Long live QSemaphoreReleaser!

This is a simple RAII class that makes semaphore releasing
reliable in the face of exceptions and early returns.

This code originates from KDTools' KDSemaphoreReleaser[1], but
has been extensively reworked to support C++11 move semantics.

[1] https://docs.kdab.com/kdtools/2.3.0/class_k_d_semaphore_releaser.html

[ChangeLog][QtCore][QSemaphore] Added a new RAII class, QSemaphoreReleaser,
to reliably perform release() calls.

Change-Id: I6aff64d37cc0882b17c4419817bde60b542f34d9
Reviewed-by: Mitch Curtis <mitch.curtis@qt.io>
Reviewed-by: David Faure <david.faure@kdab.com>
Reviewed-by: Thiago Macieira <thiago.macieira@intel.com>

1 files changed, 39 insertions, 0 deletions

diff --git a/src/corelib/thread/qsemaphore.h b/src/corelib/thread/qsemaphore.h
index adb9d73e50..a92740c8ce 100644
--- a/src/corelib/thread/qsemaphore.h
+++ b/src/corelib/thread/qsemaphore.h
@@ -69,6 +69,45 @@ private:
     QSemaphorePrivate *d;
 };
 
+class QSemaphoreReleaser
+{
+    QSemaphore *m_sem = nullptr;
+    int m_n;
+public:
+    QSemaphoreReleaser() = default;
+    explicit QSemaphoreReleaser(QSemaphore &sem, int n = 1) Q_DECL_NOTHROW
+        : m_sem(&sem), m_n(n) {}
+    explicit QSemaphoreReleaser(QSemaphore *sem, int n = 1) Q_DECL_NOTHROW
+        : m_sem(sem), m_n(n) {}
+    QSemaphoreReleaser(QSemaphoreReleaser &&other) Q_DECL_NOTHROW
+        : m_sem(other.m_sem), m_n(other.m_n)
+    { other.m_sem = nullptr; }
+    QSemaphoreReleaser &operator=(QSemaphoreReleaser &&other) Q_DECL_NOTHROW
+    { QSemaphoreReleaser moved(std::move(other)); swap(moved); return *this; }
+
+    ~QSemaphoreReleaser()
+    {
+        if (m_sem)
+            m_sem->release(m_n);
+    }
+
+    void swap(QSemaphoreReleaser &other) Q_DECL_NOTHROW
+    {
+        qSwap(m_sem, other.m_sem);
+        qSwap(m_n, other.m_n);
+    }
+
+    QSemaphore *semaphore() const Q_DECL_NOTHROW
+    { return m_sem; }
+
+    QSemaphore *cancel() Q_DECL_NOTHROW
+    {
+        QSemaphore *old = m_sem;
+        m_sem = nullptr;
+        return old;
+    }
+};
+
 #endif // QT_NO_THREAD
 
 QT_END_NAMESPACE