QHostInfo: un-QObject-ify QHostInfoLookupManager

QObjects are not even reentrant, but this class must be thread-safe,
so it's always ... icky ... to have to analyze a "thread-safe
QObject", because for all intents and purposes, that's an oxymoron.

The QObject-ness isn't even used, except for defining a private slot,
connected to QCoreApplication::destroyed(). That slot just calls
waitForDone() on QThreadPool, which is a QObject itself, so use it
as the context object for the signal-slot connection, using a lambda
as slot.

So, strip the class of it's base class, convert the private slot to a
lambda and connect to that. Finally, remove the moveToThread() call,
because this new class can be destroyed from any thread, not just the
main one.

Change-Id: I0e33983aa7afd0ad621ece4afd10d9e4adad38c1
Reviewed-by: Volker Hilsheimer <volker.hilsheimer@qt.io>

src/network/kernel/qhostinfo.cpp

@@ -907,9 +907,10 @@ void QHostInfoRunnable::run()
 
 QHostInfoLookupManager::QHostInfoLookupManager() : wasDeleted(false)
 {
-    moveToThread(QCoreApplicationPrivate::mainThread());
 #if QT_CONFIG(thread)
-    connect(QCoreApplication::instance(), SIGNAL(destroyed()), SLOT(waitForThreadPoolDone()), Qt::DirectConnection);
+    QObject::connect(QCoreApplication::instance(), &QObject::destroyed,
+                     &threadPool, [&](QObject *) { threadPool.waitForDone(); },
+                     Qt::DirectConnection);
     threadPool.setMaxThreadCount(20); // do up to 20 DNS lookups in parallel
 #endif
 }

src/network/kernel/qhostinfo_p.h

@@ -208,9 +208,8 @@ public:
 };
 
 
-class QHostInfoLookupManager : public QObject
+class QHostInfoLookupManager
 {
-    Q_OBJECT
 public:
     QHostInfoLookupManager();
     ~QHostInfoLookupManager();
@@ -246,11 +245,6 @@ protected:
 
 private:
     void rescheduleWithMutexHeld();
-
-private slots:
-#if QT_CONFIG(thread)
-    void waitForThreadPoolDone() { threadPool.waitForDone(); }
-#endif
 };
 
 QT_END_NAMESPACE