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>

2 files changed, 12 insertions, 4 deletions

diff --git a/src/corelib/global/qglobal.h b/src/corelib/global/qglobal.h
index 80f59d92d0..77ca63803f 100644
--- a/src/corelib/global/qglobal.h
+++ b/src/corelib/global/qglobal.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.
@@ -612,7 +612,8 @@ using qsizetype = QIntegerForSizeof<std::size_t>::Signed;
 #  define Q_ALWAYS_INLINE inline
 #endif
 
-#if defined(Q_CC_GNU) && defined(Q_OS_WIN)
+#if defined(Q_CC_GNU) && defined(Q_OS_WIN) && !defined(QT_NO_DATA_RELOCATION)
+// ### Qt6: you can remove me
 #  define QT_INIT_METAOBJECT __attribute__((init_priority(101)))
 #else
 #  define QT_INIT_METAOBJECT
diff --git a/src/corelib/global/qsystemdetection.h b/src/corelib/global/qsystemdetection.h
index 02e2f77c6b..4ebbe16ead 100644
--- a/src/corelib/global/qsystemdetection.h
+++ b/src/corelib/global/qsystemdetection.h
@@ -1,6 +1,7 @@
 /****************************************************************************
 **
-** Copyright (C) 2016 The Qt Company Ltd.
+** 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.
@@ -180,6 +181,12 @@
 #if defined(Q_OS_WIN32) || defined(Q_OS_WIN64) || defined(Q_OS_WINRT)
 #  define Q_OS_WINDOWS
 #  define Q_OS_WIN
+#  if QT_VERSION >= QT_VERSION_CHECK(6, 0, 0)
+// On Windows, pointers to dllimport'ed variables are not constant expressions,
+// so to keep to certain initializations (like QMetaObject) constexpr, we need
+// to use functions instead.
+#    define QT_NO_DATA_RELOCATION
+#  endif
 #endif
 
 #if defined(Q_OS_WIN)