Add a safer way to use QThreadPool::reserveThread

Add startOnReservedThread that specifically releases a reserved thread
and uses it atomically for a given task. This can make a positive
number of reserved threads work.

Change-Id: I4bd1dced24bb46fcb365f12cbc9c7905dc66cdf1
Reviewed-by: David Faure <david.faure@kdab.com>

1 files changed, 52 insertions, 2 deletions

diff --git a/tests/auto/corelib/thread/qthreadpool/tst_qthreadpool.cpp b/tests/auto/corelib/thread/qthreadpool/tst_qthreadpool.cpp
index 3ce4ee09b3..404ebb68a1 100644
--- a/tests/auto/corelib/thread/qthreadpool/tst_qthreadpool.cpp
+++ b/tests/auto/corelib/thread/qthreadpool/tst_qthreadpool.cpp
@@ -89,6 +89,7 @@ private slots:
     void releaseThread_data();
     void releaseThread();
     void reserveAndStart();
+    void reserveAndStart2();
     void releaseAndBlock();
     void start();
     void tryStart();
@@ -729,17 +730,66 @@ void tst_QThreadPool::reserveAndStart() // QTBUG-21051
     // start() will wake up the waiting thread.
     threadpool->start(&task);
     QTRY_COMPARE(threadpool->activeThreadCount(), 2);
+    QTRY_COMPARE(task.count.loadRelaxed(), 2);
+    WaitingTask task2;
+    // startOnReservedThread() will try to take the reserved task, but end up waiting instead
+    threadpool->startOnReservedThread(&task2);
+    QTRY_COMPARE(threadpool->activeThreadCount(), 1);
     task.waitForStarted.acquire();
     task.waitBeforeDone.release();
-    QTRY_COMPARE(task.count.loadRelaxed(), 2);
     QTRY_COMPARE(threadpool->activeThreadCount(), 1);
+    task2.waitForStarted.acquire();
+    task2.waitBeforeDone.release();
 
-    threadpool->releaseThread();
     QTRY_COMPARE(threadpool->activeThreadCount(), 0);
 
     threadpool->setMaxThreadCount(savedLimit);
 }
 
+void tst_QThreadPool::reserveAndStart2()
+{
+    class WaitingTask : public QRunnable
+    {
+    public:
+        QSemaphore waitBeforeDone;
+
+        WaitingTask() { setAutoDelete(false); }
+
+        void run() override
+        {
+            waitBeforeDone.acquire();
+        }
+    };
+
+    // Set up
+    QThreadPool *threadpool = QThreadPool::globalInstance();
+    int savedLimit = threadpool->maxThreadCount();
+    threadpool->setMaxThreadCount(2);
+
+    // reserve
+    threadpool->reserveThread();
+
+    // start two task, to get a running thread and one queued
+    WaitingTask task1, task2, task3;
+    threadpool->start(&task1);
+    // one running thread, one reserved:
+    QCOMPARE(threadpool->activeThreadCount(), 2);
+    // task2 starts queued
+    threadpool->start(&task2);
+    QCOMPARE(threadpool->activeThreadCount(), 2);
+    // startOnReservedThread() will take the reserved thread however, bypassing the queue
+    threadpool->startOnReservedThread(&task3);
+    // two running threads, none reserved:
+    QCOMPARE(threadpool->activeThreadCount(), 2);
+    task3.waitBeforeDone.release();
+    // task3 can finish even if all other tasks are blocking
+    // then task2 will use the previously reserved thread
+    task2.waitBeforeDone.release();
+    QTRY_COMPARE(threadpool->activeThreadCount(), 1);
+    task1.waitBeforeDone.release();
+    QTRY_COMPARE(threadpool->activeThreadCount(), 0);
+}
+
 void tst_QThreadPool::releaseAndBlock()
 {
     class WaitingTask : public QRunnable