Make qmltest/SignalSpy.qml aware of recursive qtest_update() calls

Apparently looking up a property can cause it to be evaluated (and its
signal handler called directly). From the debugging the following is
visible:

1) assume signalName is set first and target has a pending binding (not
   evaluated yet)
2) once signalName setter is triggered, it calls the relevant signal
   handler - onSignalNameChanged, which subsequently calls qtest_update()
3) in qtest_update():
  a. we first check whether there's any old state
     "if (qtest_prevTarget != null)" -> there's none yet, because we entered
     this function for the first time ever -> jump to:
     "if (target != null && signalName != "")"
  b. once at "target != null" the engine attempts to get the value of the
     `target` property. if the property `target` happens to have an unevaluated
     binding, it is evaluated immediately.
[the logic is in QObjectWrapper::getQmlProperty() which calls
 QObjectWrapper::getProperty() and that one calls
 QQmlData::flushPendingBinding()]
  c. the binding evaluation causes onTargetChanged signal handler to trigger
     which in turn again enters qtest_update
4) in the "recursive" call of qtest_update():
  a. same as 3.a.
  b. target is now evaluated and signalName is evaluated as well (due to
     being already set at step 2)
  c. "if (target != null && signalName != "")" succeeds and we proceed to
     connect to spy.qtest_activated
  d. we return from this function and get back to the outer qtest_update()
5) we are now back at 3.c in qtest_update():
  a. target is evaluated, signalName is known, so
     "if (target != null && signalName != "")" succeeds -> again, we proceed
     to connect to spy.qtest_activated (for the second time now - see 4.c)
  b. we return from qtest_update() (the outer one) back to onSignalNameChanged
6) the end. bottom line:
  - two signals were connected to target[signalName]
  - zero signals were disconnected

This seems like a very nasty thing to have in a UI-centric language
but for now let's just attempt to fix the SignalSpy class

Pick-to: 6.2
Task-number: QTBUG-98722
Change-Id: I70f11000b8383e6a8fc82d0034c62a2094f6d832
Reviewed-by: Fabian Kosmale <fabian.kosmale@qt.io>

1 files changed, 8 insertions, 1 deletions

diff --git a/src/qmltest/SignalSpy.qml b/src/qmltest/SignalSpy.qml
index 52ed83e261..7257e4f3f3 100644
--- a/src/qmltest/SignalSpy.qml
+++ b/src/qmltest/SignalSpy.qml
@@ -1,6 +1,6 @@
 /****************************************************************************
 **
-** Copyright (C) 2016 The Qt Company Ltd.
+** Copyright (C) 2021 The Qt Company Ltd.
 ** Contact: https://www.qt.io/licensing/
 **
 ** This file is part of the test suite of the Qt Toolkit.
@@ -218,9 +218,14 @@ Item {
     /*! \internal */
     property bool qtest_valid:false
     /*! \internal */
+    property bool qtest_reentrancy_guard: false
 
     /*! \internal */
     function qtest_update() {
+        if (qtest_reentrancy_guard)
+            return;
+        qtest_reentrancy_guard = true;
+
         if (qtest_prevTarget != null) {
             var prevHandlerName = qtest_signalHandlerName(qtest_prevSignalName)
             var prevFunc = qtest_prevTarget[prevHandlerName]
@@ -251,6 +256,8 @@ Item {
         } else {
             spy.qtest_valid = false
         }
+
+        qtest_reentrancy_guard = false;
     }
 
     /*! \internal */