Long live QAtomicScopedValueRollback (private API)!

QScopedValueRollback has a few users that apply it on QAtomicInt,
which happens to work as QAtomicInt is copy-constructible and its
ctors are implicit.

But that's of course nonsense. We don't need to store the oldValue in
an atomic, nor do we need to pass the new value into the ctor as an
atomic.

So, add a QAtomicScopedValueRollback which works on std::atomic as
well as the Qt atomics, but distinguishes between the reference (which
is atomic) and the value (which isn't), and use it in one of the
users, tst_QList.

Keep it private until we know whether there's an actual need for this.

The test is a copy of tst_qscopedvaluefallback, so the occasional
oddity (like atomic op*=) should be ignored.

Task-number: QTBUG-103835
Change-Id: I3c05b3e51f465698657a02ca5521ed465386e9a6
Reviewed-by: Qt CI Bot <qt_ci_bot@qt-project.org>
Reviewed-by: Fabian Kosmale <fabian.kosmale@qt.io>

src/corelib/CMakeLists.txt

@@ -226,6 +226,7 @@ qt_internal_add_module(Core
         tools/qarraydata.cpp tools/qarraydata.h
         tools/qarraydataops.h
         tools/qarraydatapointer.h
+        tools/qatomicscopedvaluerollback_p.h
         tools/qbitarray.cpp tools/qbitarray.h
         tools/qcache.h
         tools/qcontainerfwd.h

src/corelib/tools/qatomicscopedvaluerollback_p.h

@@ -0,0 +1,95 @@
+// Copyright (C) 2022 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 QATOMICSCOPEDVALUEROLLBACK_P_H
+#define QATOMICSCOPEDVALUEROLLBACK_P_H
+
+#include <QtCore/qglobal.h>
+#include <QtCore/qatomic.h>
+
+#include <atomic>
+
+QT_BEGIN_NAMESPACE
+
+template <typename T>
+class [[nodiscard]] QAtomicScopedValueRollback
+{
+    std::atomic<T> &m_atomic;
+    T m_value;
+    std::memory_order m_mo;
+
+    Q_DISABLE_COPY_MOVE(QAtomicScopedValueRollback)
+
+    constexpr std::memory_order store_part(std::memory_order mo) noexcept
+    {
+        switch (mo) {
+        case std::memory_order_relaxed:
+        case std::memory_order_consume:
+        case std::memory_order_acquire: return std::memory_order_relaxed;
+        case std::memory_order_release:
+        case std::memory_order_acq_rel: return std::memory_order_release;
+        case std::memory_order_seq_cst: return std::memory_order_seq_cst;
+        }
+        // GCC 8.x does not tread __builtin_unreachable() as constexpr
+#if !defined(Q_CC_GNU_ONLY) || (Q_CC_GNU >= 900)
+        Q_UNREACHABLE();
+#endif
+        return std::memory_order_seq_cst;
+    }
+public:
+    //
+    // std::atomic:
+    //
+    explicit constexpr
+    QAtomicScopedValueRollback(std::atomic<T> &var,
+                               std::memory_order mo = std::memory_order_seq_cst)
+        : m_atomic(var), m_value(var.load(mo)), m_mo(mo) {}
+
+    explicit constexpr
+    QAtomicScopedValueRollback(std::atomic<T> &var, T value,
+                               std::memory_order mo = std::memory_order_seq_cst)
+        : m_atomic(var), m_value(var.exchange(value, mo)), m_mo(mo) {}
+
+    //
+    // Q(Basic)AtomicInteger:
+    //
+    explicit constexpr
+    QAtomicScopedValueRollback(QBasicAtomicInteger<T> &var,
+                               std::memory_order mo = std::memory_order_seq_cst)
+        : QAtomicScopedValueRollback(var._q_value, mo) {}
+
+    explicit constexpr
+    QAtomicScopedValueRollback(QBasicAtomicInteger<T> &var, T value,
+                               std::memory_order mo = std::memory_order_seq_cst)
+        : QAtomicScopedValueRollback(var._q_value, value, mo) {}
+
+    //
+    // Q(Basic)AtomicPointer:
+    //
+    explicit constexpr
+    QAtomicScopedValueRollback(QBasicAtomicPointer<std::remove_pointer_t<T>> &var,
+                               std::memory_order mo = std::memory_order_seq_cst)
+        : QAtomicScopedValueRollback(var._q_value, mo) {}
+
+    explicit constexpr
+    QAtomicScopedValueRollback(QBasicAtomicPointer<std::remove_pointer_t<T>> &var, T value,
+                               std::memory_order mo = std::memory_order_seq_cst)
+        : QAtomicScopedValueRollback(var._q_value, value, mo) {}
+
+#if __cpp_constexpr >= 201907L
+    constexpr
+#endif
+    ~QAtomicScopedValueRollback()
+    {
+        m_atomic.store(m_value, store_part(m_mo));
+    }
+
+    constexpr void commit()
+    {
+        m_value = m_atomic.load(m_mo);
+    }
+};
+
+QT_END_NAMESPACE
+
+#endif // QATOMICASCOPEDVALUEROLLBACK_P_H

