data-device: send INVALID_FINISH when operation != dnd

The documentation of wl_data_offer::finish states that it should be
used to signify that a drag and drop operation is completed. So send
WL_DATA_OFFER_ERROR_INVALID_FINISH when the client calls the finish
request but the operation isn't dnd.

Signed-off-by: Harish Krupo <harishkrupo@gmail.com>

include/libweston/libweston.h

@@ -427,6 +427,7 @@ struct weston_data_source {
 	struct weston_seat *seat;
 	bool accepted;
 	bool actions_set;
+	bool set_selection;
 	uint32_t dnd_actions;
 	enum wl_data_device_manager_dnd_action current_dnd_action;
 	enum wl_data_device_manager_dnd_action compositor_action;

libweston/data-device.c

@@ -220,6 +220,13 @@ data_offer_finish(struct wl_client *client, struct wl_resource *resource)
 	if (!offer->source || offer->source->offer != offer)
 		return;
 
+	if (offer->source->set_selection) {
+		wl_resource_post_error(offer->resource,
+				       WL_DATA_OFFER_ERROR_INVALID_FINISH,
+				       "finish only valid for drag n drop");
+		return;
+	}
+
 	/* Disallow finish while we have a grab driving drag-and-drop, or
 	 * if the negotiation is not at the right stage
 	 */
@@ -1145,6 +1152,7 @@ weston_seat_set_selection(struct weston_seat *seat,
 
 	seat->selection_data_source = source;
 	seat->selection_serial = serial;
+	source->set_selection = true;
 
 	if (keyboard)
 		focus = keyboard->focus;
@@ -1267,6 +1275,7 @@ create_data_source(struct wl_client *client,
 	source->dnd_actions = 0;
 	source->current_dnd_action = WL_DATA_DEVICE_MANAGER_DND_ACTION_NONE;
 	source->compositor_action = WL_DATA_DEVICE_MANAGER_DND_ACTION_NONE;
+	source->set_selection = false;
 
 	wl_array_init(&source->mime_types);