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

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.

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 'configure --prefix=/usr/local/emacs-trunk'

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Memory information:
((conses 16 65080 9343)
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bug#37231: 27.0.50; concurrent XML validation yields an invalid file

[Lars Ingebrigtsen]

Vincent Lefevre <vincent@vinc17.net> writes:

> 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.

I can confirm that this bugs out.

I know nothing about the nxml code, but I did notice that about half of
the rng variables are defined thusly:

(defvar rng-validate-up-to-date-end nil
  "Last position where validation is known to be up to date.")
(make-variable-buffer-local 'rng-validate-up-to-date-end)

But the ones that it reports as failing here depends on variables like
`rng-pending-contents', which are not buffer-local.  So to test blindly,
I made that also buffer-local...  and then the entire process failed
with a backtrace, so that's not the solution.  :-)

But I do think it's really suspicious that some of those variables are
buffer-local while others aren't.  Perhaps somebody who knows how that
machinery works could have a peek at it.

-- 
(domestic pets only, the antidote for overdose, milk.)
   bloggy blog: http://lars.ingebrigtsen.no