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

[Gregory Heytings]

> > > And for the stack overflow I haven't yet found its origin.
> >
> > There is no stack overflow here, AFAIU.  It's simply that the prepended 
> > regexp matches one or more (without any upper bound) characters except 
> > "<>\n", which means that we backtrack _a lot_ when the line is long.
>
> There is clearly a stack overflow since the OP showed stack overflow 
> errors in *Messages*.

Ah yes, I misunderstood what you meant.  I thought you were talking about 
a stack overflow bug in the regexp engine.

> And the stack overflow is in the rest of the regexp: the `+?` repetition 
> uses only ever 1 stack slot no matter how long a match we consider 
> (contrary to the `+` and `*` repetitions which use N stack slots for the 
> N repetitions of the longest match).

Indeed.  That's the bug in the bug.  But it's the '+?' repetition which 
causes the "infloop", right?