QThreadPool::cancel() to remove individual jobs from the job queue.

[ChangeLog][QtCore][QThreadPool] Added QThreadPool::cancel() which allows
removing from the job queue a job that hasn't been started yet.

Change-Id: Ib8f1c1f32a34f5eec8338c641d820b928e470164
Reviewed-by: Nick Shaforostoff <shafff@ukr.net>
Reviewed-by: Olivier Goffart <ogoffart@woboq.com>

src/corelib/thread/qfutureinterface.cpp

@@ -292,7 +292,7 @@ void QFutureInterfaceBase::waitForResult(int resultIndex)
 
     // To avoid deadlocks and reduce the number of threads used, try to
     // run the runnable in the current thread.
-    d->pool()->d_func()->stealRunnable(d->runnable);
+    d->pool()->d_func()->stealAndRunRunnable(d->runnable);
 
     lock.relock();
 
@@ -313,7 +313,7 @@ void QFutureInterfaceBase::waitForFinished()
     lock.unlock();
 
     if (!alreadyFinished) {
-        d->pool()->d_func()->stealRunnable(d->runnable);
+        d->pool()->d_func()->stealAndRunRunnable(d->runnable);
 
         lock.relock();
 

src/corelib/thread/qthreadpool.cpp

@@ -311,14 +311,12 @@ void QThreadPoolPrivate::clear()
 /*!
     \internal
     Searches for \a runnable in the queue, removes it from the queue and
-    runs it if found. This function does not return until the runnable
-    has completed.
+    returns \c true if it was found in the queue
 */
-void QThreadPoolPrivate::stealRunnable(QRunnable *runnable)
+bool QThreadPoolPrivate::stealRunnable(QRunnable *runnable)
 {
     if (runnable == 0)
-        return;
-    bool found = false;
+        return false;
     {
         QMutexLocker locker(&mutex);
         QList<QPair<QRunnable *, int> >::iterator it = queue.begin();
@@ -326,17 +324,26 @@ void QThreadPoolPrivate::stealRunnable(QRunnable *runnable)
 
         while (it != end) {
             if (it->first == runnable) {
-                found = true;
                 queue.erase(it);
-                break;
+                return true;
             }
             ++it;
         }
     }
 
-    if (!found)
-        return;
+    return false;
+}
 
+    /*!
+     \internal
+     Searches for \a runnable in the queue, removes it from the queue and
+     runs it if found. This function does not return until the runnable
+     has completed.
+     */
+void QThreadPoolPrivate::stealAndRunRunnable(QRunnable *runnable)
+{
+    if (!stealRunnable(runnable))
+        return;
     const bool autoDelete = runnable->autoDelete();
     bool del = autoDelete && !--runnable->ref;
 
@@ -628,6 +635,25 @@ void QThreadPool::clear()
     d->clear();
 }
 
+/*!
+    \since 5.5
+
+    Removes the specified \a runnable from the queue if it is not yet started.
+    The runnables for which \l{QRunnable::autoDelete()}{runnable->autoDelete()}
+    returns \c true are deleted.
+
+    \sa start()
+*/
+void QThreadPool::cancel(QRunnable *runnable)
+{
+    Q_D(QThreadPool);
+    if (!d->stealRunnable(runnable))
+        return;
+    if (runnable->autoDelete() && !--runnable->ref) {
+        delete runnable;
+    }
+}
+
 QT_END_NAMESPACE
 
 #endif

src/corelib/thread/qthreadpool.h

@@ -77,6 +77,7 @@ public:
     bool waitForDone(int msecs = -1);
 
     void clear();
+    void cancel(QRunnable *runnable);
 };
 
 QT_END_NAMESPACE

src/corelib/thread/qthreadpool_p.h

@@ -76,7 +76,8 @@ public:
     void reset();
     bool waitForDone(int msecs);
     void clear();
-    void stealRunnable(QRunnable *);
+    bool stealRunnable(QRunnable *runnable);
+    void stealAndRunRunnable(QRunnable *runnable);
 
     mutable QMutex mutex;
     QSet<QThreadPoolThread *> allThreads;

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

@@ -92,6 +92,7 @@ private slots:
     void priorityStart();
     void waitForDone();
     void clear();
+    void cancel();
     void waitForDoneTimeout();
     void destroyingWaitsForTasksToFinish();
     void stressTest();
@@ -958,6 +959,56 @@ void tst_QThreadPool::clear()
     QCOMPARE(count.load(), threadPool.maxThreadCount());
 }
 
+void tst_QThreadPool::cancel()
+{
+    QSemaphore sem(0);
+    class BlockingRunnable : public QRunnable
+    {
+    public:
+        QSemaphore & sem;
+        int & dtorCounter;
+        int & runCounter;
+        int dummy;
+        BlockingRunnable(QSemaphore & s, int & c, int & r) : sem(s), dtorCounter(c), runCounter(r){}
+        ~BlockingRunnable(){dtorCounter++;}
+        void run()
+        {
+            runCounter++;
+            sem.acquire();
+            count.ref();
+        }
+    };
+    typedef BlockingRunnable* BlockingRunnablePtr;
+
+    QThreadPool threadPool;
+    threadPool.setMaxThreadCount(3);
+    int runs = 2 * threadPool.maxThreadCount();
+    BlockingRunnablePtr* runnables = new BlockingRunnablePtr[runs];
+    count.store(0);
+    int dtorCounter = 0;
+    int runCounter = 0;
+    for (int i = 0; i < runs; i++) {
+        runnables[i] = new BlockingRunnable(sem, dtorCounter, runCounter);
+        runnables[i]->setAutoDelete(i != 0 && i != (runs-1)); //one which will run and one which will not
+        threadPool.cancel(runnables[i]); //verify NOOP for jobs not in the queue
+        threadPool.start(runnables[i]);
+    }
+    for (int i = 0; i < runs; i++) {
+        threadPool.cancel(runnables[i]);
+    }
+    runnables[0]->dummy = 0; //valgrind will catch this if cancel() is crazy enough to delete currently running jobs
+    runnables[runs-1]->dummy = 0;
+    QCOMPARE(dtorCounter, runs-threadPool.maxThreadCount()-1);
+    sem.release(threadPool.maxThreadCount());
+    threadPool.waitForDone();
+    QCOMPARE(runCounter, threadPool.maxThreadCount());
+    QCOMPARE(count.load(), threadPool.maxThreadCount());
+    QCOMPARE(dtorCounter, runs-2);
+    delete runnables[0]; //if the pool deletes them then we'll get double-free crash
+    delete runnables[runs-1];
+    delete[] runnables;
+}
+
 void tst_QThreadPool::destroyingWaitsForTasksToFinish()
 {
     QTime total, pass;