bug#38035: 27.0.50; Trivial errors in process filters can render Emacs unusable

[Eli Zaretskii]

> From: Richard Stallman <rms@gnu.org>
> Date: Sun, 03 Nov 2019 08:21:12 -0500
> Cc: 38035@debbugs.gnu.org
> 
> The special code to make errors in filters visible is an important
> feature.  It prevents confusion, whereby users don't understand
> what is failing because they don't see the error messages.
> 
> It also makes possible, as you've pointed out, one more way you can
> screw yourself with a perverse Lisp program -- but is it worth trying
> to fix that?

The current idea for a fix is to let the error be displayed several
times, until and unless it is clear that the error is repeatedly
signaled with a very high frequency, and if so, disable the filter for
that process.

I think this strikes the right balance between showing the error to
users and still leaving Emacs in a usable state.