Add support to set thread priority to QThreadPool

Currently, QThreadPool's generated threads inherit the priority from the
thread they are created and that cannot be changed. This merge request
adds a property to QThreadPool so that the priority of the threads can
be different.

The default behavior does not change.

[ChangeLog][QtCore][QThreadPool] QThreadPool can now be configured to
use a different thread priority when creating new threads than the one
it inherits from the thread it was created in. This will only apply to
the threads started after the property is changed.

Fixes: QTBUG-3481
Change-Id: Ic98d4312d055a3357771abb656516ebd0715918d
Reviewed-by: Thiago Macieira <thiago.macieira@intel.com>

src/corelib/thread/qthreadpool.cpp

@@ -194,7 +194,7 @@ bool QThreadPoolPrivate::tryStart(QRunnable *task)
         ++activeThreads;
 
         thread->runnable = task;
-        thread->start();
+        thread->start(threadPriority);
         return true;
     }
 
@@ -265,7 +265,7 @@ void QThreadPoolPrivate::startThread(QRunnable *runnable)
     ++activeThreads;
 
     thread->runnable = runnable;
-    thread.take()->start();
+    thread.take()->start(threadPriority);
 }
 
 /*!
@@ -698,6 +698,32 @@ uint QThreadPool::stackSize() const
     return d->stackSize;
 }
 
+/*! \property QThreadPool::threadPriority
+    \brief the thread priority for new worker threads.
+
+    The value of the property is only used when the thread pool starts
+    new threads. Changing it has no effect for already running threads.
+
+    The default value is QThread::InheritPriority, which makes QThread
+    use the same priority as the one the QThreadPool object lives in.
+
+    \sa QThread::ThreadPriority
+
+    \since 6.2
+*/
+
+void QThreadPool::setThreadPriority(QThread::Priority priority)
+{
+    Q_D(QThreadPool);
+    d->threadPriority = priority;
+}
+
+QThread::Priority QThreadPool::threadPriority() const
+{
+    Q_D(const QThreadPool);
+    return d->threadPriority;
+}
+
 /*!
     Releases a thread previously reserved by a call to reserveThread().
 

src/corelib/thread/qthreadpool.h

@@ -60,6 +60,7 @@ class Q_CORE_EXPORT QThreadPool : public QObject
     Q_PROPERTY(int maxThreadCount READ maxThreadCount WRITE setMaxThreadCount)
     Q_PROPERTY(int activeThreadCount READ activeThreadCount)
     Q_PROPERTY(uint stackSize READ stackSize WRITE setStackSize)
+    Q_PROPERTY(QThread::Priority threadPriority READ threadPriority WRITE setThreadPriority)
     friend class QFutureInterfaceBase;
 
 public:
@@ -85,6 +86,9 @@ public:
     void setStackSize(uint stackSize);
     uint stackSize() const;
 
+    void setThreadPriority(QThread::Priority priority);
+    QThread::Priority threadPriority() const;
+
     void reserveThread();
     void releaseThread();
 

src/corelib/thread/qthreadpool_p.h

@@ -178,6 +178,7 @@ public:
     int reservedThreads = 0;
     int activeThreads = 0;
     uint stackSize = 0;
+    QThread::Priority threadPriority = QThread::InheritPriority;
 };
 
 QT_END_NAMESPACE

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

@@ -75,6 +75,7 @@ private slots:
     void singleton();
     void destruction();
     void threadRecycling();
+    void threadPriority();
     void expiryTimeout();
     void expiryTimeoutRace();
 #ifndef QT_NO_EXCEPTIONS
@@ -323,6 +324,25 @@ void tst_QThreadPool::threadRecycling()
     QCOMPARE(thread2, thread3);
 }
 
+/*
+    Test that the thread priority from the thread created by the pool matches
+    the one configured on the pool.
+*/
+void tst_QThreadPool::threadPriority()
+{
+    QThread::Priority priority = QThread::HighPriority;
+    QThreadPool threadPool;
+    threadPool.setThreadPriority(priority);
+
+    threadPool.start(new ThreadRecorderTask());
+    threadRecyclingSemaphore.acquire();
+    QThread *thread = recycledThread;
+
+    QTest::qSleep(100);
+
+    QCOMPARE(thread->priority(), priority);
+}
+
 class ExpiryTimeoutTask : public QRunnable
 {
 public: