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.

Pick-to: 5.15
Fixes: QTBUG-87092
Change-Id: I7611edab90520454278502a58621e299f9cd1f6e
Reviewed-by: Sona Kurazyan <sona.kurazyan@qt.io>

1 files changed, 3 insertions, 3 deletions

diff --git a/src/corelib/thread/qthreadpool.cpp b/src/corelib/thread/qthreadpool.cpp
index 1b417aa8be..5765bf6b09 100644
--- a/src/corelib/thread/qthreadpool.cpp
+++ b/src/corelib/thread/qthreadpool.cpp
@@ -325,7 +325,8 @@ bool QThreadPoolPrivate::waitForDone(int msecs)
 void QThreadPoolPrivate::clear()
 {
     QMutexLocker locker(&mutex);
-    for (QueuePage *page : qAsConst(queue)) {
+    while (!queue.isEmpty()) {
+        auto *page = queue.takeLast();
         while (!page->isFinished()) {
             QRunnable *r = page->pop();
             if (r && r->autoDelete()) {
@@ -335,9 +336,8 @@ void QThreadPoolPrivate::clear()
                 locker.relock();
             }
         }
+        delete page;
     }
-    qDeleteAll(queue);
-    queue.clear();
 }
 
 /*!