tests/auto/corelib/tools/CMakeLists.txt

@@ -1,4 +1,5 @@
 # Generated from tools.pro.
+add_subdirectory(qatomicscopedvaluerollback)
 if(NOT INTEGRITY)
     add_subdirectory(collections)
 endif()

tests/auto/corelib/tools/qatomicscopedvaluerollback/CMakeLists.txt

@@ -0,0 +1,6 @@
+qt_internal_add_test(tst_qatomicscopedvaluerollback
+    SOURCES
+        tst_qatomicscopedvaluerollback.cpp
+    PUBLIC_LIBRARIES
+        Qt::CorePrivate
+)

tests/auto/corelib/tools/qatomicscopedvaluerollback/tst_qatomicscopedvaluerollback.cpp

@@ -0,0 +1,157 @@
+// Copyright (C) 2022 The Qt Company Ltd.
+// SPDX-License-Identifier: LicenseRef-Qt-Commercial OR GPL-3.0-only WITH Qt-GPL-exception-1.0
+
+#include <QtCore/private/qatomicscopedvaluerollback_p.h>
+
+#include <QTest>
+
+class tst_QAtomicScopedValueRollback : public QObject
+{
+    Q_OBJECT
+
+private Q_SLOTS:
+    void leavingScope();
+    void leavingScopeAfterCommit();
+    void rollbackToPreviousCommit();
+    void exceptions();
+    void earlyExitScope();
+private:
+    void earlyExitScope_helper(int exitpoint, std::atomic<int> &member);
+};
+
+void tst_QAtomicScopedValueRollback::leavingScope()
+{
+    QAtomicInt i = 0;
+    QBasicAtomicInteger<bool> b = false;
+    std::atomic<bool> b2 = false;
+
+    //test rollback on going out of scope
+    {
+        QAtomicScopedValueRollback ri(i);
+        QAtomicScopedValueRollback rb(b);
+        QAtomicScopedValueRollback rb2(b2, true);
+        QCOMPARE(b.loadRelaxed(), false);
+        QCOMPARE(b2, true);
+        QCOMPARE(i.loadRelaxed(), 0);
+        b.storeRelaxed(true);
+        i.storeRelaxed(1);
+        QCOMPARE(b.loadRelaxed(), true);
+        QCOMPARE(i.loadRelaxed(), 1);
+    }
+    QCOMPARE(b.loadRelaxed(), false);
+    QCOMPARE(b2, false);
+    QCOMPARE(i.loadRelaxed(), 0);
+}
+
+void tst_QAtomicScopedValueRollback::leavingScopeAfterCommit()
+{
+    std::atomic<int> i = 0;
+    QAtomicInteger<bool> b = false;
+
+    //test rollback on going out of scope
+    {
+        QAtomicScopedValueRollback ri(i);
+        QAtomicScopedValueRollback rb(b);
+        QCOMPARE(b.loadRelaxed(), false);
+        QCOMPARE(i, 0);
+        b.storeRelaxed(true);
+        i = 1;
+        QCOMPARE(b.loadRelaxed(), true);
+        QCOMPARE(i, 1);
+        ri.commit();
+        rb.commit();
+    }
+    QCOMPARE(b.loadRelaxed(), true);
+    QCOMPARE(i, 1);
+}
+
+void tst_QAtomicScopedValueRollback::rollbackToPreviousCommit()
+{
+    QBasicAtomicInt i = 0;
+    {
+        QAtomicScopedValueRollback ri(i);
+        i++;
+        ri.commit();
+        i++;
+    }
+    QCOMPARE(i.loadRelaxed(), 1);
+    {
+        QAtomicScopedValueRollback ri1(i);
+        i++;
+        ri1.commit();
+        i++;
+        ri1.commit();
+        i++;
+    }
