Fix race condition in QThreadPool::clear

Since we drop the lock while deleting threads, we need to handle
the queue possibly being accessed and changed by the pool threads
while clear() is running.

Fixes: QTBUG-87092
Change-Id: I7611edab90520454278502a58621e299f9cd1f6e
Reviewed-by: Sona Kurazyan <sona.kurazyan@qt.io>
(cherry picked from commit fe36d47b371b71ad5fec30d4b5d7bf0baa0205ea)
Reviewed-by: Allan Sandfeld Jensen <allan.jensen@qt.io>

1 files changed, 26 insertions, 0 deletions

diff --git a/tests/auto/corelib/thread/qthreadpool/tst_qthreadpool.cpp b/tests/auto/corelib/thread/qthreadpool/tst_qthreadpool.cpp
index 49ce918caf..cd245030db 100644
--- a/tests/auto/corelib/thread/qthreadpool/tst_qthreadpool.cpp
+++ b/tests/auto/corelib/thread/qthreadpool/tst_qthreadpool.cpp
@@ -93,6 +93,7 @@ private slots:
     void priorityStart();
     void waitForDone();
     void clear();
+    void clearWithAutoDelete();
 #if QT_DEPRECATED_SINCE(5, 9)
     void cancel();
 #endif
@@ -976,6 +977,31 @@ void tst_QThreadPool::clear()
     QCOMPARE(count.loadRelaxed(), threadPool.maxThreadCount());
 }
 
+void tst_QThreadPool::clearWithAutoDelete()
+{
+    class MyRunnable : public QRunnable
+    {
+    public:
+        MyRunnable() {}
+        void run() override { QThread::usleep(30); }
+    };
+
+    QThreadPool threadPool;
+    threadPool.setMaxThreadCount(4);
+    const int loopCount = 20;
+    const int batchSize = 500;
+    // Should not crash see QTBUG-87092
+    for (int i = 0; i < loopCount; i++) {
+        threadPool.clear();
+        for (int j = 0; j < batchSize; j++) {
+            auto *runnable = new MyRunnable();
+            runnable->setAutoDelete(true);
+            threadPool.start(runnable);
+        }
+    }
+    QVERIFY(threadPool.waitForDone());
+}
+
 #if QT_DEPRECATED_SINCE(5, 9)
 void tst_QThreadPool::cancel()
 {