backends/x11: explicitly free the outputs

...before releasing the connection!

this was randomly causing crashes because of use-after-disconnect
problems. since we would tear down the connection in ~X11WindowedBackend
but outputs wouldn't get cleaned up until QObject children cleanup of
the X11WindowedBackend object (or more precisely the OutputBase). this
would then result in ~X11WindowedOutput accessing a backend connection
that had already been closed

to help debug this type of problem moving forward let's also reset the
connection to nullptr during destruction. it's kinda pointless but it
makes it easier to spot use-after-disconnect

BUG: 466183

src/backends/x11/windowed/x11_windowed_backend.cpp

@@ -3,6 +3,7 @@
     This file is part of the KDE project.
 
     SPDX-FileCopyrightText: 2015 Martin Gräßlin <mgraesslin@kde.org>
+    SPDX-FileCopyrightText: 2023 Harald Sitter <sitter@kde.org>
 
     SPDX-License-Identifier: GPL-2.0-or-later
 */
@@ -160,6 +161,7 @@ X11WindowedBackend::X11WindowedBackend(const X11WindowedBackendOptions &options)
 
 X11WindowedBackend::~X11WindowedBackend()
 {
+    destroyOutputs();
     m_pointerDevice.reset();
     m_keyboardDevice.reset();
     m_touchDevice.reset();
@@ -172,6 +174,7 @@ X11WindowedBackend::~X11WindowedBackend()
             xcb_key_symbols_free(m_keySymbols);
         }
         xcb_disconnect(m_connection);
+        m_connection = nullptr;
     }
 }
 
@@ -691,4 +694,14 @@ Outputs X11WindowedBackend::outputs() const
     return m_outputs;
 }
 
+void X11WindowedBackend::destroyOutputs()
+{
+    while (!m_outputs.isEmpty()) {
+        auto output = m_outputs.takeLast();
+        output->updateEnabled(false);
+        Q_EMIT outputRemoved(output);
+        delete output;
+    }
+}
+
 } // namespace KWin

src/backends/x11/windowed/x11_windowed_backend.h

@@ -129,6 +129,7 @@ private:
     void updateSize(xcb_configure_notify_event_t *event);
     void initXInput();
     X11WindowedOutput *findOutput(xcb_window_t window) const;
+    void destroyOutputs();
 
     X11WindowedBackendOptions m_options;
     xcb_connection_t *m_connection = nullptr;