QEventLoopLocker: rewrite to hold public classes

... instead of Private ones, at the cost of having to befriend of all
the lockable classes, because we need access to their d_func()'s.

This simplifies the code, because we don't need the manual QClass to
QClassPrivate mapping (o2p) anymore, we can just use d_func(). This also
paves the way to make QEventLoopLocker almost completely inline and use
a 3-pointer form of QBiPointer, once available, to hide the bit
fiddling. We couldn't make such a change if the class continued to hold
pointers to QClassPrivate's.

Pick-to: 6.6
Task-number: QTBUG-114793
Change-Id: Id300e4d45d6cacabe090a46cd6433c5ead3c8b0c
Reviewed-by: Thiago Macieira <thiago.macieira@intel.com>
Reviewed-by: Fabian Kosmale <fabian.kosmale@qt.io>

src/corelib/kernel/qcoreapplication.h

@@ -33,6 +33,7 @@ class QTranslator;
 class QPostEventList;
 class QAbstractEventDispatcher;
 class QAbstractNativeEventFilter;
+class QEventLoopLocker;
 
 #if QT_CONFIG(permissions) || defined(Q_QDOC)
 class QPermission;
@@ -59,6 +60,7 @@ class Q_CORE_EXPORT QCoreApplication
 #endif
 
     Q_DECLARE_PRIVATE(QCoreApplication)
+    friend class QEventLoopLocker;
 public:
     enum { ApplicationFlags = QT_VERSION
     };

src/corelib/kernel/qeventloop.cpp

@@ -309,19 +309,10 @@ bool QEventLoop::event(QEvent *event)
 void QEventLoop::quit()
 { exit(0); }
 
-
-namespace {
 // If any of these trigger, the Type bits will interfere with the pointer values:
-static_assert(alignof(QEventLoopPrivate) >= 4);
-static_assert(alignof(QThreadPrivate) >= 4);
-static_assert(alignof(QCoreApplicationPrivate) >= 4);
-
-template <typename Private>
-Private *o2p(QObject *o)
-{
-    return static_cast<Private*>(QObjectPrivate::get(o));
-}
-} // unnamed namespace
+static_assert(alignof(QEventLoop) >= 4);
+static_assert(alignof(QThread) >= 4);
+static_assert(alignof(QCoreApplication) >= 4);
 
 /*!
     \class QEventLoopLocker
@@ -351,8 +342,7 @@ Private *o2p(QObject *o)
     \sa QCoreApplication::quit(), QCoreApplication::isQuitLockEnabled()
  */
 QEventLoopLocker::QEventLoopLocker() noexcept
-    : QEventLoopLocker{o2p<QCoreApplicationPrivate>(QCoreApplication::instance()),
-                       Type::Application}
+    : QEventLoopLocker{QCoreApplication::instance(), Type::Application}
 {
 
 }
@@ -365,7 +355,7 @@ QEventLoopLocker::QEventLoopLocker() noexcept
     \sa QEventLoop::quit()
  */
 QEventLoopLocker::QEventLoopLocker(QEventLoop *loop) noexcept
-    : QEventLoopLocker{o2p<QEventLoopPrivate>(loop), Type::EventLoop}
+    : QEventLoopLocker{loop, Type::EventLoop}
 {
 
 }
@@ -378,7 +368,7 @@ QEventLoopLocker::QEventLoopLocker(QEventLoop *loop) noexcept
     \sa QThread::quit()
  */
 QEventLoopLocker::QEventLoopLocker(QThread *thread) noexcept
-    : QEventLoopLocker{o2p<QThreadPrivate>(thread), Type::Thread}
+    : QEventLoopLocker{thread, Type::Thread}
 {
 
 }
@@ -425,7 +415,7 @@ QEventLoopLocker::QEventLoopLocker(QThread *thread) noexcept
  */
 QEventLoopLocker::~QEventLoopLocker()
 {
-    visit([](auto p) { p->deref(); });
+    visit([](auto p) { p->d_func()->deref(); });
 }
 
 /*!
@@ -434,7 +424,7 @@ QEventLoopLocker::~QEventLoopLocker()
 QEventLoopLocker::QEventLoopLocker(void *ptr, Type t) noexcept
     : p{quintptr(ptr) | quintptr(t)}
 {
-    visit([](auto p) { p->ref(); });
+    visit([](auto p) { p->d_func()->ref(); });
 }
 
 /*!
@@ -447,9 +437,9 @@ void QEventLoopLocker::visit(Func f) const
     if (!ptr)
         return;
     switch (type()) {
-    case Type::EventLoop:   return f(static_cast<QEventLoopPrivate *>(ptr));
-    case Type::Thread:      return f(static_cast<QThreadPrivate *>(ptr));
-    case Type::Application: return f(static_cast<QCoreApplicationPrivate *>(ptr));
+    case Type::EventLoop:   return f(static_cast<QEventLoop *>(ptr));
+    case Type::Thread:      return f(static_cast<QThread *>(ptr));
+    case Type::Application: return f(static_cast<QCoreApplication *>(ptr));
     }
     Q_UNREACHABLE();
 }

src/corelib/kernel/qeventloop.h

@@ -9,12 +9,14 @@
 
 QT_BEGIN_NAMESPACE
 
+class QEventLoopLocker;
 class QEventLoopPrivate;
 
 class Q_CORE_EXPORT QEventLoop : public QObject
 {
     Q_OBJECT
     Q_DECLARE_PRIVATE(QEventLoop)
+    friend class QEventLoopLocker;
 
 public:
     explicit QEventLoop(QObject *parent = nullptr);

src/corelib/thread/qthread.h

@@ -24,6 +24,7 @@ QT_BEGIN_NAMESPACE
 class QThreadData;
 class QThreadPrivate;
 class QAbstractEventDispatcher;
+class QEventLoopLocker;
 
 class Q_CORE_EXPORT QThread : public QObject
 {
@@ -109,6 +110,7 @@ protected:
 
 private:
     Q_DECLARE_PRIVATE(QThread)
+    friend class QEventLoopLocker;
 
 #if QT_CONFIG(cxx11_future)
     [[nodiscard]] static QThread *createThreadImpl(std::future<void> &&future);