bug#34910: 27.0.50; Too much matching reentrancy

["Mattias Engdegård" <mattiase@acm.org>]

Run, in *scratch*, with emacs -Q:

(with-temp-buffer
  (dotimes (i 10000)
    (insert "aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa"))
  (insert "b")
  (goto-char (point-min))
  (looking-at ".*b"))

Result: "Too much matching reentrancy" in the minibuffer. No traceback.
Expected: The usual "Stack overflow in regexp matcher" error and
traceback.

The culprit seems to be

commit 938d252d1c6c5e2027aa250c649deb024154f936
Author: Daniel Colascione <dancol@dancol.org>
Date:   Sat Jun 16 13:46:10 2018 -0700

    Make regex matching reentrant; update syntax during match



In GNU Emacs 27.0.50 (build 9, x86_64-pc-linux-gnu, GTK+ Version
3.24.1)
 of 2019-03-18 built on dhcppc1
Repository revision: 799eb257970b1f6aeb5833417b6dd943d6479977
Repository branch: master
Windowing system distributor 'Fedora Project', version 11.0.12004000
System Description: Fedora 29 (Workstation Edition)

Recent messages:
For information about GNU Emacs and the GNU system, type C-h C-a.

Configured using:
 'configure --prefix=/home/mattias/eminst'

Configured features:
XPM JPEG TIFF GIF PNG SOUND DBUS GSETTINGS GLIB NOTIFY INOTIFY
LIBSELINUX GNUTLS FREETYPE XFT ZLIB TOOLKIT_SCROLL_BARS GTK3 X11 XDBE
XIM THREADS PDUMPER GMP

Important settings:
  value of $LC_MONETARY: sv_SE.UTF-8
  value of $LC_NUMERIC: sv_SE.UTF-8
  value of $LC_TIME: sv_SE.UTF-8
  value of $LANG: sv_SE.UTF-8
  value of $XMODIFIERS: @im=ibus
  locale-coding-system: utf-8-unix

Major mode: Lisp Interaction

Minor modes in effect:
  tooltip-mode: t
  global-eldoc-mode: t
  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
  line-number-mode: t
  transient-mark-mode: t

Load-path shadows:
None found.

Features:
(shadow sort mail-extr emacsbug message rmc puny seq byte-opt gv
bytecomp byte-compile cconv dired dired-loaddefs format-spec rfc822 mml
easymenu mml-sec password-cache epa derived epg epg-config gnus-util
rmail rmail-loaddefs time-date 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 elec-pair
mule-util 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 menu-bar
rfn-eshadow isearch timer select scroll-bar mouse jit-lock font-lock
syntax facemenu font-core term/tty-colors frame 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 minibuffer
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 dynamic-setting system-font-setting font-render-setting
move-toolbar gtk x-toolkit x multi-tty make-network-process emacs)

Memory information:
((conses 16 44594 10186)
 (symbols 48 5845 2)
 (strings 32 14860 1818)
 (string-bytes 1 490684)
 (vectors 16 9450)
 (vector-slots 8 116818 14096)
 (floats 8 17 58)
 (intervals 56 186 0)
 (buffers 992 11))