QThread/Unix: make QThreadPrivate::finish() be called much later

We need it to run after all the thread-local destructors have run, to
ensure that some user code hasn't run after QThreadPrivate::finish() has
finished. We achieve that by making it get called from a thread-local
destructor itself, in the form of a qScopeGuard.

This ought to have been done since C++11 thread_local with non-trivial
destructors became available. However, it only started showing up after
commit 4a93285b166ceceaea2e10c8fc6a254d2f7093b9 began using thread_local
inside Qt itself. The visible symptom was that QThreadPrivate::finish()
had already destroyed the thread's event dispatcher, but some user code
ran later and expected it to still exist (or, worse, recreated it, via
QEventLoop → QThreadData::ensureEventDispatcher).

Fixes: QTBUG-117996
Pick-to: 6.7
Change-Id: I8f3ce163ccc5408cac39fffd178d682e5bfa6955
Reviewed-by: Volker Hilsheimer <volker.hilsheimer@qt.io>

1 files changed, 7 insertions, 7 deletions

diff --git a/src/corelib/thread/qthread_unix.cpp b/src/corelib/thread/qthread_unix.cpp
index 617a5ebf28..556f05018f 100644
--- a/src/corelib/thread/qthread_unix.cpp
+++ b/src/corelib/thread/qthread_unix.cpp
@@ -282,8 +282,12 @@ void *QThreadPrivate::start(void *arg)
 #ifdef PTHREAD_CANCEL_DISABLE
     pthread_setcancelstate(PTHREAD_CANCEL_DISABLE, nullptr);
 #endif
-    pthread_cleanup_push(QThreadPrivate::finish, arg);
-
+#if !defined(Q_OS_QNX)
+    // On QNX, calling finish() from a thread_local destructor causes the C
+    // library to hang.
+    static thread_local
+#endif
+            auto cleanup = qScopeGuard([=] { finish(arg); });
     terminate_on_exception([&] {
         QThread *thr = reinterpret_cast<QThread *>(arg);
         QThreadData *data = QThreadData::get2(thr);
@@ -328,11 +332,7 @@ void *QThreadPrivate::start(void *arg)
         thr->run();
     });
 
-    // This pop runs finish() below. It's outside the try/catch (and has its
-    // own try/catch) to prevent finish() to be run in case an exception is
-    // thrown.
-    pthread_cleanup_pop(1);
-
+    // The qScopeGuard above call runs finish() below.
     return nullptr;
 }