Turn qApp into a normal Python variable, finally

After a long odyssey of more or less unpythonic compromizes,
the qApp "macro" would finally be moved into a normal
variable without surprizes.

This was only possible since we removed qApp from QtWidgets
and other modules. Otherwise,

    from PySide2.QtWidgets import *

would pull qApp, being the constant "None", into main and
shadow the true qApp variable in the builtins.

By inserting qApp into the builtins, only, we make sure that
this variable is always freshly looked up, without making it
change its contents.

DONE...
+ change the singleton code to normal
+ rename to MakeQAppWrapper
+ simplify the implementation
+ fix new bug concerning duplicate applications
+ check very much for refcounting bugs
+ review the rest of the implementation and further simplify

Note... The Q*Application variable will not be turned back into
a GC variable. This is not worth the effort.

Fixes: PYSIDE-571
Change-Id: Idbd158c083318e6b0dfe48d62485c68c90e944de
Reviewed-by: Qt CI Bot <qt_ci_bot@qt-project.org>
Reviewed-by: Cristian Maureira-Fredes <cristian.maureira-fredes@qt.io>

1 files changed, 19 insertions, 1 deletions

diff --git a/sources/pyside2/libpyside/pyside.cpp b/sources/pyside2/libpyside/pyside.cpp
index eeb4de037..d22958398 100644
--- a/sources/pyside2/libpyside/pyside.cpp
+++ b/sources/pyside2/libpyside/pyside.cpp
@@ -94,6 +94,7 @@ void init(PyObject *module)
     MetaFunction::init(module);
     // Init signal manager, so it will register some meta types used by QVariant.
     SignalManager::instance();
+    initQApp();
 }
 
 static bool _setProperty(PyObject *qObj, PyObject *name, PyObject *value, bool *accept)
@@ -214,7 +215,7 @@ void destroyQCoreApplication()
     delete app;
     Py_END_ALLOW_THREADS
     // PYSIDE-571: make sure to create a singleton deleted qApp.
-    MakeSingletonQAppWrapper(NULL);
+    Py_DECREF(MakeQAppWrapper(nullptr));
 }
 
 std::size_t getSizeOfQObject(SbkObjectType *type)
@@ -299,6 +300,23 @@ void initQObjectSubType(SbkObjectType *type, PyObject *args, PyObject * /* kwds
     initDynamicMetaObject(type, userData->mo.update(), userData->cppObjSize);
 }
 
+void initQApp()
+{
+    /*
+     * qApp will not be initialized when embedding is active.
+     * That means that qApp exists already when PySide is initialized.
+     * We could solve that by creating a qApp variable, but in embedded
+     * mode, we also have the effect that the first assignment to qApp
+     * is persistent! Therefore, we can never be sure to have created
+     * qApp late enough to get the right type for the instance.
+     *
+     * I would appreciate very much if someone could explain or even fix
+     * this issue. It exists only when a pre-existing application exists.
+     */
+    if (!qApp)
+        Py_DECREF(MakeQAppWrapper(nullptr));
+}
+
 PyObject *getMetaDataFromQObject(QObject *cppSelf, PyObject *self, PyObject *name)
 {
     PyObject *attr = PyObject_GenericGetAttr(self, name);