Fix crash when stopping screencast session twice

src/dbus/mutter_screen_cast.rs

@@ -27,6 +27,7 @@ pub struct Session {
     to_niri: calloop::channel::Sender<ScreenCastToNiri>,
     #[allow(clippy::type_complexity)]
     streams: Arc<Mutex<Vec<(Stream, InterfaceRef<Stream>)>>>,
+    stopped: Arc<AtomicBool>,
 }
 
 #[derive(Debug, Default, Deserialize, Type, Clone, Copy)]
@@ -135,6 +136,11 @@ impl Session {
     ) {
         debug!("stop");
 
+        if self.stopped.swap(true, Ordering::SeqCst) {
+            // Already stopped.
+            return;
+        }
+
         Session::closed(&ctxt).await.unwrap();
 
         if let Err(err) = self.to_niri.send(ScreenCastToNiri::StopCast {
@@ -255,6 +261,7 @@ impl Session {
             ipc_outputs,
             streams: Arc::new(Mutex::new(vec![])),
             to_niri,
+            stopped: Arc::new(AtomicBool::new(false)),
         }
     }
 }