xwayland/selection: simplify incremental transfer control flow

Previously, if the Wayland client died before an incremental transfer
was complete, the logs would be spammed by "write error to target fd" as
wlroots entered some control flow wherein it'd continually try
scheduling further writes to the already-dead pipe.

This commit contains no behavioral changes, but introduces explicit
handling for draining the X11 selection in case of Wayland client death.

xwayland/selection/incoming.c

@@ -11,12 +11,21 @@
 #include "xwayland/selection.h"
 #include "xwayland/xwm.h"
 
+static void xwm_notify_ready_for_next_incr_chunk(
+		struct wlr_xwm_selection_transfer *transfer) {
+	struct wlr_xwm *xwm = transfer->selection->xwm;
+	assert(transfer->incr);
+	wlr_log(WLR_DEBUG, "deleting property");
+	xcb_delete_property(xwm->xcb_conn, transfer->selection->window,
+		xwm->atoms[WL_SELECTION]);
+	xcb_flush(xwm->xcb_conn);
+}
+
 /**
  * Write the X11 selection to a Wayland client.
  */
 static int xwm_data_source_write(int fd, uint32_t mask, void *data) {
 	struct wlr_xwm_selection_transfer *transfer = data;
-	struct wlr_xwm *xwm = transfer->selection->xwm;
 
 	char *property = xcb_get_property_value(transfer->property_reply);
 	int remainder = xcb_get_property_value_length(transfer->property_reply) -
@@ -35,16 +44,14 @@ static int xwm_data_source_write(int fd, uint32_t mask, void *data) {
 		transfer->property_start + len,
 		len, xcb_get_property_value_length(transfer->property_reply));
 
-	transfer->property_start += len;
-	if (len == remainder) {
+	if (len < remainder) {
+		transfer->property_start += len;
+	} else {
 		xwm_selection_transfer_destroy_property_reply(transfer);
 		xwm_selection_transfer_remove_event_source(transfer);
 
 		if (transfer->incr) {
-			wlr_log(WLR_DEBUG, "deleting property");
-			xcb_delete_property(xwm->xcb_conn, transfer->selection->window,
-				xwm->atoms[WL_SELECTION]);
-			xcb_flush(xwm->xcb_conn);
+			xwm_notify_ready_for_next_incr_chunk(transfer);
 		} else {
 			wlr_log(WLR_DEBUG, "transfer complete");
 			xwm_selection_transfer_close_wl_client_fd(transfer);
@@ -97,11 +104,20 @@ void xwm_get_incr_chunk(struct wlr_xwm_selection_transfer *transfer) {
 	//dump_property(xwm, xwm->atoms[WL_SELECTION], reply);
 
 	if (xcb_get_property_value_length(reply) > 0) {
-		/* Reply's ownership is transferred to xwm, which is responsible
-		 * for freeing it */
-		xwm_write_property(transfer, reply);
+		// Reply's ownership is transferred to xwm, which is responsible
+		// for freeing it.
+		if (transfer->wl_client_fd >= 0) {
+			// Wayland client is alive, property will be freed once it has finished
+			// reading it.
+			xwm_write_property(transfer, reply);
+		} else {
+			// Wayland client closed its pipe prematurely (or died). Continue draining
+			// the X11 client.
+			xwm_notify_ready_for_next_incr_chunk(transfer);
+			free(reply);
+		}
 	} else {
-		wlr_log(WLR_DEBUG, "transfer complete");
+		wlr_log(WLR_DEBUG, "incremental transfer complete");
 		xwm_selection_transfer_close_wl_client_fd(transfer);
 		free(reply);
 	}