxed();
+    if (!canSuspend(state))
+        return;
+
+    // Note: expecting that Suspending and Suspended are mutually exclusive
+    if (!(state & Suspended)) {
+        // switch state in case this is the first invocation
+        switch_from_to(d->state, Suspending, Suspended);
+        d->sendCallOut(QFutureCallOutEvent(QFutureCallOutEvent::Suspended));
+    }
+
+    // decrease active thread count since this thread will wait.
+    const ThreadPoolThreadReleaser releaser(d->pool());
+    d->pausedWaitCondition.wait(&d->m_mutex);
+}
+
 int QFutureInterfaceBase::progressValue() const
 {
     const QMutexLocker lock(&d->m_mutex);
@@ -361,6 +392,11 @@ bool QFutureInterfaceBase::queryState(State state) const
     return d->state.loadRelaxed() & state;
 }
 
+int QFutureInterfaceBase::loadState() const
+{
+    return d->state.loadRelaxed();
+}
+
 void QFutureInterfaceBase::waitForResult(int resultIndex)
 {
     d->m_exceptionStore.throwPossibleException();
diff --git a/src/corelib/thread/qfutureinterface.h b/src/corelib/thread/qfutureinterface.h
index c802be762b2..75c4cae0ca0 100644
--- a/src/corelib/thread/qfutureinterface.h
+++ b/src/corelib/thread/qfutureinterface.h
@@ -140,6 +140,7 @@ public:
     bool isThrottled() const;
     bool isResultReadyAt(int index) const;
     bool isValid() const;
+    int loadState() const;
 
     void cancel();
     void setSuspended(bool suspend);
@@ -151,6 +152,7 @@ public:
     bool waitForNextResult();
     void waitForResult(int resultIndex);
     void waitForResume();
+    void suspendIfRequested();
 
     QMutex &mutex() const;
     QtPrivate::ExceptionStore &exceptionStore();
@@ -233,6 +235,7 @@ public:
 
     inline void reportResult(const T *result, int index = -1);
     inline void reportAndMoveResult(T &&result, int index = -1);
+    inline void reportResult(T &&result, int index = -1);
     inline void reportResult(const T &result, int index = -1);
     inline void reportResults(const QVector<T> &results, int beginIndex = -1, int count = -1);
     inline void reportFinished(const T *result);
@@ -285,6 +288,12 @@ void QFutureInterface<T>::reportAndMoveResult(T &&result, int index)
         reportResultsReady(insertIndex, store.count());
 }
 
