bug#42931: 27.1; json-pretty-print-buffer on ~2MB line causes core dump

[Phil Sainty <psainty@orcon.net.nz>]

(I presume this is to do with the native JSON support, as Emacs 26.3
copes fine with the same command on the example files.)

Using the example JSON file from
https://emacs.stackexchange.com/questions/598/how-do-i-prevent-extremely-long-lines-making-emacs-slow

which you can fetch with:

wget https://github.com/Wilfred/ReVo-utilities/blob/a4bdc40dd2656c496defc461fc19c403c8306d9f/revo-export/dictionary.json?raw=true -O one_line.json

and then safely open in Emacs 27 with:

emacs -Q -f global-so-long-mode one_line.json

C-x C-q to make the buffer writeable.

M-x json-pretty-print-buffer

On my system, Emacs hangs for quite a while and then core dumps.

That's an 18MB line.  If I trim it down to ~2MB I still see the same
thing.  You can do that with (write-region 1 2000151 "two_mb.json")
and then appending a single '}' at the end of the new file to make
it valid JSON.

If I trim back to ~1MB the command succeeds.
(write-region 1 1000088 "one_mb.json") and then append '}]}}'

The smaller files are a bit nicer for comparisons with Emacs 26.3,
which *does* cope with the 18MB file, but processes the smaller ones
much faster (and much faster than it takes Emacs 27.1 to fail).


I also note that, when forgetting to toggle the read-only buffer
state first, Emacs 26.3 immediately issues the "json-pretty-print:
Buffer is read-only" error, whereas Emacs 27.1 evidentially tries
to do all the work, and (for a file small enough to not cause it
to crash in the process) only notices the buffer read-only state
once it tries to replace the contents "replace-region-contents:
Buffer is read-only".


-Phil

p.s. If you're unable to replicate this and wish me to use gdb,
please give step by step instructions for the entire process.





In GNU Emacs 27.1 (build 1, x86_64-pc-linux-gnu, X toolkit, Xaw3d scroll bars)
 of 2020-08-12 built on shodan
Windowing system distributor 'The X.Org Foundation', version 11.0.12008000
System Description: Ubuntu 18.04.5 LTS

Recent messages:
For information about GNU Emacs and the GNU system, type C-h C-a.
Quit [2 times]
Loading json...done
delete-backward-char: Text is read-only [2 times]
Quit [2 times]
Mark activated

Configured using:
 'configure --prefix=/home/phil/emacs/27.1/usr/local
 --with-x-toolkit=lucid --without-sound'

Configured features:
XAW3D XPM JPEG TIFF GIF PNG RSVG DBUS GSETTINGS GLIB NOTIFY INOTIFY
GNUTLS LIBXML2 FREETYPE HARFBUZZ XFT ZLIB TOOLKIT_SCROLL_BARS LUCID X11
XDBE XIM MODULES THREADS JSON PDUMPER LCMS2 GMP

Important settings:
  value of $LANG: en_NZ.UTF-8
  locale-coding-system: utf-8-unix

Major mode: Dired by name

Minor modes in effect:
  tooltip-mode: t
  global-eldoc-mode: t
  electric-indent-mode: t
  mouse-wheel-mode: t
  tool-bar-mode: t
  menu-bar-mode: t
  file-name-shadow-mode: t
  global-font-lock-mode: t
  font-lock-mode: t
  blink-cursor-mode: t
  auto-composition-mode: t
  auto-encryption-mode: t
  auto-compression-mode: t
  buffer-read-only: t
  line-number-mode: t
  transient-mark-mode: t

Load-path shadows:
None found.

Features:
(shadow sort mail-extr json map emacsbug message rmc puny format-spec
rfc822 mml easymenu mml-sec password-cache epa derived epg epg-config
gnus-util rmail rmail-loaddefs text-property-search time-date subr-x seq
byte-opt gv bytecomp byte-compile cconv mm-decode mm-bodies mm-encode
mail-parse rfc2231 mailabbrev gmm-utils mailheader cl-loaddefs cl-lib
sendmail rfc2047 rfc2045 ietf-drums mm-util mail-prsvr mail-utils dired
dired-loaddefs advice tooltip eldoc electric uniquify ediff-hook
vc-hooks lisp-float-type mwheel term/x-win x-win term/common-win x-dnd
tool-bar dnd fontset image regexp-opt fringe tabulated-list replace
newcomment text-mode elisp-mode lisp-mode prog-mode register page
tab-bar menu-bar rfn-eshadow isearch timer select scroll-bar mouse
jit-lock font-lock syntax facemenu font-core term/tty-colors frame
minibuffer cl-generic cham georgian utf-8-lang misc-lang vietnamese
tibetan thai tai-viet lao korean japanese eucjp-ms cp51932 hebrew greek
romanian slovak czech european ethiopic indian cyrillic chinese
composite charscript charprop case-table epa-hook jka-cmpr-hook help
simple abbrev obarray cl-preloaded nadvice loaddefs button faces
cus-face macroexp files text-properties overlay sha1 md5 base64 format
env code-pages mule custom widget hashtable-print-readable backquote
threads dbusbind inotify lcms2 dynamic-setting system-font-setting
font-render-setting x-toolkit x multi-tty make-network-process emacs)

Memory information:
((conses 16 50018 10992)
 (symbols 48 6273 1)
 (strings 32 17137 1060)
 (string-bytes 1 545762)
 (vectors 16 9965)
 (vector-slots 8 132814 16180)
 (floats 8 26 42)
 (intervals 56 300 0)
 (buffers 1000 14))