bug#6214: 23.1; json-read-string crashes emacs with long string

[Carl Worth]

> Please describe exactly what actions triggered the bug
> and the precise symptoms of the bug:

A user of the emacs-based mail client, Notmuch [*], found that
attempting to display a particular message would consistently
causes a segmentation fault in emacs.

I tracked this down to calling `json-read-string' with a very long
string, (roughly 1 million characters). Rather than including that
enormous string in this message, here's a little snippet of emacs lisp
that creates and reads such a string. So, if evaluated, this code should
trigger the segmentation fault, (assuming a copy of GPLv3 exists at
/usr/share/emacs/23.1/etc/COPYING---adjust the filename if necessary).

;; Caution: Evaluating the block below has been known to crash emacs
(with-temp-buffer
  (require 'json)
  ;; First we just need a lot of text. 32 copies of GPLv3 seems to do it
  (dotimes (i 32)
    (insert-file "/usr/share/emacs/23.1/etc/COPYING"))
  ;; Now create a buffer with a json-encoded version of the text
  (let ((json-string (json-encode-string (buffer-string))))
    (with-temp-buffer
      (insert json-string)
      (goto-char (point-min))
      ;; And try to read the string. This triggers the segfault.
      (json-read-string))))

> If Emacs crashed, and you have the Emacs process in the gdb debugger,
> please include the output from the following gdb commands:
>    `bt full' and `xbacktrace'.

I haven't attempted to debug this within gdb yet, (I'll have to get my
hands on a build of emacs with debugging symbols first). But I wanted to
share things right away, so that perhaps someone else could do further
debugging and follow up.

In the meantime, notmuch folks, if you've got a good idea for modifying
notmuch to avoid this bug I'd be glad to hear it. Adjust followups to
include the notmuch list and not the gnu.org bug address as appropriate.

-Carl

[*] http://notmuchmail.org

PS. Here are some of the details provided by `report-emacs-bug':

In GNU Emacs 23.1.1 (i486-pc-linux-gnu, GTK+ Version 2.18.2)
 of 2010-01-26 on raven, modified by Debian
Windowing system distributor `The X.Org Foundation', version 11.0.10799001
configured using `configure  '--build=i486-linux-gnu' '--host=i486-linux-gnu' 
'--prefix=/usr' '--sharedstatedir=/var/lib' '--libexecdir=/usr/lib' 
'--localstatedir=/var/lib' '--infodir=/usr/share/info' 
'--mandir=/usr/share/man' '--with-pop=yes' 
'--enable-locallisppath=/etc/emacs23:/etc/emacs:/usr/local/share/emacs/23.1/site-lisp:/usr/local/share/emacs/site-lisp:/usr/share/emacs/23.1/site-lisp:/usr/share/emacs/site-lisp:/usr/share/emacs/23.1/leim'
 '--with-x=yes' '--with-x-toolkit=gtk' '--with-toolkit-scroll-bars' 
'build_alias=i486-linux-gnu' 'host_alias=i486-linux-gnu' 'CFLAGS=-DDEBIAN -g 
-O2' 'LDFLAGS=-g' 'CPPFLAGS=''

Important settings:
  value of $LC_ALL: nil
  value of $LC_COLLATE: nil
  value of $LC_CTYPE: nil
  value of $LC_MESSAGES: nil
  value of $LC_MONETARY: nil
  value of $LC_NUMERIC: nil
  value of $LC_TIME: nil
  value of $LANG: en_US.UTF-8
  value of $XMODIFIERS: nil
  locale-coding-system: utf-8-unix
  default-enable-multibyte-characters: t

-- 
carl.d.worth@intel.com