Improve code robustness.

* exwm-layout.el (exwm-layout-unset-fullscreen)
	(exwm-layout-set-fullscreen): Use `user-error' rather than
	`cl-assert'.
	* exwm-input.el (exwm-input--set-focus): Silently accept unknown
	ids. (exwm-input--grab-keyboard) (exwm-input--release-keyboard):
        Silently ignore calls for windows that have no buffer.
	* exwm-manage.el (exwm-manage--kill-client): Don't throw error
	when trying to kill a vanished window.

exwm-input.el

@@ -56,6 +56,7 @@ It's updated in several occasions, and only used by `exwm-input--set-focus'.")
 
 (defun exwm-input--set-focus (id)
   "Set input focus to window ID in a proper way."
+  (when (exwm--id->buffer id)
     (with-current-buffer (exwm--id->buffer id)
       (if (and (not exwm--hints-input)
                (memq xcb:Atom:WM_TAKE_FOCUS exwm--protocols))
@@ -75,7 +76,7 @@ It's updated in several occasions, and only used by `exwm-input--set-focus'.")
             (make-instance 'xcb:SetInputFocus
                            :revert-to xcb:InputFocus:Parent :focus id
                            :time xcb:Time:CurrentTime)))
-    (xcb:flush exwm--connection)))
+      (xcb:flush exwm--connection))))
 
 (defvar exwm-input--focus-window nil "The (Emacs) window to be focused.")
 (defvar exwm-input--redirected nil
@@ -311,7 +312,7 @@ It's updated in several occasions, and only used by `exwm-input--set-focus'.")
 (defun exwm-input--grab-keyboard (&optional id)
   "Grab all key events on window ID."
   (unless id (setq id (exwm--buffer->id (window-buffer))))
-  (cl-assert id)
+  (when id
     (when (xcb:+request-checked+request-check exwm--connection
               (make-instance 'xcb:GrabKey
                              :owner-events 0 :grab-window id
@@ -320,18 +321,18 @@ It's updated in several occasions, and only used by `exwm-input--set-focus'.")
                              :pointer-mode xcb:GrabMode:Async
                              :keyboard-mode xcb:GrabMode:Async))
       (exwm--log "Failed to grab keyboard for #x%x" id))
-  (setq exwm--on-KeyPress 'exwm-input--on-KeyPress-line-mode))
+    (setq exwm--on-KeyPress 'exwm-input--on-KeyPress-line-mode)))
 
 (defun exwm-input--release-keyboard (&optional id)
   "Ungrab all key events on window ID."
   (unless id (setq id (exwm--buffer->id (window-buffer))))
-  (cl-assert id)
+  (when id
     (when (xcb:+request-checked+request-check exwm--connection
               (make-instance 'xcb:UngrabKey
                              :key xcb:Grab:Any :grab-window id
                              :modifiers xcb:ModMask:Any))
       (exwm--log "Failed to release keyboard for #x%x" id))
-  (setq exwm--on-KeyPress 'exwm-input--on-KeyPress-char-mode))
+    (setq exwm--on-KeyPress 'exwm-input--on-KeyPress-char-mode)))
 
 (defun exwm-input-grab-keyboard (&optional id)
   "Switch to line-mode."

exwm-layout.el

@@ -93,6 +93,8 @@
   "Make window ID fullscreen."
   (interactive)
   (with-current-buffer (if id (exwm--id->buffer id) (window-buffer))
+    (when exwm--fullscreen
+      (user-error "Already in full-screen mode."))
     ;; Set the floating frame fullscreen first when the client is floating
     (when exwm--floating-frame
       (let* ((outer-id (frame-parameter exwm--floating-frame 'exwm-outer-id))
@@ -130,7 +132,6 @@
                        :window exwm--id
                        :data (vector xcb:Atom:_NET_WM_STATE_FULLSCREEN)))
     (xcb:flush exwm--connection)
-    (cl-assert (not exwm--fullscreen))
     (setq exwm--fullscreen t)
     (exwm-input-release-keyboard)))
 
@@ -138,6 +139,8 @@
   "Restore window from fullscreen state."
   (interactive)
   (with-current-buffer (if id (exwm--id->buffer id) (window-buffer))
+    (unless exwm--fullscreen
+      (user-error "Not in full-screen mode."))
     ;; Restore the floating frame if the client is floating
     (when exwm--floating-frame
       (xcb:+request exwm--connection
@@ -156,7 +159,6 @@
     (xcb:+request exwm--connection
         (make-instance 'xcb:ewmh:set-_NET_WM_STATE :window exwm--id :data []))
     (xcb:flush exwm--connection)
-    (cl-assert exwm--fullscreen)
     (setq exwm--fullscreen nil)
     (exwm-input-grab-keyboard)))
 

exwm-manage.el

@@ -260,10 +260,9 @@ corresponding buffer.")
   "Kill an X client."
   (interactive)
   (unless id (setq id (exwm--buffer->id (current-buffer))))
-  (let ((pid (slot-value
+  (let* ((response (xcb:+request-unchecked+reply exwm--connection
-              (xcb:+request-unchecked+reply exwm--connection
+                       (make-instance 'xcb:ewmh:get-_NET_WM_PID :window id)))
-                  (make-instance 'xcb:ewmh:get-_NET_WM_PID :window id))
+         (pid (and response (slot-value response 'value))))
-              'value)))
     (if pid
         (signal-process pid 'SIGKILL)
       (xcb:+request exwm--connection