Fix QRunnable::ref use in QThreadPool

The reference counter could only ever be -1, 0 or 1,
as an autoDeleted QRunnable can only be in a single
thread pool.

This commits keeps the reference counting for now,
but asserts sanity, simplifies locking and fixes a
leak.

Pick-To: 5.15
Fixes: QTBUG-20251
Fixes: QTBUG-65486
Change-Id: I4de44e9a4e3710225971d1eab8f2e332513f75ad
Reviewed-by: Sona Kurazyan <sona.kurazyan@qt.io>

src/corelib/thread/qrunnable.h

@@ -47,7 +47,7 @@ QT_BEGIN_NAMESPACE
 
 class Q_CORE_EXPORT QRunnable
 {
-    int ref;
+    int ref; // Qt6: Make this a bool, or make autoDelete() virtual.
 
     friend class QThreadPool;
     friend class QThreadPoolPrivate;

src/corelib/thread/qthreadpool.cpp

@@ -88,7 +88,8 @@ void QThreadPoolThread::run()
 
         do {
             if (r) {
-                const bool autoDelete = r->autoDelete();
+                const bool del = r->autoDelete();
+                Q_ASSERT(!del || r->ref == 1);
 
 
                 // run the task
@@ -106,10 +107,10 @@ void QThreadPoolThread::run()
                     throw;
                 }
 #endif
-                locker.relock();
 
-                if (autoDelete && !--r->ref)
+                if (del)
                     delete r;
+                locker.relock();
             }
 
             // if too many threads are active, expire this thread
@@ -193,8 +194,6 @@ bool QThreadPoolPrivate::tryStart(QRunnable *task)
 
         ++activeThreads;
 
-        if (task->autoDelete())
-            ++task->ref;
         thread->runnable = task;
         thread->start();
         return true;
@@ -213,9 +212,6 @@ inline bool comparePriority(int priority, const QueuePage *p)
 void QThreadPoolPrivate::enqueueTask(QRunnable *runnable, int priority)
 {
     Q_ASSERT(runnable != nullptr);
-    if (runnable->autoDelete())
-        ++runnable->ref;
-
     for (QueuePage *page : qAsConst(queue)) {
         if (page->priority() == priority && !page->isFull()) {
             page->push(runnable);
@@ -269,8 +265,6 @@ void QThreadPoolPrivate::startThread(QRunnable *runnable)
     allThreads.insert(thread.data());
     ++activeThreads;
 
-    if (runnable->autoDelete())
-        ++runnable->ref;
     thread->runnable = runnable;
     thread.take()->start();
 }
@@ -334,8 +328,12 @@ void QThreadPoolPrivate::clear()
     for (QueuePage *page : qAsConst(queue)) {
         while (!page->isFinished()) {
             QRunnable *r = page->pop();
-            if (r && r->autoDelete() && !--r->ref)
+            if (r && r->autoDelete()) {
+                Q_ASSERT(r->ref == 1);
+                locker.unlock();
                 delete r;
+                locker.relock();
+            }
         }
     }
     qDeleteAll(queue);
@@ -365,19 +363,19 @@ bool QThreadPool::tryTake(QRunnable *runnable)
 
     if (runnable == nullptr)
         return false;
-    {
-        QMutexLocker locker(&d->mutex);
-
-        for (QueuePage *page : qAsConst(d->queue)) {
-            if (page->tryTake(runnable)) {
-                if (page->isFinished()) {
-                    d->queue.removeOne(page);
-                    delete page;
-                }
-                if (runnable->autoDelete())
-                    --runnable->ref; // undo ++ref in start()
-                return true;
+
+    QMutexLocker locker(&d->mutex);
+    for (QueuePage *page : qAsConst(d->queue)) {
+        if (page->tryTake(runnable)) {
+            if (page->isFinished()) {
+                d->queue.removeOne(page);
+                delete page;
             }
+            if (runnable->autoDelete()) {
+                Q_ASSERT(runnable->ref == 1);
+                --runnable->ref; // undo ++ref in start()
+            }
+            return true;
         }
     }
 
@@ -395,11 +393,12 @@ void QThreadPoolPrivate::stealAndRunRunnable(QRunnable *runnable)
     Q_Q(QThreadPool);
     if (!q->tryTake(runnable))
         return;
-    const bool del = runnable->autoDelete() && !runnable->ref; // tryTake already deref'ed
+    const bool del = runnable->autoDelete();
 
     runnable->run();
 
     if (del) {
+        Q_ASSERT(runnable->ref == 0); // tryTake already deref'ed
         delete runnable;
     }
 }
@@ -503,6 +502,11 @@ void QThreadPool::start(QRunnable *runnable, int priority)
 
     Q_D(QThreadPool);
     QMutexLocker locker(&d->mutex);
+    if (runnable->autoDelete()) {
+        Q_ASSERT(runnable->ref == 0);
+        ++runnable->ref;
+    }
+
     if (!d->tryStart(runnable)) {
         d->enqueueTask(runnable, priority);
 
@@ -548,9 +552,23 @@ bool QThreadPool::tryStart(QRunnable *runnable)
     if (!runnable)
         return false;
 
+    if (runnable->autoDelete()) {
+        Q_ASSERT(runnable->ref == 0);
+        ++runnable->ref;
+    }
+
     Q_D(QThreadPool);
     QMutexLocker locker(&d->mutex);
-    return d->tryStart(runnable);
+    if (d->tryStart(runnable))
+        return true;
+
+    // Undo the reference above as we did not start the runnable and
+    // take over ownership.
+    if (runnable->autoDelete()) {
+        --runnable->ref;
+        Q_ASSERT(runnable->ref == 0);
+    }
+    return false;
 }
 
 /*!