bug#37231: 27.0.50; concurrent XML validation yields an invalid file

[Vincent Lefevre]

Consider a big, valid XML file a.xml, e.g. using a vacuous schema, and
copy it to b.xml (thus a.xml and b.xml are the same file, though they
don't need to be identical for the test).

Open the files at the same time with: emacs -Q a.xml b.xml

Then one of them is regarded as invalid.

Note: This also works with "emacs -Q a.xml" then C-x 5 2 C-x f b.xml
while XML validation on a.xml hasn't finished.

The file a.xml must be big enough so that XML validation takes enough
time (or something like that). For instance, I can reproduce the bug
on my machine with a.xml being

<?xml version="1.0" encoding="utf-8"?>
<root>
  <a/>
  <a/>
  <a/>
  <a/>
  <a/>
  <a/>
  <a/>
...
  <a/>
  <a/>
</root>

where there are 3000 <a/>.


In GNU Emacs 27.0.50 (build 1, x86_64-pc-linux-gnu, GTK+ Version 3.24.10)
 of 2019-08-23 built on zira
Repository revision: 7b0d49854675eae962c6baf8ae6fd90a2c87889b
Repository branch: master
Windowing system distributor 'The X.Org Foundation', version 11.0.12004000
System Description: Debian GNU/Linux bullseye/sid

Recent messages:
Loading /home/vinc17/share/emacs/site-lisp/mutteditor.el (source)...done
Loading time...done
For information about GNU Emacs and the GNU system, type C-h C-a.

Configured using:
 'configure --prefix=/usr/local/emacs-trunk'

Configured features:
XPM JPEG TIFF GIF PNG RSVG SOUND GPM DBUS GSETTINGS GLIB NOTIFY INOTIFY
ACL LIBSELINUX GNUTLS LIBXML2 FREETYPE HARFBUZZ M17N_FLT LIBOTF XFT ZLIB
TOOLKIT_SCROLL_BARS GTK3 X11 XDBE XIM THREADS LIBSYSTEMD PDUMPER LCMS2
GMP

Important settings:
  value of $LC_COLLATE: POSIX
  value of $LC_CTYPE: en_US.UTF-8
  value of $LC_TIME: en_DK
  value of $LANG: POSIX
  locale-coding-system: utf-8-unix

Major mode: Lisp Interaction

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

Load-path shadows:
None found.

Features:
(shadow sort mail-extr warnings emacsbug message rmc puny dired
dired-loaddefs format-spec rfc822 mml easymenu mml-sec password-cache
epa derived epg epg-config gnus-util rmail rmail-loaddefs
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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
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Memory information:
((conses 16 65080 9343)
 (symbols 48 8743 1)
 (strings 32 20691 2807)
 (string-bytes 1 697512)
 (vectors 16 11221)
 (vector-slots 8 142606 9736)
 (floats 8 24 22)
 (intervals 56 217 0)
 (buffers 992 12))