Debugger: Add safety net to avoid infinite lookups

Do not lookup the same item twice without intermediate stepping.

Dumpers could announce the existence of children but when asked
for them bail out or produced similar inconsistent output.
Better not depend on it.

Task-number: QTCREATORBUG-15352
Change-Id: I38532d08bb438b12b6eb202a06ff610670b1069f
Reviewed-by: Eike Ziller <eike.ziller@qt.io>
Reviewed-by: Christian Stenger <christian.stenger@theqtcompany.com>

src/plugins/debugger/debuggerengine.cpp

@@ -282,6 +282,7 @@ public slots:
 
     void resetLocation()
     {
+        m_lookupRequests.clear();
         m_locationTimer.stop();
         m_locationMark.reset();
         m_stackHandler.resetLocation();
@@ -337,6 +338,9 @@ public:
 
     Utils::FileInProjectFinder m_fileFinder;
     QByteArray m_qtNamespace;
+
+    // Safety net to avoid infinite lookups.
+    QSet<QByteArray> m_lookupRequests; // FIXME: Integrate properly.
 };
 
 
@@ -2011,6 +2015,23 @@ bool DebuggerEngine::canHandleToolTip(const DebuggerToolTipContext &context) con
 
 void DebuggerEngine::updateItem(const QByteArray &iname)
 {
+    if (d->m_lookupRequests.contains(iname)) {
+        showMessage(QString::fromLatin1("IGNORING REPEATED REQUEST TO EXPAND " + iname));
+        WatchHandler *handler = watchHandler();
+        WatchItem *item = handler->findItem(iname);
+        if (!item->hasChildren()) {
+            handler->notifyUpdateStarted({iname});
+            item->setValue(decodeData({}, "notaccessible"));
+            item->setHasChildren(false);
+            item->outdated = false;
+            item->update();
+            handler->notifyUpdateFinished();
+            return;
+        }
+        // We could legitimately end up here after expanding + closing + re-expaning an item.
+    }
+    d->m_lookupRequests.insert(iname);
+
     UpdateParameters params;
     params.partialVariable = iname;
     doUpdateLocals(params);