[kwin_wayland] Pass socket file descriptor to QtWayland

WaylandServer allows to create a ClientConnection which is intended for
QtWayland. This allows us to easily identify our "own" surfaces. The
created file descriptor is set as env variable WAYLAND_SOCKET prior to
creating the Application. Wayland will unset it after connecting, so we
don't need to unset it. This removes the hack of setting and resetting
the WAYLAND_DISPLAY environment variable.

main_wayland.cpp

@@ -318,12 +318,8 @@ KWIN_EXPORT int kdemain(int argc, char * argv[])
     if (signal(SIGHUP, KWin::sighandler) == SIG_IGN)
         signal(SIGHUP, SIG_IGN);
 
-    // we want QtWayland to connect to our Wayland display, but the WaylandBackend to the existing Wayland backend
-    // so fiddling around with the env variables.
-    const QByteArray systemDisplay = qgetenv("WAYLAND_DISPLAY");
-    qputenv("WAYLAND_DISPLAY", waylandSocket.isEmpty() ? QByteArrayLiteral("wayland-0") : waylandSocket);
+    qputenv("WAYLAND_SOCKET", QByteArray::number(server->createQtConnection()));
     KWin::ApplicationWayland a(argc, argv);
-    qputenv("WAYLAND_DISPLAY", systemDisplay);
     a.setupTranslator();
 
     server->setParent(&a);

wayland_server.cpp

@@ -102,4 +102,15 @@ int WaylandServer::createXWaylandConnection()
     return sx[1];
 }
 
+int WaylandServer::createQtConnection()
+{
+    int sx[2];
+    if (socketpair(AF_UNIX, SOCK_STREAM | SOCK_CLOEXEC, 0, sx) < 0) {
+        qCWarning(KWIN_CORE) << "Could not create socket";
+        return -1;
+    }
+    m_qtConnection = m_display->createClient(sx[0]);
+    return sx[1];
+}
+
 }

wayland_server.h

@@ -66,12 +66,18 @@ public:
      **/
     int createXWaylandConnection();
 
+    /**
+     * @returns file descriptor for QtWayland
+     **/
+    int createQtConnection();
+
 private:
     KWayland::Server::Display *m_display = nullptr;
     KWayland::Server::CompositorInterface *m_compositor = nullptr;
     KWayland::Server::SeatInterface *m_seat = nullptr;
     KWayland::Server::ShellInterface *m_shell = nullptr;
     KWayland::Server::ClientConnection *m_xwaylandConnection = nullptr;
+    KWayland::Server::ClientConnection *m_qtConnection = nullptr;
     KWIN_SINGLETON(WaylandServer)
 };