eval'ing message-use-idna defcustom hangs emacs

[Eric Abrahamsen <eric@ericabrahamsen.net>]

Yesterday, emacs started hanging when I loaded gnus. The problem turned
out to be this defcustom from message.el:

;; (defcustom message-use-idna (and (condition-case nil (require 'idna)
;; 				   (file-error))
;; 				 (mm-coding-system-p 'utf-8)
;; 				 (executable-find idna-program)
;; 				 (string= (idna-to-ascii "räksmörgås")
;; 					  "xn--rksmrgs-5wao1o")
;; 				 t)
;;   "Whether to encode non-ASCII in domain names into ASCII according to IDNA.
;; GNU Libidn, and in particular the elisp package \"idna.el\" and
;; the external program \"idn\", must be installed for this
;; functionality to work."
;;   :version "22.1"
;;   :group 'message-headers
;;   :link '(custom-manual "(message)IDNA")
;;   :type '(choice (const :tag "Ask" ask)
;; 		 (const :tag "Never" nil)
;; 		 (const :tag "Always" t)))

idna can be required, and the executable is found (/usr/bin/idn).
Archlinux's libidn package is at 1.28-2. I'm using git gnus and
emacs-version "24.3.1". I don't think anything has changed recently.

It seems that the process call to idna never returns. I set
toggle-debug-on-quit, and hit C-g during the hang, and get this
backtrace:

Debugger entered--Lisp error: (quit)
  accept-process-output(#<process idna> 1)
  (while (and (eq (process-status idna-to-ascii-process) (quote run)) (null idna-to-ascii-response)) (accept-process-output idna-to-ascii-process 1))
  idna-to-ascii-response()
  (setq string (idna-to-ascii-response))
  (if (null proc) (error "Cannot start idn application (to-ascii)") (idna-to-ascii-response-clear) (process-send-string proc (concat str "\n")) (setq string (idna-to-ascii-response)) (if (and string (string= (substring string (1- (length string))) "\n")) (substring string 0 (1- (length string))) string))
  (let ((proc (idna-to-ascii-process)) string) (if (null proc) (error "Cannot start idn application (to-ascii)") (idna-to-ascii-response-clear) (process-send-string proc (concat str "\n")) (setq string (idna-to-ascii-response)) (if (and string (string= (substring string (1- (length string))) "\n")) (substring string 0 (1- (length string))) string)))
  idna-to-ascii("räksmörgås")
  eval((idna-to-ascii "räksmörgås") nil)
  eval-last-sexp-1(nil)
  eval-last-sexp(nil)
  call-interactively(eval-last-sexp nil nil)

I've just commented out the defcustom and set the variable directly to
t. That seems to work, but I hope someone can shed some light on the
above!

Thanks,
Eric