QObject: attempt to fix a deadlock introduced by an earlier fix

Commit 71b4d4f150 is likely the source of
the issue. It fixed a race on disconnection, but kept the call to
disconnectNotify() (which is user code) inside the locked section. My
analysis is that by construction the sender object can't be undergoing
concurrent deletion anyway at this point. All call sites
(QObject::disconnect or the signal-slot activations but before the slot
is activated) imply that the user code that reached here cannot itself
be racing the deletion.

There may be one race condition left: if the same signal was connected
earlier to a slot via queued connection and that slot deletes the sender
asynchronously. A synchronous deletion is handled by doActivate(), so
the single-shot connection is never activated in the first place, but an
asynchronous deletion could race past that check and delete the sender
while QObjectPrivate::removeConnection is running. However, I'd call
this a mistake in user code.

[ChangeLog][QtCore][QObject] Fixed a regression from 6.3 that caused
QObject::isSignalConnected() to deadlock if called from inside
disconnectNotify().

Fixes: QTBUG-106025
Pick-to: 5.15 6.2 6.3 6.4
Change-Id: Ic6547f8247454b47baa8fffd170fe0bdb62cfcaf
Reviewed-by: Qt CI Bot <qt_ci_bot@qt-project.org>
Reviewed-by: Lars Knoll <lars@knoll.priv.no>

src/corelib/kernel/qobject.cpp

@@ -5311,11 +5311,20 @@ bool QObjectPrivate::disconnect(const QObject *sender, int signal_index, const Q
 /*!
     \internal
     \threadsafe
+
+    Thread-safety warning: this function may be called from any thread and is
+    thread-safe, \b{so long as the sender is not being deleted}. At the time of
+    this writing, this function is called from QObject::disconnect() and from
+    the multiple places where a single-shot connection is activated; in both
+    cases, the construction of the user code is already such that the sender
+    object cannot be undergoing deletion in another thread.
 */
 inline bool QObjectPrivate::removeConnection(QObjectPrivate::Connection *c)
 {
     if (!c)
         return false;
+
+    // double-checked locking on this pointer
     QObject *receiver = c->receiver.loadRelaxed();
     if (!receiver)
         return false;
@@ -5336,7 +5345,6 @@ inline bool QObjectPrivate::removeConnection(QObjectPrivate::Connection *c)
         Q_ASSERT(connections);
         connections->removeConnection(c);
 
-        c->sender->disconnectNotify(QMetaObjectPrivate::signal(c->sender->metaObject(), c->signal_index));
         // We must not hold the receiver mutex, else we risk dead-locking; we also only need the sender mutex
         // It is however vital to hold the senderMutex before calling cleanOrphanedConnections, as otherwise
         // another thread might modify/delete the connection
@@ -5348,6 +5356,8 @@ inline bool QObjectPrivate::removeConnection(QObjectPrivate::Connection *c)
         locker.dismiss(); // so we dismiss the QOrderedMutexLocker
     }
 
+    // this is safe if the condition in the documentation is correct
+    c->sender->disconnectNotify(QMetaObjectPrivate::signal(c->sender->metaObject(), c->signal_index));
     return true;
 }