bug#20074: edebug tracing can't be stopped with 'S'

[Stefan Monnier <monnier@iro.umontreal.ca>]

>    (defun forever ()    (interactive) (while t (message "doing nothing")))
> i have occasionally accidentally written ELisp i feel is analogous: to wit,
> code that doesn't need to wait for relatively 'slow' events, such as disk IO
> or user input.

In this case, "message" is a "slow event" which does I/O, so it's a bad
example, but I see what you mean.

> On those occasions, Emacs itself has become unresponsive to any of my
> attempts at input, and i've had to terminate the Emacs process itself.

Normally this should never happen.  In practice there are various cases
where this can happen for two reasons:
- A bug.  Some of those can be difficult to fix, tho.
- `inhibit-quit' is non-nil while running this inf-loop.  In this case,
  indeed, it's usually a bug in the Elisp code rather than in the
  C code: code that runs with inhibit-quit (e.g. code run from a timer)
  should be careful to avoid long run-times.  This isn't great, but it's
  what we have so far.


        Stefan