+template<typename T>
+void QFutureInterface<T>::reportResult(T &&result, int index)
+{
+    reportAndMoveResult(std::move(result), index);
+}
+
 template <typename T>
 inline void QFutureInterface<T>::reportResult(const T &result, int index)
 {
diff --git a/src/corelib/thread/qpromise.h b/src/corelib/thread/qpromise.h
new file mode 100644
index 00000000000..15aea16f29b
--- /dev/null
+++ b/src/corelib/thread/qpromise.h
@@ -0,0 +1,128 @@
+/****************************************************************************
+**
+** Copyright (C) 2020 The Qt Company Ltd.
+** Contact: https://www.qt.io/licensing/
+**
+** This file is part of the QtCore module of the Qt Toolkit.
+**
+** $QT_BEGIN_LICENSE:LGPL$
+** Commercial License Usage
+** Licensees holding valid commercial Qt licenses may use this file in
+** accordance with the commercial license agreement provided with the
+** Software or, alternatively, in accordance with the terms contained in
+** a written agreement between you and The Qt Company. For licensing terms
+** and conditions see https://www.qt.io/terms-conditions. For further
+** information use the contact form at https://www.qt.io/contact-us.
+**
+** GNU Lesser General Public License Usage
+** Alternatively, this file may be used under the terms of the GNU Lesser
+** General Public License version 3 as published by the Free Software
+** Foundation and appearing in the file LICENSE.LGPL3 included in the
+** packaging of this file. Please review the following information to
+** ensure the GNU Lesser General Public License version 3 requirements
+** will be met: https://www.gnu.org/licenses/lgpl-3.0.html.
+**
+** GNU General Public License Usage
+** Alternatively, this file may be used under the terms of the GNU
+** General Public License version 2.0 or (at your option) the GNU General
+** Public license version 3 or any later version approved by the KDE Free
+** Qt Foundation. The licenses are as published by the Free Software
+** Foundation and appearing in the file LICENSE.GPL2 and LICENSE.GPL3
+** included in the packaging of this file. Please review the following
+** information to ensure the GNU General Public License requirements will
+** be met: https://www.gnu.org/licenses/gpl-2.0.html and
+** https://www.gnu.org/licenses/gpl-3.0.html.
+**
+** $QT_END_LICENSE$
+**
+****************************************************************************/
+
+#ifndef QPROMISE_H
+#define QPROMISE_H
+
+#include <QtCore/qglobal.h>
+#include <QtCore/qfuture.h>
+
+QT_REQUIRE_CONFIG(future);
+
+QT_BEGIN_NAMESPACE
+
+template<typename T>
+class QPromise
+{
+    static_assert (std::is_copy_constructible_v<T>
+                   || std::is_move_constructible_v<T>
+                   || std::is_same_v<T, void>,
+                   "Type with copy or move constructors or type void is required");
+public:
+    QPromise() = default;
+    Q_DISABLE_COPY(QPromise)
+    QPromise(QPromise<T> &&other) : d(other.d)
+    {
+        // In constructor, there's no need to perform swap(). Assign new
+        // QFutureInterface to other.d instead which is slightly cheaper.
+        other.d = QFutureInterface<T>();
+    }
+    QPromise& operator=(QPromise<T> &&other)
+    {
+        swap(other);
+        return *this;
+    }
+    ~QPromise()
+    {
+        const int state = d.loadState();
+        // If QFutureInterface has no state, there is nothing to be done
+        if (state == static_cast<int>(QFutureInterfaceBase::State::NoState))
+            return;
+        // Otherwise, if computation is not finished at this point, cancel
+        // potential waits
+        if (!(state & QFutureInterfaceBase::State::Finished)) {
+            d.cancel();
+            reportFinished();  // required to finalize the state
+        }
+    }
+
+    // Core QPromise APIs
+    QFuture<T> future() const { return d.future(); }
+    template<typename U = T,
+             typename = QtPrivate::EnableForNonVoid<std::decay_t<U>>,
+             typename = QtPrivate::EnableIfSameOrConvertible<std::decay_t<U>, std::decay_t<T>>>
+    void addResult(U &&result, int index = -1)
+    {
+        d.reportResult(std::forward<U>(result), index);
+    }
+#ifndef QT_NO_EXCEPTIONS
+    void setException(const QException &e) { d.reportException(e); }
+    void setException(std::exception_ptr e) { d.reportException(e); }
+#endif
+    void reportStarted() { d.reportStarted(); }
+    void reportFinished() { d.reportFinished(); }
+
+    void suspendIfRequested() { d.suspendIfRequested(); }
+
+    bool isCanceled() const { return d.isCanceled(); }
+
+    // Progress methods
+    void setProgressRange(int minimum, int maximum) { d.setProgressRange(minimum, maximum); }
+    void setProgressValue(int progressValue) { d.setProgressValue(progressValue); }
+    void setProgressValueAndText(int progressValue, const QString &progressText)
+    {
+        d.setProgressValueAndText(progressValue, progressText);
+    }
+
+private:
+    mutable QFutureInterface<T> d = QFutureInterface<T>();
+
+    void swap(QPromise<T> &other)
+    {
+        // Note: copy operations are expensive! They trigger several atomic
+        //       reference counts
+        auto tmp = this->d;
+        this->d = other.d;
+        other.d = tmp;
+    }
+};
+
+QT_END_NAMESPACE
+
+#endif  // QPROMISE_H
diff --git a/src/corelib/thread/thread.pri b/src/corelib/thread/thread.pri
index e3d791fee7b..d36c8011ef4 100644
--- a/src/corelib/thread/thread.pri
+++ b/src/corelib/thread/thread.pri
@@ -72,7 +72,8 @@ qtConfig(future) {
         thread/qfuturesynchronizer.h \
         thread/qfuturewatcher.h \
         thread/qfuturewatcher_p.h \
-        thread/qresultstore.h
+        thread/qresultstore.h \
+        thread/qpromise.h
 
     SOURCES += \
         thread/qexception.cpp \