Library for XDG things (was Re: [PATCH] checking eww download directory)

[Mark Oteiza]

On 28/01/17 at 09:53am, Eli Zaretskii wrote:
> > From: Mark Oteiza <address@hidden>
> > Date: Fri, 27 Jan 2017 15:17:10 -0500
> > Cc: address@hidden
> > 
> > > How hard would it be to integrate with the xdg-user-dirs standard
> > > instead? On many GNU/Linux systems, running "xdg-user-dir DOWNLOAD" is
> > > enough (http://stackoverflow.com/questions/13161226/).  This would
> > > make eww consistent with Firefox, too (they take the slightly more
> > > painful route of parsing ~/.config/user-dirs.dirs:
> > > https://dxr.mozilla.org/mozilla-release/source/xpcom/io/SpecialSystemDirectory.cpp#256)
> > 
> > An xdg.el would be nice to have for elisp that interfaces
> > with XDG-following things.  mpc.el and image-dired.el are two files that
> > come to mind: mpd follows [0], image-dired
> > supports [1].  Actually, it looks like those are the only two files with
> > XDG bits, and I put them there.  How about that.
> > 
> > Anyways, these aren't difficult to implement.  If it would be a welcome
> > addition I'd have a go at writing it.
> > 
> > [0] https://standards.freedesktop.org/basedir-spec/basedir-spec-latest.html
> > [1] 
> > https://specifications.freedesktop.org/thumbnail-spec/thumbnail-spec-latest.html
> 
> This has come up before.  I think this would be welcome, but one issue
> that didn't get resolved is how much of that should be automatic
> (i.e. should Emacs automatically search certain directories for
> certain kinds of files), and if so, how to split the imaginary xdg.el
> into two parts, with the automatic part living in some preloaded file
> (probably files.el).  There's also the issue of whether users and/or
> Lisp programs should be able to disable this search (e.g., by binding
> some variable).

Sounds like a daunting goal.  I imagine having XDG bits preloaded would
only really be necessary if Emacs decided it was going to follow the
base directory spec (and probably only on GNU/Linux).

In the mean time, the following is a small library of some useful
functions. At least for the base directory spec it remains rather low
level as the standard gives a lot of freedom to how applications want to
handle multiple locations of config/data:

;;; xdg.el --- XDG specification and standard library -*- lexical-binding: t -*-

;; Copyright (C) 2017  Mark Oteiza <address@hidden>

;; Author: Mark Oteiza <address@hidden>
;; Keywords: files, data

;; This program is free software; you can redistribute it and/or
;; modify it under the terms of the GNU General Public License
;; as published by the Free Software Foundation; either version 3
;; of the License, or (at your option) any later version.

;; This program is distributed in the hope that it will be useful,
;; but WITHOUT ANY WARRANTY; without even the implied warranty of
;; MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
;; GNU General Public License for more details.

;; You should have received a copy of the GNU General Public License
;; along with this program. If not, see <http://www.gnu.org/licenses/>.

;;; Commentary:

;; Library providing some convenience functions for the following XDG
;; standards and specifications
;;
;; - XDG Base Directory Specification
;; - Thumbnail Managing Standard
;; - xdg-user-dirs configuration

;;; Code:


;; XDG Base Directory Specification
;; https://standards.freedesktop.org/basedir-spec/basedir-spec-latest.html

(defmacro xdg--dir-home (environ default-path)
  (declare (debug (stringp stringp)))
  (let ((env (make-symbol "env")))
    `(let ((,env (getenv ,environ)))
       (if (or (null ,env) (not (file-name-absolute-p ,env)))
           (expand-file-name ,default-path)
         ,env))))

(defun xdg-config-home ()
  "Return the base directory for user specific configuration files."
  (xdg--dir-home "XDG_CONFIG_HOME" "~/.config"))

(defun xdg-cache-home ()
  "Return the base directory for user specific cache files."
  (xdg--dir-home "XDG_CACHE_HOME" "~/.cache"))

(defun xdg-data-home ()
  "Return the base directory for user specific data files."
  (xdg--dir-home "XDG_DATA_HOME" "~/.local/share"))

(defun xdg-runtime-dir ()
  "Return the value of $XDG_RUNTIME_DIR."
  (getenv "XDG_RUNTIME_DIR"))

(defun xdg-config-dirs ()
  "Return the config directory search path as a list."
  (let ((env (getenv "XDG_CONFIG_DIRS")))
    (if (or (null env) (string= env ""))
        '("/etc/xdg")
      (parse-colon-path env))))

(defun xdg-data-dirs ()
  "Return the data directory search path as a list."
  (let ((env (getenv "XDG_CONFIG_DIRS")))
    (if (or (null env) (string= env ""))
        '("/usr/local/share/" "/usr/share/")
      (parse-colon-path env))))


;; Thumbnail Managing Standard
;; 
https://specifications.freedesktop.org/thumbnail-spec/thumbnail-spec-latest.html

(defun xdg-thumb-uri (filename)
  "Return the canonical URI for FILENAME.
If FILENAME has absolute path /foo/bar.jpg, its canonical URI is
file:///foo/bar.jpg"
  (concat "file://" (expand-file-name filename)))

(defun xdg-thumb-name (filename)
  "Return the appropriate thumbnail filename for FILENAME."
  (concat (md5 (xdg-thumb-uri filename)) ".png"))

(defun xdg-thumb-mtime (filename)
  "Return modification time of FILENAME as integral seconds from the epoch."
  (floor (float-time (nth 5 (file-attributes filename)))))


;; XDG User Directories
;; https://www.freedesktop.org/wiki/Software/xdg-user-dirs/

(defconst xdg-line-regexp
  (eval-when-compile
    (rx "XDG_"
        (group-n 1 (or "DESKTOP" "DOWNLOAD" "TEMPLATES" "PUBLICSHARE"
                       "DOCUMENTS" "MUSIC" "PICTURES" "VIDEOS"))
        "_DIR=\""
        (group-n 2 (or "/" "$HOME/") (*? (or (not (any "\"")) "\\\"")))
        "\""))
  "Regexp matching non-comment lines in xdg-user-dirs config files.")

(defvar xdg-user-dirs nil
  "Alist of directory keys and values.")

(defun xdg--user-dirs-parse-line ()
  "Return pair of user-dirs key to directory value in LINE, otherwise nil.
This should be called at the beginning of a line."
  (skip-chars-forward "[:blank:]")
  (when (and (/= (following-char) ?#)
             (looking-at xdg-line-regexp))
    (let ((k (match-string 1))
          (v (match-string 2)))
      (when (and k v) (cons k v)))))

(defun xdg--user-dirs-parse-file (filename)
  "Return alist of xdg-user-dirs from FILENAME."
  (let (elt res)
    (with-temp-buffer
      (insert-file-contents filename)
      (goto-char (point-min))
      (while (not (eobp))
        (setq elt (xdg--user-dirs-parse-line))
        (when (consp elt) (push elt res))
        (forward-line)))
    res))

(defun xdg-user-dir (name)
  "Return the path of user directory referred to by NAME."
  (when (null xdg-user-dirs)
    (setq xdg-user-dirs
          (xdg--user-dirs-parse-file
           (expand-file-name "user-dirs.dirs" (xdg-config-home)))))
  (cdr (assoc name xdg-user-dirs)))

(provide 'xdg)

;;; xdg.el ends here