bug#30421: 25.3; desktop.el: Steal lock when no living "emacs" process owns it

[Pierre Neidhardt <ambrevar@gmail.com>]

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Did not work.  Recipe:

- init.el:

	(require 'desktop)
	
	(defun desktop-read (&optional dirname)
	  "Read and process the desktop file in directory DIRNAME.
	Look for a desktop file in DIRNAME, or if DIRNAME is omitted, look in
	directories listed in `desktop-path'.  If a desktop file is found, it
	is processed and `desktop-after-read-hook' is run.  If no desktop file
	is found, clear the desktop and run `desktop-no-desktop-file-hook'.
	This function is a no-op when Emacs is running in batch mode.
	It returns t if a desktop file was loaded, nil otherwise."
	  (interactive)
	  (unless noninteractive
	    (setq desktop-dirname
	          (file-name-as-directory
	           (expand-file-name
	            (or
	             ;; If DIRNAME is specified, use it.
	             (and (< 0 (length dirname)) dirname)
	             ;; Otherwise search desktop file in desktop-path.
	             (let ((dirs desktop-path))
	               (while (and dirs
	                           (not (file-exists-p
	                                 (desktop-full-file-name (car dirs)))))
	                 (setq dirs (cdr dirs)))
	               (and dirs (car dirs)))
	             ;; If not found and `desktop-path' is non-nil, use its first element.
	             (and desktop-path (car desktop-path))
	             ;; Default: .emacs.d.
	             user-emacs-directory))))
	    (if (file-exists-p (desktop-full-file-name))
	        ;; Desktop file found, but is it already in use?
	        (let ((desktop-first-buffer nil)
	              (desktop-buffer-ok-count 0)
	              (desktop-buffer-fail-count 0)
	              (owner (desktop-owner))
	              ;; Avoid desktop saving during evaluation of desktop buffer.
	              (desktop-save nil)
	              (desktop-autosave-was-enabled))
	          (if (and owner
	                   (memq desktop-load-locked-desktop '(nil ask))
	                   (or (null desktop-load-locked-desktop)
	                       ;; Without a visible frame, Emacs daemon cannot
	                       ;; prompt the user so we don't load.
	                       (and (daemonp) (= (length (visible-frame-list)) 1))
	                       (not (y-or-n-p (format "Warning: desktop file appears to be in use by PID %s.\n\
	Using it may cause conflicts.  Use it anyway? " owner)))))
	              (let ((default-directory desktop-dirname))
	                (setq desktop-dirname nil)
	                (run-hooks 'desktop-not-loaded-hook)
	                (unless desktop-dirname
	                  (message "Desktop file in use; not loaded.")))
	            (desktop-lazy-abort)
	            ;; Temporarily disable the autosave that will leave it
	            ;; disabled when loading the desktop fails with errors,
	            ;; thus not overwriting the desktop with broken contents.
	            (setq desktop-autosave-was-enabled
	                  (memq 'desktop-auto-save-set-timer (default-toplevel-value 'window-configuration-change-hook)))
	            (desktop-auto-save-disable)
	            ;; Evaluate desktop buffer and remember when it was modified.
	            (load (desktop-full-file-name) t t t)
	            (setq desktop-file-modtime (nth 5 (file-attributes (desktop-full-file-name))))
	            ;; If it wasn't already, mark it as in-use, to bother other
	            ;; desktop instances.
	            (unless (eq (emacs-pid) owner)
	              (condition-case nil
	                  (desktop-claim-lock)
	                (file-error (message "Couldn't record use of desktop file")
	                            (sit-for 1))))
	
	            (unless (desktop-restoring-frameset-p)
	              ;; `desktop-create-buffer' puts buffers at end of the buffer list.
	              ;; We want buffers existing prior to evaluating the desktop (and
	              ;; not reused) to be placed at the end of the buffer list, so we
	              ;; move them here.
	              (mapc 'bury-buffer
	                    (nreverse (cdr (memq desktop-first-buffer (nreverse (buffer-list))))))
	              (switch-to-buffer (car (buffer-list))))
	            (run-hooks 'desktop-delay-hook)
	            (setq desktop-delay-hook nil)
	            (desktop-restore-frameset)
	            (run-hooks 'desktop-after-read-hook)
	            (message "Desktop: %s%d buffer%s restored%s%s."
	                     (if desktop-saved-frameset
	                         (let ((fn (length (frameset-states desktop-saved-frameset))))
	                           (format "%d frame%s, "
	                                   fn (if (= fn 1) "" "s")))
	                       "")
	                     desktop-buffer-ok-count
	                     (if (= 1 desktop-buffer-ok-count) "" "s")
	                     (if (< 0 desktop-buffer-fail-count)
	                         (format ", %d failed to restore" desktop-buffer-fail-count)
	                       "")
	                     (if desktop-buffer-args-list
	                         (format ", %d to restore lazily"
	                                 (length desktop-buffer-args-list))
	                       ""))
	            (unless (desktop-restoring-frameset-p)
	              ;; Bury the *Messages* buffer to not reshow it when burying
	              ;; the buffer we switched to above.
	              (when (buffer-live-p (get-buffer "*Messages*"))
	                (bury-buffer "*Messages*"))
	              ;; Clear all windows' previous and next buffers, these have
	              ;; been corrupted by the `switch-to-buffer' calls in
	              ;; `desktop-restore-file-buffer' (bug#11556).  This is a
	              ;; brute force fix and should be replaced by a more subtle
	              ;; strategy eventually.
	              (walk-window-tree (lambda (window)
	                                  (set-window-prev-buffers window nil)
	                                  (set-window-next-buffers window nil))))
	            (setq desktop-saved-frameset nil)
	            (if desktop-autosave-was-enabled (desktop-auto-save-enable))
	            t))
	      ;; No desktop file found.
	      (let ((default-directory desktop-dirname))
	        (run-hooks 'desktop-no-desktop-file-hook))
	      (message "No desktop file.")
	      nil)))
	
	(when (daemonp)
	  (defun ambrevar/desktop-init (frame)
	    (when (frame-parameter frame 'client)
	      (desktop-save-mode)
	      (desktop-read)
	      (remove-hook 'after-make-frame-functions 'ambrevar/desktop-init)))
	  (add-hook 'after-make-frame-functions 'ambrevar/desktop-init))

- Leave a ~/.emacs.d/.emacs.desktop behind.
- Kill all emacs instances.
- `echo 0 > ~/.emacs.d/.emacs.desktop.lock`
- Optional: Switch to a TTY.
- Start `emacsclient -a '' -t` or `emacsclient -a '' -c` if not in a TTY.
- Emacs gives a blank screen (with a blinking cursor in a TTY). Press `n`.
- See the *Messages* buffer: the `n` was answering the desktop question.

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