QThreadPool: Fix regression from Qt 4 in dealing with priority starts

The optimisation done in cbaf52b09971edf6f3e1458f7dd677b80a1568ed for Qt
5.0 got the order wrong of the comparison. The queue must be sorted in
decreasing priority order. But since higher numbers mean higher
priority, that means the queue must be sorted in decreasing priority
number order.

Task-number: QTBUG-29163
Change-Id: Iaf3424b9bb445bf5c71518927f37253cead454f3
Reviewed-by: Konstantin Ritt <ritt.ks@gmail.com>

1 files changed, 53 insertions, 0 deletions

diff --git a/tests/auto/corelib/thread/qthreadpool/tst_qthreadpool.cpp b/tests/auto/corelib/thread/qthreadpool/tst_qthreadpool.cpp
index a93a94a470..3c0e132a0a 100644
--- a/tests/auto/corelib/thread/qthreadpool/tst_qthreadpool.cpp
+++ b/tests/auto/corelib/thread/qthreadpool/tst_qthreadpool.cpp
@@ -94,6 +94,8 @@ private slots:
     void tryStart();
     void tryStartPeakThreadCount();
     void tryStartCount();
+    void priorityStart_data();
+    void priorityStart();
     void waitForDone();
     void waitForDoneTimeout();
     void destroyingWaitsForTasksToFinish();
@@ -747,6 +749,57 @@ void tst_QThreadPool::tryStartCount()
     }
 }
 
+void tst_QThreadPool::priorityStart_data()
+{
+    QTest::addColumn<int>("otherCount");
+    QTest::newRow("0") << 0;
+    QTest::newRow("1") << 1;
+    QTest::newRow("2") << 2;
+}
+
+void tst_QThreadPool::priorityStart()
+{
+    class Holder : public QRunnable
+    {
+    public:
+        QSemaphore &sem;
+        Holder(QSemaphore &sem) : sem(sem) {}
+        void run()
+        {
+            sem.acquire();
+        }
+    };
+    class Runner : public QRunnable
+    {
+    public:
+        QAtomicPointer<QRunnable> &ptr;
+        Runner(QAtomicPointer<QRunnable> &ptr) : ptr(ptr) {}
+        void run()
+        {
+            ptr.testAndSetRelaxed(0, this);
+        }
+    };
+
+    QFETCH(int, otherCount);
+    QSemaphore sem;
+    QAtomicPointer<QRunnable> firstStarted;
+    QRunnable *expected;
+    QThreadPool threadPool;
+    threadPool.setMaxThreadCount(1); // start only one thread at a time
+
+    // queue the holder first
+    // We need to be sure that all threads are active when we
+    // queue the two Runners
+    threadPool.start(new Holder(sem));
+    while (otherCount--)
+        threadPool.start(new Runner(firstStarted), 0); // priority 0
+    threadPool.start(expected = new Runner(firstStarted), 1); // priority 1
+
+    sem.release();
+    QVERIFY(threadPool.waitForDone());
+    QCOMPARE(firstStarted.load(), expected);
+}
+
 void tst_QThreadPool::waitForDone()
 {
     QTime total, pass;