xwayland/selection: allow simultaneous Wayland-to-X11 transfers

There seems to be no reason why we can't service multiple Wayland-to-X11
transfers concurrently, so long as they are to different windows (or
possibly, same windows but different target properties?)

This commit removes the queuing logic, but retains the request
de-duplication from #2428.

xwayland/selection/outgoing.c

@@ -55,30 +55,11 @@ static int xwm_selection_flush_source_data(
 static void xwm_selection_transfer_start_outgoing(
 		struct wlr_xwm_selection_transfer *transfer);
 
-static struct wlr_xwm_selection_transfer *xwm_selection_transfer_get_first(
-		struct wlr_xwm_selection *selection) {
-	struct wlr_xwm_selection_transfer *first = NULL;
-	if (!wl_list_empty(&selection->outgoing)) {
-		first = wl_container_of(selection->outgoing.prev, first, link);
-	}
-
-	return first;
-}
-
 void xwm_selection_transfer_destroy_outgoing(
 		struct wlr_xwm_selection_transfer *transfer) {
-	struct wlr_xwm_selection *selection = transfer->selection;
-	bool was_first = transfer == xwm_selection_transfer_get_first(selection);
 	wl_list_remove(&transfer->link);
 	wlr_log(WLR_DEBUG, "Destroying transfer %p", transfer);
 
-	// Start next queued transfer if we just removed the active one.
-	if (was_first && !wl_list_empty(&selection->outgoing)) {
-		wlr_log(WLR_DEBUG, "Destroyed transfer was active, starting next");
-		xwm_selection_transfer_start_outgoing(
-			xwm_selection_transfer_get_first(selection));
-	}
-
 	xwm_selection_transfer_remove_event_source(transfer);
 	xwm_selection_transfer_close_wl_client_fd(transfer);
 	wl_array_release(&transfer->source_data);
@@ -232,7 +213,6 @@ static void xwm_selection_transfer_start_outgoing(
 	struct wl_event_loop *loop =
 		wl_display_get_event_loop(xwm->xwayland->wl_display);
 	wlr_log(WLR_DEBUG, "Starting transfer %p", transfer);
-	assert(transfer == xwm_selection_transfer_get_first(transfer->selection));
 	transfer->event_source = wl_event_loop_add_fd(loop, transfer->wl_client_fd,
 		WL_EVENT_READABLE, xwm_data_source_read, transfer);
 }
@@ -328,21 +308,14 @@ static bool xwm_selection_send_data(struct wlr_xwm_selection *selection,
 			wlr_log(WLR_DEBUG, "Destroying stale transfer %p", outgoing);
 			xwm_selection_send_notify(selection->xwm, &outgoing->request, false);
 			xwm_selection_transfer_destroy_outgoing(outgoing);
+		} else {
+			wlr_log(WLR_DEBUG, "Transfer %p still running", outgoing);
 		}
 	}
 
 	wl_list_insert(&selection->outgoing, &transfer->link);
 
-	// We can only handle one transfer at a time
-	if (wl_list_length(&selection->outgoing) == 1) {
-		wlr_log(WLR_DEBUG, "No transfer active, starting %p now", transfer);
 	xwm_selection_transfer_start_outgoing(transfer);
-	} else {
-		struct wlr_xwm_selection_transfer *outgoing;
-		wl_list_for_each(outgoing, &selection->outgoing, link) {
-			wlr_log(WLR_DEBUG, "Transfer %p still queued", outgoing);
-		}
-	}
 
 	return true;
 }