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>

3 files changed, 30 insertions, 0 deletions

diff --git a/src/corelib/thread/qthreadpool.cpp b/src/corelib/thread/qthreadpool.cpp
index a7d52f9652..bee6790705 100644
--- a/src/corelib/thread/qthreadpool.cpp
+++ b/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
diff --git a/src/corelib/thread/qthreadpool.h b/src/corelib/thread/qthreadpool.h
index ffc16dedbe..22a42c2272 100644
--- a/src/corelib/thread/qthreadpool.h
+++ b/src/corelib/thread/qthreadpool.h
@@ -83,6 +83,8 @@ public:
     void releaseThread();
 
     bool waitForDone(int msecs = -1);
+
+    void clear();
 };
 
 QT_END_NAMESPACE
diff --git a/src/corelib/thread/qthreadpool_p.h b/src/corelib/thread/qthreadpool_p.h
index 754d754e74..ba77f7e57c 100644
--- a/src/corelib/thread/qthreadpool_p.h
+++ b/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;