QFutureInterface: allow to work with a QThreadPool != globalInstance()

Background:
It is often necessary/advisable to schedule tasks on thread pools !=
globalInstance(). As Herb Sutter writes in
http://www.drdobbs.com/parallel/use-thread-pools-correctly-keep-tasks-sh/216500409
and the Qt Training Material stresses, tasks you schedule on a (global)
thread pool should be non-blocking, which currently rules out using any of
the QtConcurrent functions for, say, file I/O.

Nonetheless it's often convenient to have thread pools also for file I/O, as
the thumbnail viewer exercise in the Qt Training Material shows. In this
case, you'd use a dedicated thead pool, leaving the global thread pool for
CPU-bound tasks.

Yet, none of the QtConcurrent functions allow to pick the QThreadPool
instance on which to schedule the work created with them.

This patch prepares for them to do so.

This is the first part of the forward-port of
https://qt.gitorious.org/qt/qt/merge_requests/1281.

Implement by using a new QThreadPool* member that defaults to nullptr,
and adding setThreadPool to set this member, then using it in lieu of
QThreadPool::globalInstance() everywhere.

I chose to leave m_pool == nullptr to mean globalInstance() to avoid
creating the global instance whenever a QFuture is created, even if the
future represents the result of a calculation not run on the global thread
pool.

[ChangeLog][QtCore][QFuture] Can now be used with any QThreadPool, not
just globalInstance().

Task-number: QTBUG-17220
Change-Id: I4e1dc18d55cf60141b2fa3d14e2d44a3e9e74858
Reviewed-by: Olivier Goffart <ogoffart@woboq.com>

src/corelib/thread/qfutureinterface.cpp

@@ -48,7 +48,6 @@
 
 #include <QtCore/qatomic.h>
 #include <QtCore/qthread.h>
-#include <QtCore/qthreadpool.h>
 #include <private/qthreadpool_p.h>
 
 QT_BEGIN_NAMESPACE
@@ -195,7 +194,7 @@ void QFutureInterfaceBase::waitForResume()
         return;
 
     // decrease active thread count since this thread will wait.
-    const ThreadPoolThreadReleaser releaser(QThreadPool::globalInstance());
+    const ThreadPoolThreadReleaser releaser(d->pool());
 
     d->pausedWaitCondition.wait(&d->m_mutex);
 }
@@ -301,7 +300,7 @@ void QFutureInterfaceBase::waitForResult(int resultIndex)
 
     // To avoid deadlocks and reduce the number of threads used, try to
     // run the runnable in the current thread.
-    QThreadPool::globalInstance()->d_func()->stealRunnable(d->runnable);
+    d->pool()->d_func()->stealRunnable(d->runnable);
 
     lock.relock();
 
@@ -322,7 +321,7 @@ void QFutureInterfaceBase::waitForFinished()
     lock.unlock();
 
     if (!alreadyFinished) {
-        QThreadPool::globalInstance()->d_func()->stealRunnable(d->runnable);
+        d->pool()->d_func()->stealRunnable(d->runnable);
 
         lock.relock();
 
@@ -364,6 +363,11 @@ void QFutureInterfaceBase::setRunnable(QRunnable *runnable)
     d->runnable = runnable;
 }
 
+void QFutureInterfaceBase::setThreadPool(QThreadPool *pool)
+{
+    d->m_pool = pool;
+}
+
 void QFutureInterfaceBase::setFilterMode(bool enable)
 {
     QMutexLocker locker(&d->m_mutex);
@@ -444,7 +448,7 @@ bool QFutureInterfaceBase::derefT() const
 QFutureInterfaceBasePrivate::QFutureInterfaceBasePrivate(QFutureInterfaceBase::State initialState)
     : refCount(1), m_progressValue(0), m_progressMinimum(0), m_progressMaximum(0),
       state(initialState),
-      manualProgress(false), m_expectedResultCount(0), runnable(0)
+      manualProgress(false), m_expectedResultCount(0), runnable(0), m_pool(0)
 {
     progressTime.invalidate();
 }

