Qt6: Fix uninitialized meta objects on Windows

Windows has a problem relating to cross-DLL variable relocations: they
are not supported. Since QMetaObject's link to the parent class is done
via a pointer, every QMetaObject in a DLL or in the EXE that derives
from a class from another DLL (such as QObject) will be dynamically
initialized.

This commit changes the meta object pointers in QMetaObject::d from raw
pointers to a wrapper class SuperData, which is almost entirely source-
compatible with the pointer itself. On all systems except for Windows
with Qt 6, it's binary compatible with the current implementation.

But for Windows with Qt 6, this commit will store both the raw pointer
and a pointer to a function that returns the QMetaObject, with one of
them non-null only. For all meta objects constructed by moc, we store
the function pointer, which allows the staticMetaObject to be statically
intialized. For dynamic meta objects (QMetaObjectBuilder, QtDBus, QtQml,
ActiveQt), we'll store the actual raw pointer.

[ChangeLog][QtCore][QMetaObject] Some internal members of the
QMetaObject class have changed types. Those members are not public API
and thus should not cause source incompatibilities.

The macro QT_NO_DATA_RELOCATION existed in Qt 4 but was called
Q_NO_DATA_RELOCATION and only applied to Symbian. It was removed in
commit 24a72c4efa929648d3afd95b3c269a95ecf46e57 ("qglobal: Remove
symbian specific features").

Task-number: QTBUG-38876
Fixes: QTBUG-69963
Change-Id: Id92f4a61915b49ddaee6fffd14ae1cf615525e92
Reviewed-by: Simon Hausmann <simon.hausmann@qt.io>

1 files changed, 33 insertions, 4 deletions

diff --git a/src/corelib/kernel/qobjectdefs.h b/src/corelib/kernel/qobjectdefs.h
index ef22b6e67f..dc2d832fe5 100644
--- a/src/corelib/kernel/qobjectdefs.h
+++ b/src/corelib/kernel/qobjectdefs.h
@@ -1,7 +1,7 @@
 /****************************************************************************
 **
-** Copyright (C) 2016 The Qt Company Ltd.
-** Copyright (C) 2016 Intel Corporation.
+** Copyright (C) 2019 The Qt Company Ltd.
+** Copyright (C) 2019 Intel Corporation.
 ** Contact: https://www.qt.io/licensing/
 **
 ** This file is part of the QtCore module of the Qt Toolkit.
@@ -572,13 +572,42 @@ struct Q_CORE_EXPORT QMetaObject
     int static_metacall(Call, int, void **) const;
     static int metacall(QObject *, Call, int, void **);
 
+    template <const QMetaObject &MO> static constexpr const QMetaObject *staticMetaObject()
+    {
+        return &MO;
+    }
+
+    struct SuperData {
+        const QMetaObject *direct;
+        SuperData() = default;
+        constexpr SuperData(std::nullptr_t) : direct(nullptr) {}
+        constexpr SuperData(const QMetaObject *mo) : direct(mo) {}
+
+        constexpr const QMetaObject *operator->() const { return operator const QMetaObject *(); }
+
+#ifdef QT_NO_DATA_RELOCATION
+        using Getter = const QMetaObject *(*)();
+        Getter indirect = nullptr;
+        constexpr SuperData(Getter g) : direct(nullptr), indirect(g) {}
+        constexpr operator const QMetaObject *() const
+        { return indirect ? indirect() : direct; }
+        template <const QMetaObject &MO> static constexpr SuperData link()
+        { return SuperData(QMetaObject::staticMetaObject<MO>); }
+#else
+        constexpr operator const QMetaObject *() const
+        { return direct; }
+        template <const QMetaObject &MO> static constexpr SuperData link()
+        { return SuperData(QMetaObject::staticMetaObject<MO>()); }
+#endif
+    };
+
     struct { // private data
-        const QMetaObject *superdata;
+        SuperData superdata;
         const QByteArrayData *stringdata;
         const uint *data;
         typedef void (*StaticMetacallFunction)(QObject *, QMetaObject::Call, int, void **);
         StaticMetacallFunction static_metacall;
-        const QMetaObject * const *relatedMetaObjects;
+        const SuperData *relatedMetaObjects;
         void *extradata; //reserved for future use
     } d;