xwm: Try a non-blocking write before setting up an fd watch for property data

Typically we can write it immediately without blocking, so save the overhead
of setting up an fd watch and writing the data in a callback.  For the
case where the immediate write doesn't write all data, we fallback and
set up the fd watch as usual.

This patch also consolidates setting up the async write a bit.

src/xwayland/selection.c

@@ -30,7 +30,7 @@
 #include "xwayland.h"
 
 static int
-weston_wm_write_property(int fd, uint32_t mask, void *data)
+writable_callback(int fd, uint32_t mask, void *data)
 {
 	struct weston_wm *wm = data;
 	unsigned char *property;
@@ -43,6 +43,8 @@ weston_wm_write_property(int fd, uint32_t mask, void *data)
 	len = write(fd, property + wm->property_start, remainder);
 	if (len == -1) {
 		free(wm->property_reply);
+		wm->property_reply = NULL;
+		if (wm->property_source)
 			wl_event_source_remove(wm->property_source);
 		close(fd);
 		weston_log("write error to target fd: %m\n");
@@ -56,6 +58,8 @@ weston_wm_write_property(int fd, uint32_t mask, void *data)
 	wm->property_start += len;
 	if (len == remainder) {
 		free(wm->property_reply);
+		wm->property_reply = NULL;
+		if (wm->property_source)
 			wl_event_source_remove(wm->property_source);
 
 		if (wm->incr) {
@@ -71,6 +75,21 @@ weston_wm_write_property(int fd, uint32_t mask, void *data)
 	return 1;
 }
 
+static void
+weston_wm_write_property(struct weston_wm *wm, xcb_get_property_reply_t *reply)
+{
+	wm->property_start = 0;
+	wm->property_reply = reply;
+	writable_callback(wm->data_source_fd, WL_EVENT_WRITABLE, wm);
+
+	if (wm->property_reply)
+		wm->property_source =
+			wl_event_loop_add_fd(wm->server->loop,
+					     wm->data_source_fd,
+					     WL_EVENT_WRITABLE,
+					     writable_callback, wm);
+}
+
 static void
 weston_wm_get_incr_chunk(struct weston_wm *wm)
 {
@@ -90,14 +109,7 @@ weston_wm_get_incr_chunk(struct weston_wm *wm)
 	dump_property(wm, wm->atom.wl_selection, reply);
 
 	if (xcb_get_property_value_length(reply) > 0) {
-		wm->property_start = 0;
-		wm->property_source =
-			wl_event_loop_add_fd(wm->server->loop,
-					     wm->data_source_fd,
-					     WL_EVENT_WRITABLE,
-					     weston_wm_write_property,
-					     wm);
-		wm->property_reply = reply;
+		weston_wm_write_property(wm, reply);
 	} else {
 		weston_log("transfer complete\n");
 		close(wm->data_source_fd);
@@ -223,14 +235,7 @@ weston_wm_get_selection_data(struct weston_wm *wm)
 	} else {
 		dump_property(wm, wm->atom.wl_selection, reply);
 		wm->incr = 0;
-		wm->property_start = 0;
-		wm->property_source =
-			wl_event_loop_add_fd(wm->server->loop,
-					     wm->data_source_fd,
-					     WL_EVENT_WRITABLE,
-					     weston_wm_write_property,
-					     wm);
-		wm->property_reply = reply;
+		weston_wm_write_property(wm, reply);
 	}
 }