allow to use shiboken.delete() to delete Q*Application

This function should initially not allow deleting Q*Application
with a shiboken.delete() function, but unfortunately some
competitor can do that, so we will as well.

Deletion of qApp needs knowledge about qApp.
We add a hook function that calls the pyside function
  `destroyQCoreApplication`.

A special problem was testing interactively.
See the error description for the days long journey :)

Change-Id: I34862425c2cb2cc80d6cafc22d25a867f96f3e0a
Fixes: PYSIDE-1470
Reviewed-by: Friedemann Kleint <Friedemann.Kleint@qt.io>
Reviewed-by: Cristian Maureira-Fredes <cristian.maureira-fredes@qt.io>
(cherry picked from commit 98aba46414b666c27df3904d2b4313f0a16e05dc)
Reviewed-by: Qt Cherry-pick Bot <cherrypick_bot@qt-project.org>

7 files changed, 46 insertions, 9 deletions

diff --git a/sources/pyside6/libpyside/pyside.cpp b/sources/pyside6/libpyside/pyside.cpp
index 75b3eb16f..22230703e 100644
--- a/sources/pyside6/libpyside/pyside.cpp
+++ b/sources/pyside6/libpyside/pyside.cpp
@@ -308,6 +308,9 @@ void initQApp()
      */
     if (!qApp)
         Py_DECREF(MakeQAppWrapper(nullptr));
+
+    // PYSIDE-1470: Register a function to destroy an application from shiboken.
+    setDestroyQApplication(destroyQCoreApplication);
 }
 
 PyObject *getMetaDataFromQObject(QObject *cppSelf, PyObject *self, PyObject *name)
diff --git a/sources/pyside6/tests/pysidetest/qapp_like_a_macro_test.py b/sources/pyside6/tests/pysidetest/qapp_like_a_macro_test.py
index 2a796cea8..2233b1b33 100644
--- a/sources/pyside6/tests/pysidetest/qapp_like_a_macro_test.py
+++ b/sources/pyside6/tests/pysidetest/qapp_like_a_macro_test.py
@@ -59,10 +59,11 @@ class qAppMacroTest(unittest.TestCase):
         classes = (QtCore.QCoreApplication,
                    QtGui.QGuiApplication,
                    QtWidgets.QApplication)
+        fil = sys.stderr
         for klass in classes:
-            print("created", klass([]))
+            print("CREATED", klass([]), file=fil); fil.flush()
             qApp.shutdown()
-            print("deleted qApp", qApp)
+            print("DELETED qApp", qApp, file=fil); fil.flush()
         # creating without deletion raises:
         QtCore.QCoreApplication([])
         with self.assertRaises(RuntimeError):
diff --git a/sources/shiboken6/libshiboken/basewrapper.cpp b/sources/shiboken6/libshiboken/basewrapper.cpp
index 62e2df2cf..7f927523a 100644
--- a/sources/shiboken6/libshiboken/basewrapper.cpp
+++ b/sources/shiboken6/libshiboken/basewrapper.cpp
@@ -95,6 +95,13 @@ static void SbkObjectTypeDealloc(PyObject *pyObj);
 static PyObject *SbkObjectTypeTpNew(PyTypeObject *metatype, PyObject *args, PyObject *kwds);
 
 static SelectableFeatureHook SelectFeatureSet = nullptr;
+static DestroyQAppHook DestroyQApplication = nullptr;
+
+// PYSIDE-1470: Provide a hook to kill an Application from Shiboken.
+void setDestroyQApplication(DestroyQAppHook func)
+{
+    DestroyQApplication = func;
+}
 
 static PyObject *Sbk_TypeGet___dict__(PyTypeObject *type, void *context);   // forward
 
@@ -457,6 +464,12 @@ PyObject *MakeQAppWrapper(PyTypeObject *type)
     // object already has a reference from PyObject_GC_New. But this is
     // exactly the needed reference that keeps qApp alive from alone!
     Py_INCREF(qApp_curr);
+    // PYSIDE-1470: As a side effect, the interactive "_" variable tends to
+    // create reference cycles. It was found when using gc.collect(). But using
+    // PyGC_collect() inside the C code had no effect in the interactive shell.
+    // The cycle exists only in the eval loop of the interpreter!
+    if (PyDict_GetItem(builtins, Shiboken::PyName::underscore()))
+        PyDict_SetItem(builtins, Shiboken::PyName::underscore(), Py_None);
     return qApp_curr;
 }
 
@@ -673,7 +686,12 @@ PyObject *SbkObjectTpNew(PyTypeObject *subtype, PyObject *, PyObject *)
 PyObject *SbkQAppTpNew(PyTypeObject *subtype, PyObject *, PyObject *)
 {
     auto self = reinterpret_cast<SbkObject *>(MakeQAppWrapper(subtype));
-    return self == nullptr ? nullptr : _setupNew(self, subtype);
+    if (self == nullptr)
+        return nullptr;
+    auto ret = _setupNew(self, subtype);
+    auto priv = self->d;
+    priv->isQAppSingleton = 1;
+    return ret;
 }
 
