QThreadPool - Add method clear() to remove queued QRunnable.

QThreadPool::clear() method removes all queued QRunnable.

When a large number of long-running tasks are queud in a
QThreadPool its destruction, which calls waitForDone(), can
be quite long.
QThreadPool:clear() removes (and deletes when appropriate)
all QRunnable that have yet to be started from the queue
enabling a faster interruption.

Change-Id: Ie5d6028ad3cfe7e439d1db068c8d0936ff818db9
Reviewed-by: David Faure <david.faure@kdab.com>

src/corelib/thread/qthreadpool.cpp

@@ -308,6 +308,18 @@ bool QThreadPoolPrivate::waitForDone(int msecs)
     return queue.isEmpty() && activeThreads == 0;
 }
 
+void QThreadPoolPrivate::clear()
+{
+    QMutexLocker locker(&mutex);
+    for (QList<QPair<QRunnable *, int> >::const_iterator it = queue.constBegin();
+         it != queue.constEnd(); ++it) {
+        QRunnable* r = it->first;
+        if (r->autoDelete() && !--r->ref)
+            delete r;
+    }
+    queue.clear();
+}
+
 /*!
     \internal
     Seaches for \a runnable in the queue, removes it from the queue and
@@ -609,6 +621,21 @@ bool QThreadPool::waitForDone(int msecs)
     return rc;
 }
 
+/*!
+    \since 5.2
+
+    Removes the runnables that are not yet started from the queue.
+    The runnables for which \l{QRunnable::autoDelete()}{runnable->autoDelete()}
+    returns true are deleted.
+
+    \sa start()
+*/
+void QThreadPool::clear()
+{
+    Q_D(QThreadPool);
+    d->clear();
+}
+
 QT_END_NAMESPACE
 
 #endif

src/corelib/thread/qthreadpool.h

@@ -83,6 +83,8 @@ public:
     void releaseThread();
 
     bool waitForDone(int msecs = -1);
+
+    void clear();
 };
 
 QT_END_NAMESPACE

src/corelib/thread/qthreadpool_p.h

@@ -83,6 +83,7 @@ public:
     void startThread(QRunnable *runnable = 0);
     void reset();
     bool waitForDone(int msecs);
+    void clear();
     void stealRunnable(QRunnable *);
 
     mutable QMutex mutex;

tests/auto/corelib/thread/qthreadpool/tst_qthreadpool.cpp

@@ -97,6 +97,7 @@ private slots:
     void priorityStart_data();
     void priorityStart();
     void waitForDone();
+    void clear();
     void waitForDoneTimeout();
     void destroyingWaitsForTasksToFinish();
     void stressTest();
@@ -855,6 +856,34 @@ void tst_QThreadPool::waitForDoneTimeout()
     QVERIFY(threadPool.waitForDone(400));
 }
 
+void tst_QThreadPool::clear()
+{
+    QSemaphore sem(0);
+    class BlockingRunnable : public QRunnable
+    {
+        public:
+            QSemaphore & sem;
+            BlockingRunnable(QSemaphore & sem) : sem(sem){}
+            void run()
+            {
+                sem.acquire();
+                count.ref();
+            }
+    };
+
+    QThreadPool threadPool;
+    threadPool.setMaxThreadCount(10);
+    int runs = 2 * threadPool.maxThreadCount();
+    count.store(0);
+    for (int i = 0; i <= runs; i++) {
+        threadPool.start(new BlockingRunnable(sem));
+    }
+    threadPool.clear();
+    sem.release(threadPool.maxThreadCount());
+    threadPool.waitForDone();
+    QCOMPARE(count.load(), threadPool.maxThreadCount());
+}
+
 void tst_QThreadPool::destroyingWaitsForTasksToFinish()
 {
     QTime total, pass;