Implement _NET_WM_PING for X11 (#977)

CHANGELOG.md

@@ -3,6 +3,7 @@
 - On Mac, implement `DeviceEvent::Button`.
 - Change `Event::Suspended(true / false)` to `Event::Suspended` and `Event::Resumed`.
 - On X11, fix sanity check which checks that a monitor's reported width and height (in millimeters) are non-zero when calculating the DPI factor.
+- On X11, implement `_NET_WM_PING` to allow desktop environment to kill unresponsive programs.
 - On Windows, when a window is initially invisible, it won't take focus from the existing visible windows.
 
 # 0.20.0 Alpha 1

src/platform_impl/linux/x11/event_processor.rs

@@ -131,6 +131,16 @@ impl<T: 'static> EventProcessor<T> {
                         window_id,
                         event: WindowEvent::CloseRequested,
                     });
+                } else if client_msg.data.get_long(0) as ffi::Atom == wt.net_wm_ping {
+                    let response_msg: &mut ffi::XClientMessageEvent = xev.as_mut();
+                    response_msg.window = wt.root;
+                    wt.xconn
+                        .send_event(
+                            wt.root,
+                            Some(ffi::SubstructureNotifyMask | ffi::SubstructureRedirectMask),
+                            *response_msg,
+                        )
+                        .queue();
                 } else if client_msg.message_type == self.dnd.atoms.enter {
                     let source_window = client_msg.data.get_long(0) as c_ulong;
                     let flags = client_msg.data.get_long(1);

src/platform_impl/linux/x11/mod.rs

@@ -46,6 +46,7 @@ use crate::{
 pub struct EventLoopWindowTarget<T> {
     xconn: Arc<XConnection>,
     wm_delete_window: ffi::Atom,
+    net_wm_ping: ffi::Atom,
     ime_sender: ImeSender,
     root: ffi::Window,
     ime: RefCell<Ime>,
@@ -75,6 +76,8 @@ impl<T: 'static> EventLoop<T> {
 
         let wm_delete_window = unsafe { xconn.get_atom_unchecked(b"WM_DELETE_WINDOW\0") };
 
+        let net_wm_ping = unsafe { xconn.get_atom_unchecked(b"_NET_WM_PING\0") };
+
         let dnd = Dnd::new(Arc::clone(&xconn))
             .expect("Failed to call XInternAtoms when initializing drag and drop");
 
@@ -142,6 +145,7 @@ impl<T: 'static> EventLoop<T> {
                 ime_sender,
                 xconn,
                 wm_delete_window,
+                net_wm_ping,
                 pending_redraws: Default::default(),
             }),
             _marker: ::std::marker::PhantomData,

src/platform_impl/linux/x11/window.rs

@@ -330,8 +330,9 @@ impl UnownedWindow {
                 (xconn.xlib.XSetWMProtocols)(
                     xconn.display,
                     window.xwindow,
-                    &event_loop.wm_delete_window as *const ffi::Atom as *mut ffi::Atom,
-                    1,
+                    &[event_loop.wm_delete_window, event_loop.net_wm_ping] as *const ffi::Atom
+                        as *mut ffi::Atom,
+                    2,
                 );
             } //.queue();