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

[Gregory Heytings]

> > > See my reply to Stefan.  With a 'with-restriction' of 10000 chars, the 
> > > file opens in 1.6 seconds.  I'm not sure it would make sense to add a 
> > > variable/defcustom there instead of the (admittedly somewhat 
> > > arbitrary) constant 10000, which should be large enough in practice.
> >
> > OK, then let's go with that version.
>
> OK, thanks.  Stefan, do you have any further comments/objections on that 
> version?

By the way, I noted that a variant of the regexp still produces stack 
overflows:

(with-current-buffer (get-buffer-create "*bug*")
  (erase-buffer)
  (insert (make-string 266665 ?x) "=")
  (goto-char (point-min))
  (looking-at "[^y]*=*"))

266665 overflows, 266664 does not.  Is that expected?