a11y: Do not refer to destructed QObject in Windows bridge

The crash could happen if a QWidget in the UI got deleted, and
the AT client later tried to access the widget through the cache
(qAccessibleRecentSentEvents()).

Solution: Use a QPointer as a guard.

Task-number: QTBUG-26187
Change-Id: I41eab158989fddfa147309b6bd91ac0cd1fe7b1a
Reviewed-by: Frederik Gladhorn <frederik.gladhorn@digia.com>

2 files changed, 32 insertions, 10 deletions

diff --git a/src/gui/accessible/qaccessible_win.cpp b/src/gui/accessible/qaccessible_win.cpp
index 678a528..f9f9f04 100644
--- a/src/gui/accessible/qaccessible_win.cpp
+++ b/src/gui/accessible/qaccessible_win.cpp
@@ -49,6 +49,7 @@
 #include "qsettings.h"
 #include <QtCore/qmap.h>
 #include <QtCore/qpair.h>
+#include <QtCore/qpointer.h>
 #include <QtGui/qgraphicsitem.h>
 #include <QtGui/qgraphicsscene.h>
 #include <QtGui/qgraphicsview.h>
@@ -265,7 +266,7 @@ void showDebug(const char* funcName, const QAccessibleInterface *iface)
 #endif
 
 // This stuff is used for widgets/items with no window handle:
-typedef QMap<int, QPair<QObject*,int> > NotifyMap;
+typedef QMap<int, QPair<QPointer<QObject>, int> > NotifyMap;
 Q_GLOBAL_STATIC(NotifyMap, qAccessibleRecentSentEvents)
 static int eventNum = 0;
 
@@ -414,13 +415,26 @@ void QAccessible::updateAccessibility(QObject *o, int who, Event reason)
     if (reason != MenuCommand) { // MenuCommand is faked
         if (w != o) {
             // See comment "SENDING EVENTS TO OBJECTS WITH NO WINDOW HANDLE"
-            eventNum %= 50;              //[0..49]
-            int eventId = - eventNum - 1;
-
-            qAccessibleRecentSentEvents()->insert(eventId, qMakePair(o,who));
-            ptrNotifyWinEvent(reason, wid, OBJID_CLIENT, eventId );
-
-            ++eventNum;
+            if (reason != QAccessible::ObjectDestroyed) {
+                /* In some rare occasions, the server (Qt) might get a ::get_accChild call with a
+                   childId that references an entry in the cache where there was a dangling
+                   QObject-pointer. Previously we crashed on this.
+
+                   There is no point in actually notifying the AT client that the object got destroyed,
+                   because the AT client won't query for get_accChild if the event is ObjectDestroyed
+                   anyway, and we have no other way of mapping the eventId argument to the actual
+                   child/descendant object. (Firefox seems to simply completely ignore
+                   EVENT_OBJECT_DESTROY).
+
+                   We therefore guard each QObject in the cache with a QPointer, and only notify the AT
+                   client if the type is not ObjectDestroyed.
+                */
+                eventNum %= 50;              //[0..49]
+                int eventId = - eventNum - 1;
+                qAccessibleRecentSentEvents()->insert(eventId, qMakePair(QPointer<QObject>(o), who));
+                ptrNotifyWinEvent(reason, wid, OBJID_CLIENT, eventId );
+                ++eventNum;
+            }
         } else {
             ptrNotifyWinEvent(reason, wid, OBJID_CLIENT, who);
         }
@@ -603,8 +617,8 @@ HRESULT STDMETHODCALLTYPE QWindowsEnumerate::Skip(unsigned long celt)
 struct AccessibleElement {
     AccessibleElement(int entryId, QAccessibleInterface *accessible) {
         if (entryId < 0) {
-            QPair<QObject*, int> ref = qAccessibleRecentSentEvents()->value(entryId);
-            iface = QAccessible::queryAccessibleInterface(ref.first);
+            QPair<QPointer<QObject>, int> ref = qAccessibleRecentSentEvents()->value(entryId);
+            iface = QAccessible::queryAccessibleInterface(ref.first.data());
             entry = ref.second;
             cleanupInterface = true;
         } else {
diff --git a/tests/auto/qaccessibility/tst_qaccessibility.cpp b/tests/auto/qaccessibility/tst_qaccessibility.cpp
index d47eb06..39207ec 100644
--- a/tests/auto/qaccessibility/tst_qaccessibility.cpp
+++ b/tests/auto/qaccessibility/tst_qaccessibility.cpp
@@ -436,7 +436,15 @@ void tst_QAccessibility::eventTest()
     button->hide();
     QVERIFY_EVENT(button, 0, QAccessible::ObjectHide);
 
+    // Destroy a visible widget
+    QTestAccessibility::clearEvents();
+    button->show();
+    QVERIFY_EVENT(button, 0, QAccessible::ObjectShow);
+
     delete button;
+
+    QVERIFY_EVENT(button, 0, QAccessible::ObjectHide);
+    QVERIFY_EVENT(button, 0, QAccessible::ObjectDestroyed);
 }
 
 void tst_QAccessibility::customWidget()