Don't let the daemon frame make a file visible

Question

I normally start Emacs in daemon mode and reload my last session with the desktop package. This causes whichever file desktop-read open last to remain visited in a buffer which is selected in the window which is selected in the “dummy” frame created for the daemon. As far as Emacs is concerned, this file is visible in a frame. But as far as I'm concerned, it's not.

I could fix all my code to skip the daemon's frame… if I could identify it. But it would be easier to just make the daemon frame visit a buffer whose visibility I don't particularly care about, like *Messages* or ​ *Minibuf-0.

What can I put in my init file to

• identify whether Emacs was started in daemon mode;
• identify the frame corresponding to the daemon;
• make this frame visit a harmless buffer, or kill it (what can go wrong if I do that), or otherwise not let it continue “showing” some interesting buffer?

Test by running emacs --daemon -q -l foo.emacs where foo.emacs is

;; -*-emacs-lisp-*-
(setq server-name "foo")
;; Simulate loading a desktop session
(find-file "~/.emacs")
;; Test code
(defun foo ()
  (interactive)
  (let* ((buffer (find-buffer-visiting "~/.emacs"))
         (window (get-buffer-window buffer t)))
    (if (interactive-p) (message "%S" window) window)))
;; Here goes the code I want

then I want emacsclient -s foo -e '(foo)' to return nil.

Answer 1

Immediately after starting Emacs, the daemon frame is the last one in the frame list. It has a single window which shows the last visited file.

(set-window-buffer (car (window-list (car (last (frame-list))))) "*Messages*")

In my use case, which is loading a desktop session, I need to do this after loading the session. I do this through a one-time hook.

(when (and (fboundp 'daemonp) (daemonp))
  (defun my-desktop-after-read-hook-set-window-buffer ()
    "Make the last frame in the frame list visit *Messages*.
This is intended for use after the first call to `desktop-read' in a daemon
Emacs. In this situation, the (sole) window in the daemon frame ends up
visiting the last buffer that was restored. Since this buffer is visited in
a window, some automation makes that window pop up (which has no actual
effect) instead of displaying the buffer in the current window or a new one."
    (set-window-buffer (car (window-list (car (last (frame-list)))))
                       "*Messages*")
    (remove-hook 'desktop-after-read-hook 'my-desktop-after-read-hook-set-window-buffer))
  (add-hook 'desktop-after-read-hook 'my-desktop-after-read-hook-set-window-buffer))

Ugly, but it works.