bug#13400: 23.4; overlapping process filter calls

[Noam Postavsky <npostavs@gmail.com>]

Stefan Monnier <monnier@iro.umontreal.ca> writes:

> I'm pretty sure that while there might be use-cases for recursive
> invocation of filters, this is a very exceptional situation and the
> average programmer will not expect it and would be stumped if/when
> it happens.
>
> So the default should be to prevent it, with maybe some way to override
> it to cater to the exceptional case where recursive invocation
> is to be allowed.
>
> I think we could do that by setting a property on process object during
> filter invocation to postpone further filter invocations, and then the
> process filter could locally unset this property if it wants to allow
> recursive invocations.

Yeah, I suppose that would probably be better.  Although then we have
some potential weird edge cases like what happens when when changing the
property during a recursive invocation.