data-device: implement drags with no data source for self-dnd

Properly handle a drag with no data source, i.e., don't crash and send
events only to the client that initiated the drag. This way a client can
do self drag and drop without offering anything to other clients.

TODO

@@ -78,10 +78,6 @@ EWMH
 
  - window move and resize functionality for kb and touch.
 
- - dnd loose ends: self-dnd: initiate dnd with a null data-source,
-   compositor will not offer to other clients, client has to know
-   internally what's offered and how to transfer data. no fd passing.
-
  - Protocol for specifying title bar rectangle (for moving
    unresponsive apps).  Rectangle for close button, so we can popup
    force-close dialog if application doesn't respond to ping event

protocol/wayland.xml

@@ -317,15 +317,23 @@
     <request name="start_drag">
       <description summary="start drag and drop operation">
 	This request asks the compositor to start a drag and drop
-	operation on behalf of the client.  The source argument is the
-	data source that provides the data for the eventual data
-	transfer.  The origin surface is the surface where the drag
-	originates and the client must have an active implicit grab
-	that matches the serial.  The icon surface is an optional
-	(can be nil) surface that provides an icon to be moved around
-	with the cursor.  Initially, the top-left corner of the icon
-	surface is placed at the cursor hotspot, but subsequent
-	surface.attach request can move the relative position.
+	operation on behalf of the client.
+
+	The source argument is the data source that provides the data
+	for the eventual data transfer. If source is NULL, enter, leave
+	and motion events are sent only to the client that initiated the
+	drag and the client is expected to handle the data passing
+	internally.
+
+	The origin surface is the surface where the drag originates and
+	the client must have an active implicit grab that matches the
+	serial.
+
+	The icon surface is an optional (can be nil) surface that
+	provides an icon to be moved around with the cursor.  Initially,
+	the top-left corner of the icon surface is placed at the cursor
+	hotspot, but subsequent surface.attach request can move the
+	relative position.
       </description>
       <arg name="source" type="object" interface="wl_data_source"/>
       <arg name="origin" type="object" interface="wl_surface"/>

src/data-device.c

@@ -180,7 +180,7 @@ drag_grab_focus(struct wl_pointer_grab *grab,
 		struct wl_surface *surface, wl_fixed_t x, wl_fixed_t y)
 {
 	struct wl_seat *seat = container_of(grab, struct wl_seat, drag_grab);
-	struct wl_resource *resource, *offer;
+	struct wl_resource *resource, *offer = NULL;
 	struct wl_display *display;
 	uint32_t serial;
 
@@ -191,13 +191,22 @@ drag_grab_focus(struct wl_pointer_grab *grab,
 		seat->drag_focus = NULL;
 	}
 
-	if (surface)
+	if (!surface)
+		return;
+
+	if (!seat->drag_data_source &&
+	    surface->resource.client != seat->drag_client)
+		return;
+
 	resource = find_resource(&seat->drag_resource_list,
 				 surface->resource.client);
-	if (surface && resource) {
+	if (!resource)
+		return;
+
 	display = wl_client_get_display(resource->client);
 	serial = wl_display_next_serial(display);
 
+	if (seat->drag_data_source)
 		offer = wl_data_source_send_offer(seat->drag_data_source,
 						  resource);
 
@@ -210,7 +219,6 @@ drag_grab_focus(struct wl_pointer_grab *grab,
 		      &seat->drag_focus_listener);
 	seat->drag_focus_resource = resource;
 	grab->focus = surface;
-	}
 }
 
 static void
@@ -247,6 +255,7 @@ data_device_end_drag_grab(struct wl_seat *seat)
 
 	seat->drag_data_source = NULL;
 	seat->drag_surface = NULL;
+	seat->drag_client = NULL;
 }
 
 static void
@@ -261,6 +270,7 @@ drag_grab_button(struct wl_pointer_grab *grab,
 
 	if (seat->pointer->button_count == 0 && state == 0) {
 		data_device_end_drag_grab(seat);
+		if (seat->drag_data_source)
 			wl_list_remove(&seat->drag_data_source_listener.link);
 	}
 }
@@ -304,10 +314,16 @@ data_device_start_drag(struct wl_client *client, struct wl_resource *resource,
 
 	seat->drag_grab.interface = &drag_grab_interface;
 
+	seat->drag_client = client;
+	seat->drag_data_source = NULL;
+
+	if (source_resource) {
 		seat->drag_data_source = source_resource->data;
-	seat->drag_data_source_listener.notify = destroy_data_device_source;
+		seat->drag_data_source_listener.notify =
+			destroy_data_device_source;
 		wl_signal_add(&source_resource->destroy_signal,
 			      &seat->drag_data_source_listener);
+	}
 
 	if (icon_resource) {
 		seat->drag_surface = icon_resource->data;

src/wayland-server.h

@@ -297,6 +297,7 @@ struct wl_seat {
 	struct wl_signal selection_signal;
 
 	struct wl_list drag_resource_list;
+	struct wl_client *drag_client;
 	struct wl_data_source *drag_data_source;
 	struct wl_listener drag_data_source_listener;
 	struct wl_surface *drag_focus;