bug#61514: 30.0.50; sadistically long xml line hangs emacs

[Eli Zaretskii]

> From: Stefan Monnier <monnier@iro.umontreal.ca>
> Cc: mah@everybody.org,  61514@debbugs.gnu.org
> Date: Mon, 20 Feb 2023 08:19:26 -0500
> 
> >   
> > "\\(\\(?:\\(xmlns\\)\\|[_[:alpha:]][-._[:alnum:]]*\\)\\(:[_[:alpha:]][-._[:alnum:]]*\\)?\\)[
> >  \r\t\n]*=\\(?:[ 
> > \r\t\n]*\\('[^<'&\r\n\t]*\\([&\r\n\t][^<']*\\)?'\\|\"[^<\"&\r\n\t]*\\([&\r\n\t][^<\"]*\\)?\"\\)\\(?:\\([
> >  \r\t\n]*>\\)\\|\\(?:\\([ \r\t\n]*/\\)\\(>\\)?\\)\\|\\([ \r\t\n]+\\)\\)\\)?"
> >
> > As you can see, the prepended "[^<>\n]+?" in the regexp which "hangs"
> > makes all the difference.  So the looking-at which fails reasonably
> > quickly is the first call to looking-at above, whereas the one the
> > "hangs" is the second one.
> 
> Yes, it makes a lot of sense now.
> 
> > Maybe this points out a way out of this misery?
> 
> I think it does.  E.g. there's a chance that using "[^<>\n]+?\\<"
> instead of "[^<>\n]+?"  avoids the hang

It does, thanks.

> (not sure if it's the right thing to do for all the regexp that can
> be returned by `xmltok-attribute`, tho).

How would we go about finding out?  Because other than that, changing
the regexp solves this nasty problem, and all the tests in
test/lisp/nxml/ still pass.

> And for the stack overflow I haven't yet found its origin.

Not sure what is the mystery here.  AFAIU, we look for the closing
">", don't find it, and then start looking for fewer and fewer non-'>'
characters followed by '>'.  Isn't that what happens here?