bug#7496: 23.2; copy recursive keymap cause crash

[Lars Ingebrigtsen <larsi@gnus.org>]

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

>> When I call `copy-keymap' with a keymap which contains recursive binding,
>> the emacs crashed.
>> I can reproduce it by following sexp.
>
>> (let ((map (make-sparse-keymap)))
>>   (define-key map " " map)
>>   (copy-keymap map))
>
> I'm not surprised.  There are many ways to address it:
> - try and make sure we better handle the "using up all memory" case
>   rather than crashing.  This is very difficult.  We already try to do
>   it, but clearly it's not working that well.
> - try and detect such cycles and either signal an error or reproduce the
>   same cycle in the copy.  We have added such things in several other
>   cases, so we should probably do that.

While this is a pretty obscure, Emacs shouldn't crash on stuff like
this.  I first considered whether just to check for EQ in Fcopy_keymap,
but it's possible to have nested keymaps that are mutually recursive,
so that won't work.

So I just added a recursion counter and refuse to copy when we've
reached level 100.

It does not protect against the case where the keymap is a char table
where one of the entries is the same keymap, but I don't know whether
that's a thing.

-- 
(domestic pets only, the antidote for overdose, milk.)
   bloggy blog: http://lars.ingebrigtsen.no