Add a safer way to use QThreadPool::reserveThread

Add startOnReservedThread that specifically releases a reserved thread
and uses it atomically for a given task. This can make a positive
number of reserved threads work.

Change-Id: I4bd1dced24bb46fcb365f12cbc9c7905dc66cdf1
Reviewed-by: David Faure <david.faure@kdab.com>

src/corelib/thread/qthreadpool.cpp

@@ -770,6 +770,57 @@ void QThreadPool::releaseThread()
     d->tryToStartMoreThreads();
 }
 
+/*!
+    Releases a thread previously reserved with reserveThread() and uses it
+    to run \a runnable.
+
+    Note that the thread pool takes ownership of the \a runnable if
+    \l{QRunnable::autoDelete()}{runnable->autoDelete()} returns \c true,
+    and the \a runnable will be deleted automatically by the thread
+    pool after the \l{QRunnable::run()}{runnable->run()} returns. If
+    \l{QRunnable::autoDelete()}{runnable->autoDelete()} returns \c false,
+    ownership of \a runnable remains with the caller. Note that
+    changing the auto-deletion on \a runnable after calling this
+    functions results in undefined behavior.
+
+    \note Calling this when no threads are reserved results in
+    undefined behavior.
+
+    \since 6.3
+    \sa reserveThread(), start()
+*/
+void QThreadPool::startOnReservedThread(QRunnable *runnable)
+{
+    if (!runnable)
+        return releaseThread();
+
+    Q_D(QThreadPool);
+    QMutexLocker locker(&d->mutex);
+    Q_ASSERT(d->reservedThreads > 0);
+    --d->reservedThreads;
+
+    if (!d->tryStart(runnable)) {
+        // This can only happen if we reserved max threads,
+        // and something took the one minimum thread.
+        d->enqueueTask(runnable, INT_MAX);
+    }
+}
+
+/*!
+    \overload
+    \since 6.3
+
+    Releases a thread previously reserved with reserveThread() and uses it
+    to run \a functionToRun.
+*/
+void QThreadPool::startOnReservedThread(std::function<void()> functionToRun)
+{
+    if (!functionToRun)
+        return releaseThread();
+
+    startOnReservedThread(QRunnable::create(std::move(functionToRun)));
+}
+
 /*!
     Waits up to \a msecs milliseconds for all threads to exit and removes all
     threads from the thread pool. Returns \c true if all threads were removed;

src/corelib/thread/qthreadpool.h

@@ -75,6 +75,9 @@ public:
     void start(std::function<void()> functionToRun, int priority = 0);
     bool tryStart(std::function<void()> functionToRun);
 
+    void startOnReservedThread(QRunnable *runnable);
+    void startOnReservedThread(std::function<void()> functionToRun);
+
     int expiryTimeout() const;
     void setExpiryTimeout(int expiryTimeout);
 

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

@@ -89,6 +89,7 @@ private slots:
     void releaseThread_data();
     void releaseThread();
     void reserveAndStart();
+    void reserveAndStart2();
     void releaseAndBlock();
     void start();
     void tryStart();
@@ -729,17 +730,66 @@ void tst_QThreadPool::reserveAndStart() // QTBUG-21051
     // start() will wake up the waiting thread.
     threadpool->start(&task);
     QTRY_COMPARE(threadpool->activeThreadCount(), 2);
+    QTRY_COMPARE(task.count.loadRelaxed(), 2);
+    WaitingTask task2;
+    // startOnReservedThread() will try to take the reserved task, but end up waiting instead
+    threadpool->startOnReservedThread(&task2);
+    QTRY_COMPARE(threadpool->activeThreadCount(), 1);
     task.waitForStarted.acquire();
     task.waitBeforeDone.release();
-    QTRY_COMPARE(task.count.loadRelaxed(), 2);
     QTRY_COMPARE(threadpool->activeThreadCount(), 1);
+    task2.waitForStarted.acquire();
+    task2.waitBeforeDone.release();
 
-    threadpool->releaseThread();
     QTRY_COMPARE(threadpool->activeThreadCount(), 0);
 
     threadpool->setMaxThreadCount(savedLimit);
 }
 
+void tst_QThreadPool::reserveAndStart2()
+{
+    class WaitingTask : public QRunnable
+    {
+    public:
+        QSemaphore waitBeforeDone;
+
+        WaitingTask() { setAutoDelete(false); }
+
+        void run() override
+        {
+            waitBeforeDone.acquire();
+        }
+    };
+
+    // Set up
+    QThreadPool *threadpool = QThreadPool::globalInstance();
+    int savedLimit = threadpool->maxThreadCount();
+    threadpool->setMaxThreadCount(2);
+
+    // reserve
+    threadpool->reserveThread();
+
+    // start two task, to get a running thread and one queued
+    WaitingTask task1, task2, task3;
+    threadpool->start(&task1);
+    // one running thread, one reserved:
+    QCOMPARE(threadpool->activeThreadCount(), 2);
+    // task2 starts queued
+    threadpool->start(&task2);
+    QCOMPARE(threadpool->activeThreadCount(), 2);
+    // startOnReservedThread() will take the reserved thread however, bypassing the queue
+    threadpool->startOnReservedThread(&task3);
+    // two running threads, none reserved:
+    QCOMPARE(threadpool->activeThreadCount(), 2);
+    task3.waitBeforeDone.release();
+    // task3 can finish even if all other tasks are blocking
+    // then task2 will use the previously reserved thread
+    task2.waitBeforeDone.release();
+    QTRY_COMPARE(threadpool->activeThreadCount(), 1);
+    task1.waitBeforeDone.release();
+    QTRY_COMPARE(threadpool->activeThreadCount(), 0);
+}
+
 void tst_QThreadPool::releaseAndBlock()
 {
     class WaitingTask : public QRunnable