Cocoa integration: do not use released session

QTBUG_10735_crashWithDialog started to show flakyness recently - it crashes,
but not every time (which fits the definition of UB perfectly).

While the test itself is doing weird things and puts our event dispatcher
into a weird state, our API allows to:

1. Using QDialog to enter event loop (with runModalSession under the hood), then ...
2. to call from a slot (e.g. timer-attached) QApplication::closeAllWindows() while ...
3. we are still inside that special loop and using the 'session' object, thus ...
4. on the next iteration with [NSApp runModalSession:session] we'll re-use already released
   session (released by endModalSession which in turn was called indirectly by closeAllWindows).

And Cocoa gives us a warning/hint: "Use of freed session detected. Do not call
runModalSession: after calling endModalSesion:."

Task-number: QTBUG-62589
Change-Id: Ie651cee1fba43cfd2b0fc44af5eddc5fd52e2907
Reviewed-by: Tor Arne Vestbø <tor.arne.vestbo@qt.io>
Reviewed-by: Morten Johan Sørvig <morten.sorvig@qt.io>
Reviewed-by: Gabriel de Dietrich <gabriel.dedietrich@qt.io>

1 files changed, 11 insertions, 1 deletions

diff --git a/src/plugins/platforms/cocoa/qcocoaeventdispatcher.mm b/src/plugins/platforms/cocoa/qcocoaeventdispatcher.mm
index d2f985ec87..b22f1b1f54 100644
--- a/src/plugins/platforms/cocoa/qcocoaeventdispatcher.mm
+++ b/src/plugins/platforms/cocoa/qcocoaeventdispatcher.mm
@@ -401,8 +401,18 @@ bool QCocoaEventDispatcher::processEvents(QEventLoop::ProcessEventsFlags flags)
             // [NSApp run], which is the normal code path for cocoa applications.
             if (NSModalSession session = d->currentModalSession()) {
                 QBoolBlocker execGuard(d->currentExecIsNSAppRun, false);
-                while ([NSApp runModalSession:session] == NSModalResponseContinue && !d->interrupt)
+                while ([NSApp runModalSession:session] == NSModalResponseContinue && !d->interrupt) {
                     qt_mac_waitForMoreEvents(NSModalPanelRunLoopMode);
+                    if (session != d->currentModalSessionCached) {
+                        // It's possible to release the current modal session
+                        // while we are in this loop, for example, by closing all
+                        // windows from a slot via QApplication::closeAllWindows.
+                        // In this case we cannot use 'session' anymore. A warning
+                        // from Cocoa is: "Use of freed session detected. Do not
+                        // call runModalSession: after calling endModalSesion:."
+                        break;
+                    }
+                }
 
                 if (!d->interrupt && session == d->currentModalSessionCached) {
                     // Someone called [NSApp stopModal:] from outside the event