Fix QtDBus deadlock inside kded/kiod

Whenever a message spy was installed, we failed to actually process
looped-back messages by queueing them for processing by the spy. That
had as a consequence that the caller got an error reply. Worse, since
the message had been queued, QtDBus would attempt to deliver it later.
Since that message had isLocal==true, bad things happened inside the
manager thread.

The correct solution is not to queue the message for the filter. If the
message is local, then simply deliver directly, as we're still in the
user's thread. This used to be the behavior in Qt 5.5.

Task-number: QTBUG-51676
Change-Id: I1dc112894cde7121e8ce302ae51b438ade1ff612
Reviewed-by: David Faure <david.faure@kdab.com>
Reviewed-by: Dmitry Shachnev <mitya57@gmail.com>
Reviewed-by: Jan Kundrát <jkt@kde.org>

src/dbus/qdbusintegrator.cpp

@@ -480,6 +480,11 @@ QDBusSpyCallEvent::~QDBusSpyCallEvent()
 }
 
 void QDBusSpyCallEvent::placeMetaCall(QObject *)
+{
+    invokeSpyHooks(msg, hooks, hookCount);
+}
+
+inline void QDBusSpyCallEvent::invokeSpyHooks(const QDBusMessage &msg, const Hook *hooks, int hookCount)
 {
     // call the spy hook list
     for (int i = 0; i < hookCount; ++i)
@@ -509,7 +514,12 @@ bool QDBusConnectionPrivate::handleMessage(const QDBusMessage &amsg)
 {
     if (!ref.load())
         return false;
-    if (!dispatchEnabled && !QDBusMessagePrivate::isLocal(amsg)) {
+
+    // local message are always delivered, regardless of filtering
+    // or whether the dispatcher is enabled
+    bool isLocal = QDBusMessagePrivate::isLocal(amsg);
+
+    if (!dispatchEnabled && !isLocal) {
         // queue messages only, we'll handle them later
         qDBusDebug() << this << "delivery is suspended";
         pendingMessages << amsg;
@@ -523,14 +533,24 @@ bool QDBusConnectionPrivate::handleMessage(const QDBusMessage &amsg)
         // let them see the signal too
         return false;
     case QDBusMessage::MethodCallMessage:
-        // run it through the spy filters (if any) before the regular processing
+        // run it through the spy filters (if any) before the regular processing:
+        // a) if it's a local message, we're in the caller's thread, so invoke the filter directly
+        // b) if it's an external message, post to the main thread
         if (Q_UNLIKELY(qDBusSpyHookList.exists()) && qApp) {
             const QDBusSpyHookList &list = *qDBusSpyHookList;
-            qDBusDebug() << this << "invoking message spies";
+            if (isLocal) {
+                Q_ASSERT(QThread::currentThread() != thread());
+                qDBusDebug() << this << "invoking message spies directly";
+                QDBusSpyCallEvent::invokeSpyHooks(amsg, list.constData(), list.size());
+            } else {
+                qDBusDebug() << this << "invoking message spies via event";
                 QCoreApplication::postEvent(qApp, new QDBusSpyCallEvent(this, QDBusConnection(this),
                                                                         amsg, list.constData(), list.size()));
+
+                // we'll be called back, so return
                 return true;
             }
+        }
 
         handleObjectCall(amsg);
         return true;
@@ -1451,9 +1471,9 @@ void QDBusConnectionPrivate::handleObjectCall(const QDBusMessage &msg)
     // that means the dispatchLock mutex is locked
     // must not call out to user code in that case
     //
-    // however, if the message is internal, handleMessage was called
-    // directly and no lock is in place. We can therefore call out to
-    // user code, if necessary
+    // however, if the message is internal, handleMessage was called directly
+    // (user's thread) and no lock is in place. We can therefore call out to
+    // user code, if necessary.
     ObjectTreeNode result;
     int usedLength;
     QThread *objThread = 0;
@@ -1492,12 +1512,14 @@ void QDBusConnectionPrivate::handleObjectCall(const QDBusMessage &msg)
                                                            usedLength, msg));
             return;
         } else if (objThread != QThread::currentThread()) {
-            // synchronize with other thread
+            // looped-back message, targeting another thread:
+            // synchronize with it
             postEventToThread(HandleObjectCallPostEventAction, result.obj,
                               new QDBusActivateObjectEvent(QDBusConnection(this), this, result,
                                                            usedLength, msg, &sem));
             semWait = true;
         } else {
+            // looped-back message, targeting current thread
             semWait = false;
         }
     } // release the lock

src/dbus/qdbusintegrator_p.h

@@ -145,6 +145,7 @@ public:
     {}
     ~QDBusSpyCallEvent();
     void placeMetaCall(QObject *) Q_DECL_OVERRIDE;
+    static inline void invokeSpyHooks(const QDBusMessage &msg, const Hook *hooks, int hookCount);
 
     QDBusConnection conn;   // keeps the refcount in QDBusConnectionPrivate up
     QDBusMessage msg;