src/corelib/thread/qfutureinterface.h

@@ -54,6 +54,7 @@ QT_BEGIN_NAMESPACE
 
 
 template <typename T> class QFuture;
+class QThreadPool;
 class QFutureInterfaceBasePrivate;
 class QFutureWatcherBase;
 class QFutureWatcherBasePrivate;
@@ -85,6 +86,7 @@ public:
     void reportResultsReady(int beginIndex, int endIndex);
 
     void setRunnable(QRunnable *runnable);
+    void setThreadPool(QThreadPool *pool);
     void setFilterMode(bool enable);
     void setProgressRange(int minimum, int maximum);
     int progressMinimum() const;

src/corelib/thread/qfutureinterface_p.h

@@ -58,6 +58,7 @@
 #include <QtCore/qlist.h>
 #include <QtCore/qwaitcondition.h>
 #include <QtCore/qrunnable.h>
+#include <QtCore/qthreadpool.h>
 
 QT_BEGIN_NAMESPACE
 
@@ -169,6 +170,10 @@ public:
     QtPrivate::ExceptionStore m_exceptionStore;
     QString m_progressText;
     QRunnable *runnable;
+    QThreadPool *m_pool;
+
+    inline QThreadPool *pool() const
+    { return m_pool ? m_pool : QThreadPool::globalInstance(); }
 
     // Internal functions that does not change the mutex state.
     // The mutex must be locked when calling these.

tests/auto/corelib/thread/qfuture/tst_qfuture.cpp

@@ -47,6 +47,7 @@
 #include <qfuture.h>
 #include <qfuturewatcher.h>
 #include <qresultstore.h>
+#include <qthreadpool.h>
 #include <qexception.h>
 #include <private/qfutureinterface_p.h>
 
@@ -80,6 +81,7 @@ private slots:
     void exceptions();
     void nestedExceptions();
 #endif
+    void nonGlobalThreadPool();
 };
 
 void tst_QFuture::resultStore()
@@ -1444,6 +1446,53 @@ void tst_QFuture::nestedExceptions()
     QVERIFY(MyClass::caught);
 }
 
+void tst_QFuture::nonGlobalThreadPool()
+{
+    static Q_CONSTEXPR int Answer = 42;
+
+    struct UselessTask : QRunnable, QFutureInterface<int>
+    {
+        QFuture<int> start(QThreadPool *pool)
+        {
+            setRunnable(this);
+            setThreadPool(pool);
+            reportStarted();
+            QFuture<int> f = future();
+            pool->start(this);
+            return f;
+        }
+
+        void run() Q_DECL_OVERRIDE
+        {
+            const int ms = 100 + (qrand() % 100 - 100/2);
+            QThread::msleep(ms);
+            reportResult(Answer);
+            reportFinished();
+        }
+    };
+
+    QThreadPool pool;
+
+    const int numTasks = QThread::idealThreadCount();
+
+    QVector<QFuture<int> > futures;
+    futures.reserve(numTasks);
+
+    for (int i = 0; i < numTasks; ++i)
+        futures.push_back((new UselessTask)->start(&pool));
+
+    QVERIFY(!pool.waitForDone(0)); // pool is busy (meaning our tasks did end up executing there)
+
+    QVERIFY(pool.waitForDone(10000)); // max sleep time in UselessTask::run is 150ms, so 10s should be enough
+                                      // (and the call returns as soon as all tasks finished anyway, so the
+                                      // maximum wait time only matters when the test fails)
+
+    Q_FOREACH (const QFuture<int> &future, futures) {
+        QVERIFY(future.isFinished());
+        QCOMPARE(future.result(), Answer);
+    }
+}
+
 #endif // QT_NO_EXCEPTIONS
 
 QTEST_MAIN(tst_QFuture)