xwayland/selection: introduce `xwm_selection_transfer_init`

Currently, all this does is initialize `wl_client_fd` to -1, so that
comparisons with 0 are meaningful.

include/xwayland/selection.h

@@ -47,6 +47,7 @@ void xwm_selection_transfer_close_wl_client_fd(
 	struct wlr_xwm_selection_transfer *transfer);
 void xwm_selection_transfer_destroy_property_reply(
 	struct wlr_xwm_selection_transfer *transfer);
+void xwm_selection_transfer_init(struct wlr_xwm_selection_transfer *transfer);
 void xwm_selection_transfer_finish(struct wlr_xwm_selection_transfer *transfer);
 bool xwm_selection_transfer_get_selection_property(
 	struct wlr_xwm_selection_transfer *transfer, bool delete);

xwayland/selection/incoming.c

@@ -155,7 +155,7 @@ static void source_send(struct wlr_xwm_selection *selection,
 	// lose track of the current `transfer->wl_client_fd` and use-after-free
 	// during cleanup. This doesn't happen often, but bail now to avoid a
 	// compositor crash later.
-	if (transfer->wl_client_fd > 0) {
+	if (transfer->wl_client_fd >= 0) {
 		wlr_log(WLR_ERROR, "source_send fd %d, but %d already in progress", fd,
 			transfer->wl_client_fd);
 		if (transfer->wl_client_fd != fd) {
@@ -404,7 +404,7 @@ void xwm_handle_selection_notify(struct wlr_xwm *xwm,
 			wlr_log(WLR_DEBUG, "denying write access to clipboard: "
 				"no xwayland surface focused");
 			// Would leak this transfer otherwise. Should never happen.
-			assert(selection->incoming.wl_client_fd <= 0);
+			assert(selection->incoming.wl_client_fd < 0);
 			return;
 		}
 

xwayland/selection/outgoing.c

@@ -297,6 +297,7 @@ static bool xwm_selection_send_data(struct wlr_xwm_selection *selection,
 		return false;
 	}
 
+	xwm_selection_transfer_init(transfer);
 	transfer->selection = selection;
 	transfer->request = *req;
 	wl_array_init(&transfer->source_data);

xwayland/selection/selection.c

@@ -32,6 +32,10 @@ void xwm_selection_transfer_destroy_property_reply(
 	transfer->property_reply = NULL;
 }
 
+void xwm_selection_transfer_init(struct wlr_xwm_selection_transfer *transfer) {
+	transfer->wl_client_fd = -1;
+}
+
 void xwm_selection_transfer_finish(
 		struct wlr_xwm_selection_transfer *transfer) {
 	transfer->incr = false;
@@ -182,6 +186,7 @@ static void selection_init(struct wlr_xwm *xwm,
 	selection->window = xwm->selection_window;
 	selection->incoming.selection = selection;
 	wl_list_init(&selection->outgoing);
+	xwm_selection_transfer_init(&selection->incoming);
 
 	uint32_t mask =
 		XCB_XFIXES_SELECTION_EVENT_MASK_SET_SELECTION_OWNER |