QThreadPool: Fix restarting of expired threads

Ensure that expired threads have actually finished before attempting
to restart them. Calling start() on a thread that is not yet finished
does nothing.

Add a regression test into tst_qthreadpool that attempts to trigger
reuse of expired threads and verifies that all submitted tasks
execute.

Fixes: QTBUG-72872
Pick-to: 6.2
Change-Id: I2109b628b8a4e91491115dc56aebf3eb249646b5
Reviewed-by: Allan Sandfeld Jensen <allan.jensen@qt.io>
Reviewed-by: Edward Welbourne <edward.welbourne@qt.io>

1 files changed, 5 insertions, 0 deletions

diff --git a/src/corelib/thread/qthreadpool.cpp b/src/corelib/thread/qthreadpool.cpp
index 9185554b1f..4d7d7a2250 100644
--- a/src/corelib/thread/qthreadpool.cpp
+++ b/src/corelib/thread/qthreadpool.cpp
@@ -194,6 +194,11 @@ bool QThreadPoolPrivate::tryStart(QRunnable *task)
         ++activeThreads;
 
         thread->runnable = task;
+
+        // Ensure that the thread has actually finished, otherwise the following
+        // start() has no effect.
+        thread->wait();
+        Q_ASSERT(thread->isFinished());
         thread->start(threadPriority);
         return true;
     }