+    QCOMPARE(i.loadRelaxed(), 3);
+}
+
+void tst_QAtomicScopedValueRollback::exceptions()
+{
+    std::atomic<bool> b = false;
+    bool caught = false;
+    QT_TRY
+    {
+        QAtomicScopedValueRollback rb(b);
+        b = true;
+        QT_THROW(std::bad_alloc()); //if Qt compiled without exceptions this is noop
+        rb.commit(); //if Qt compiled without exceptions, true is committed
+    }
+    QT_CATCH(...)
+    {
+        caught = true;
+    }
+    QCOMPARE(b, !caught); //expect false if exception was thrown, true otherwise
+}
+
+void tst_QAtomicScopedValueRollback::earlyExitScope()
+{
+    QAtomicInt ai = 0;
+    std::atomic<int> aj = 0;
+    while (true) {
+        QAtomicScopedValueRollback ri(ai);
+        ++ai;
+        aj = ai.loadRelaxed();
+        if (ai.loadRelaxed() > 8) break;
+        ri.commit();
+    }
+    QCOMPARE(ai.loadRelaxed(), 8);
+    QCOMPARE(aj.load(), 9);
+
+    for (int i = 0; i < 5; ++i) {
+        aj = 1;
+        earlyExitScope_helper(i, aj);
+        QCOMPARE(aj.load(), 1 << i);
+    }
+}
+
+static void operator*=(std::atomic<int> &lhs, int rhs)
+{
+    int expected = lhs.load();
+    while (!lhs.compare_exchange_weak(expected, expected * rhs))
+        ;
+}
+
+void tst_QAtomicScopedValueRollback::earlyExitScope_helper(int exitpoint, std::atomic<int>& member)
+{
+    QAtomicScopedValueRollback r(member);
+    member *= 2;
+    if (exitpoint == 0)
+        return;
+    r.commit();
+    member *= 2;
+    if (exitpoint == 1)
+        return;
+    r.commit();
+    member *= 2;
+    if (exitpoint == 2)
+        return;
+    r.commit();
+    member *= 2;
+    if (exitpoint == 3)
+        return;
+    r.commit();
+}
+
+QTEST_MAIN(tst_QAtomicScopedValueRollback)
+#include "tst_qatomicscopedvaluerollback.moc"

tests/auto/corelib/tools/qlist/CMakeLists.txt

@@ -7,6 +7,8 @@
 qt_internal_add_test(tst_qlist
     SOURCES
         tst_qlist.cpp
+    PUBLIC_LIBRARIES
+        Qt::CorePrivate
 )
 
 ## Scopes:

tests/auto/corelib/tools/qlist/tst_qlist.cpp

@@ -5,7 +5,7 @@
 #include <QAtomicInt>
 #include <QThread>
 #include <QSemaphore>
-#include <QScopedValueRollback>
+#include <private/qatomicscopedvaluerollback_p.h>
 #include <qlist.h>
 
 
@@ -3181,7 +3181,7 @@ void tst_QList::emplaceReturnsIterator()
 
 void tst_QList::emplaceFront() const
 {
-    QScopedValueRollback<QAtomicInt> rollback(Movable::counter, 0);
+    QAtomicScopedValueRollback rollback(Movable::counter, 0);
 
     QList<Movable> vec;
     vec.emplaceFront('b');
@@ -3206,7 +3206,7 @@ void tst_QList::emplaceFrontReturnsRef() const
 
 void tst_QList::emplaceBack()
 {
-    QScopedValueRollback<QAtomicInt> rollback(Movable::counter, 0);
+    QAtomicScopedValueRollback rollback(Movable::counter, 0);
 
     QList<Movable> vec;