auto-update-gtags.el

(require 'vc-git)
(provide 'auto-update-gtags)

(defvar auto-update-gtags/interval (* 60 60) "interval seconds")
(defvar auto-update-gtags/command "gtags")
(defun auto-update-gtags/file-need-update-p (file interval)
  (and (file-writable-p file)
       (or (not (file-exists-p file))
           (> (- (float-time) (float-time (nth 5 (file-attributes file))))
              interval))))

(defun auto-update-gtags/update-if-needed (file interval &rest shell-command-args)
  (when (auto-update-gtags/file-need-update-p file interval)
    (apply 'start-process-shell-command shell-command-args)))

(defun auto-update-gtags/find-git-root ()
  (vc-git-root (or (buffer-file-name) default-directory)))


(defun auto-update-gtags/update-gtags-if-needed ()
  (unless (and (fboundp 'tramp-tramp-file-p) (tramp-tramp-file-p buffer-file-name))
    (let ((default-directory (auto-update-gtags/find-git-root)))
      (when default-directory
        (auto-update-gtags/update-if-needed
         "GTAGS"
         auto-update-gtags/interval
         "update gtags" nil auto-update-gtags/command)))))

(add-to-list 'find-file-hook 'auto-update-gtags/update-gtags-if-needed)