 PyObject *SbkDummyNew(PyTypeObject *type, PyObject *, PyObject *)
@@ -1154,6 +1172,12 @@ bool wasCreatedByPython(SbkObject *pyObj)
 
 void callCppDestructors(SbkObject *pyObj)
 {
+    auto priv = pyObj->d;
+    if (priv->isQAppSingleton && DestroyQApplication) {
+        // PYSIDE-1470: Allow to destroy the application from Shiboken.
+        DestroyQApplication();
+        return;
+    }
     PyTypeObject *type = Py_TYPE(pyObj);
     SbkObjectTypePrivate *sotp = PepType_SOTP(type);
     if (sotp->is_multicpp) {
@@ -1166,18 +1190,19 @@ void callCppDestructors(SbkObject *pyObj)
         sotp->cpp_dtor(pyObj->d->cptr[0]);
     }
 
+    if (priv->validCppObject && priv->containsCppWrapper) {
+        BindingManager::instance().releaseWrapper(pyObj);
+    }
+
     /* invalidate needs to be called before deleting pointer array because
        it needs to delete entries for them from the BindingManager hash table;
        also release wrapper explicitly if object contains C++ wrapper because
        invalidate doesn't */
     invalidate(pyObj);
-    if (pyObj->d->validCppObject && pyObj->d->containsCppWrapper) {
-      BindingManager::instance().releaseWrapper(pyObj);
-    }
 
-    delete[] pyObj->d->cptr;
-    pyObj->d->cptr = nullptr;
-    pyObj->d->validCppObject = false;
+    delete[] priv->cptr;
+    priv->cptr = nullptr;
+    priv->validCppObject = false;
 }
 
 bool hasOwnership(SbkObject *pyObj)
diff --git a/sources/shiboken6/libshiboken/basewrapper.h b/sources/shiboken6/libshiboken/basewrapper.h
index 364f6c5cb..2f0c22e9f 100644
--- a/sources/shiboken6/libshiboken/basewrapper.h
+++ b/sources/shiboken6/libshiboken/basewrapper.h
@@ -106,6 +106,9 @@ LIBSHIBOKEN_API void SbkObjectType_SetReserved(PyTypeObject *type, int value);
 LIBSHIBOKEN_API const char **SbkObjectType_GetPropertyStrings(PyTypeObject *type);
 LIBSHIBOKEN_API void SbkObjectType_SetPropertyStrings(PyTypeObject *type, const char **strings);
 
+/// PYSIDE-1470: Set the function to kill a Q*Application.
+typedef void(*DestroyQAppHook)();
+LIBSHIBOKEN_API void setDestroyQApplication(DestroyQAppHook func);
 
 extern LIBSHIBOKEN_API PyTypeObject *SbkObjectType_TypeF(void);
 extern LIBSHIBOKEN_API SbkObjectType *SbkObject_TypeF(void);
diff --git a/sources/shiboken6/libshiboken/basewrapper_p.h b/sources/shiboken6/libshiboken/basewrapper_p.h
index 64f7941b7..60fba13c5 100644
--- a/sources/shiboken6/libshiboken/basewrapper_p.h
+++ b/sources/shiboken6/libshiboken/basewrapper_p.h
@@ -97,6 +97,9 @@ struct SbkObjectPrivate
     unsigned int validCppObject : 1;
     /// Marked as true when the object constructor was called
     unsigned int cppObjectCreated : 1;
+    /// PYSIDE-1470: Marked as true if this is the Q*Application singleton.
+    /// This bit allows app deletion from shiboken?.delete() .
+    unsigned int isQAppSingleton : 1;
     /// Information about the object parents and children, may be null.
     Shiboken::ParentInfo *parentInfo;
     /// Manage reference count of objects that are referred to but not owned from.
diff --git a/sources/shiboken6/libshiboken/sbkstaticstrings.cpp b/sources/shiboken6/libshiboken/sbkstaticstrings.cpp
index 35fc2a410..fd34c0eeb 100644
--- a/sources/shiboken6/libshiboken/sbkstaticstrings.cpp
+++ b/sources/shiboken6/libshiboken/sbkstaticstrings.cpp
@@ -61,6 +61,7 @@ STATIC_STRING_IMPL(name, "name")
 STATIC_STRING_IMPL(qApp, "qApp")
 STATIC_STRING_IMPL(result, "result")
 STATIC_STRING_IMPL(select_id, "select_id")
+STATIC_STRING_IMPL(underscore, "_")
 STATIC_STRING_IMPL(value, "value")
 STATIC_STRING_IMPL(values, "values")
 
diff --git a/sources/shiboken6/libshiboken/sbkstaticstrings.h b/sources/shiboken6/libshiboken/sbkstaticstrings.h
index d835d286c..4aaef814f 100644
--- a/sources/shiboken6/libshiboken/sbkstaticstrings.h
+++ b/sources/shiboken6/libshiboken/sbkstaticstrings.h
@@ -59,6 +59,7 @@ LIBSHIBOKEN_API PyObject *multi();
 LIBSHIBOKEN_API PyObject *name();
 LIBSHIBOKEN_API PyObject *result();
 LIBSHIBOKEN_API PyObject *select_id();
+LIBSHIBOKEN_API PyObject *underscore();
 LIBSHIBOKEN_API PyObject *value();
 LIBSHIBOKEN_API PyObject *values();
 } // namespace PyName