Re: auth-source backend stuff

[Eric Abrahamsen <eric@ericabrahamsen.net>]

Ted Zlatanov <tzz@lifelogs.com> writes:

> On Sat, 21 Nov 2015 11:55:16 +0800 Eric Abrahamsen <eric@ericabrahamsen.net> wrote: 
>
> EA> Ted Zlatanov <tzz@lifelogs.com> writes:
>>> On Tue, 17 Nov 2015 13:06:47 +0800 Eric Abrahamsen <eric@ericabrahamsen.net> wrote: 
>>> 
> EA> I see that auth-source.el is written to provide backend subclassing, but
> EA> I've found the system as a whole a bit confusing to grasp. I've got a
> EA> couple of questions I'm hoping someone can provide pointers for.
>>> 
>>> I wrote docs for it here:
>>> http://thread.gmane.org/gmane.emacs.devel/143198/focus=144282 but the
>>> demand hasn't been high for custom backends.
>>> 
>>> I think the :create stuff is what confuses people the most.
>
> EA> Thanks for this! It was also enough to clue me in to the fact that
> EA> auth-source has its own info manual, which I hadn't realized. Between
> EA> the various explanations I ought to be able to make this work.
>
> I would really appreciate it if you added your experience to the
> developer section of the auth-source manual, and especially let me know
> if you think the code needs changing.  As far as I know Daiki Ueno is
> the only one to have added a backend, so with you we'd have enough
> experience to improve the code and simplify or expand it as needed.

I have to admit it's still a bit hazy to me. Do I really have to edit
the auth-source.el source code in order to add a new backend? I can see
the utility of allowing users to refer to backend types using a simple
string, but this makes it hard to add ad-hoc backends from outside the
code base. Could `auth-source-backend-parse' be edited to look like:

(defun auth-source-backend-parse (entry)
  (if (child-of-class-p entry 'auth-source-backend)
    entry
    (auth-source-backend-parse-parameters
      ....)))

That would allow me to instantiate a backend in my own config code, and
then just slap it into `auth-sources'.

My other comment (admittedly totally unhelpful at this point) is that it
seems a bit odd to be using classes, but then not using subclassing, or
generic methods. The :type slot seems to be doing work that should be
done by subclassing, and the :create-function and :search-function slots
seem like they ought to be generic methods. Generic methods would
probably also be the right solution for a unified prompt interface.

Perhaps there are other considerations that prevent this -- I'm not that
familiar with the code. And, practically, it would probably be too much
work to re-write at that level, now. But I thought I'd mention it
anyway!

Right now the :initform arg to :create-function and :search-function is
"ignore". I'm not certain, but it looks like this isn't working the way
it's supposed to -- I was obliged to actually write stump functions
before the class definition could be eval'ed. It appears that the
current way to do this is to set the :allow-nil-initforms class-level
option.

Otherwise, here's how far I've gotten with this. The create function
seems like it will be a nightmare of prompting, so I'm punting on that
for now. Users would almost certainly have to create their own mapping
between host/port/user combinations and pass entries, as I doubt
anyone's going to want to structure their pass entries so that they're
somehow programmatically findable given the host and user. Here's code
that doesn't currently work, but I wish it would:

(defun my-stub-create-function ()
  nil)

(defcustom my-server-to-pass-entry-map
  ;; Does :host correspond to the Gnus server name, or to the actual
  ;; nnimap-address host?
  '((:host "mail.ericabrahamsen.net"
	   :user "eric@ericabrahamsen.net" :entry "email/ea")
    (:host "mail.paper-republic.org"
	   :user "eric@paper-republic.org" :entry "email/pr")))

(defun* my-pass-search-function (&rest
                                    spec
                                    &key backend create delete
                                    max
                                    &allow-other-keys)
  (let ((pass-arg
	 (catch 'found
	   (dolist (m my-server-to-pass-entry-map)
	     (when (and (string= (plist-get m :host)
				 (plist-get spec :host))
			(string= (plist-get m :user)
				 (plist-get spec :user)))
	       (throw 'found (plist-get m :entry)))))))
    (when pass-arg
      (shell-command-to-string (format "pass show %s" pass-arg)))))

(setq my-halfassed-pass-backend
      (make-instance 'auth-source-backend
		     :type 'pass-utility
		     :source "dummy string"
		     :create-function 'my-stub-create-function
		     :search-function 'my-pass-search-function))

(setq auth-sources (list my-halfassed-pass-backend))


I won't waste time apologizing for this awful code, but this is the bare
outline of what I'm trying to do. The other search functions for other
backends are pretty enormously complex, and I don't yet know why.

Thanks!
Eric