bug#20995: 24.5; uncomment XML file results in high CPU utilisation / hanging

[Sam Halliday]

I just upgraded from 24.4 to 24.5 on Debian Jessie (self compiled) and
if I open an XML file containing the below information, and attempt to
M-: uncomment the last comment line in the file, Emacs eats 100%
CPU and I have to kill it. I can confirm that it works OK in 24.4:

=======================
<configuration scan="true" scanPeriod="3 seconds"> 
  <property name="PATTERN" value="%d{dd MMM yy HH:mm:ss.SSS} %logger{0} 
[%X{CONTEXT_ID} %thread] %-5level - %msg%n" /> 
  <turboFilter class="ch.qos.logback.classic.turbo.DuplicateMessageFilter"> 
    <cacheSize>1048576</cacheSize> 
  </turboFilter> 
  <appender name="file" class="ch.qos.logback.core.FileAppender"> 
    <file>test-output.log</file> 
    <append>false</append> 
    <encoder> 
      <pattern>${PATTERN}</pattern> 
    </encoder> 
  </appender> 
  <root level="${root-level:-INFO}"> 
    <appender-ref ref="file" /> 
  </root> 
  <logger name="org.apache" level="WARN" /> 
  <!-- <logger name="akka" level="DEBUG" /> --> 
  <!-- <logger name="testing" level="DEBUG" /> --> 

  <!-- <logger name="foo.bar" level="DEBUG" /> --> 
</configuration> 
=======================

The debugging below is probably useless. I encourage you to try this
yourself, ensuring that nXML mode is enabled.

I share my Emacs configuration, it is available here:

  https://github.com/fommil/dotfiles/tree/master/.emacs.d



In GNU Emacs 24.5.1 (x86_64-unknown-linux-gnu, GTK+ Version 3.14.5)
 of 2015-07-05 on Samskara
Windowing system distributor `The X.Org Foundation', version 11.0.11604000
System Description:     Debian GNU/Linux 8.1 (jessie)

Configured using:
 `configure --prefix=/opt/emacs-24.5'

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

Major mode: Lisp Interaction

Minor modes in effect:
  persistent-scratch-autosave-mode: t
  global-auto-revert-mode: t
  global-subword-mode: t
  subword-mode: t
  show-paren-mode: t
  mouse-wheel-mode: t
  file-name-shadow-mode: t
  global-font-lock-mode: t
  font-lock-mode: t
  blink-cursor-mode: t
  auto-compression-mode: t
  column-number-mode: t
  line-number-mode: t
  transient-mark-mode: t

Recent messages:
Loading /home/fommil/.emacs.d/local.el (source)...done
For information about GNU Emacs and the GNU system, type C-h C-a.

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Features:
(shadow sort face-remap darkroom flyspell ispell gnus-util mail-extr
emacsbug message format-spec rfc822 mml mml-sec mm-decode mm-bodies
mm-encode mail-parse rfc2231 mailabbrev gmm-utils mailheader sendmail
rfc2047 rfc2045 ietf-drums mm-util help-fns mail-prsvr mail-utils
darcula-theme edmacro kmacro cl-loaddefs cl-lib highlight-symbol
thingatpt persistent-scratch midnight malabar-mode-autoloads eieio-core
info easymenu package epg-config autorevert filenotify superword subword
paren time-date tooltip electric uniquify ediff-hook vc-hooks
lisp-float-type mwheel x-win x-dnd tool-bar dnd fontset image regexp-opt
fringe tabulated-list newcomment lisp-mode prog-mode register page
menu-bar rfn-eshadow timer select scroll-bar mouse jit-lock font-lock
syntax facemenu font-core frame cham georgian utf-8-lang misc-lang
vietnamese tibetan thai tai-viet lao korean japanese hebrew greek
romanian slovak czech european ethiopic indian cyrillic chinese
case-table epa-hook jka-cmpr-hook help simple abbrev minibuffer 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 make-network-process dbusbind
gfilenotify dynamic-setting system-font-setting font-render-setting
move-toolbar gtk x-toolkit x multi-tty emacs)

Memory information:
((conses 16 135483 8631)
 (symbols 48 23118 0)
 (miscs 40 54 139)
 (strings 32 30491 17032)
 (string-bytes 1 881960)
 (vectors 16 14011)
 (vector-slots 8 440483 4179)
 (floats 8 82 73)
 (intervals 56 187 0)
 (buffers 960 11)
 (heap 1024 45124 1209))

-- 
Best regards,
Sam