bug#27571: C stack overflow from `prin1' on deeply nested lisp object.

[Noam Postavsky]

Keith David Bershatsky <esq@lawlist.com> writes:

> I have determined that bug #27571 was introduced on November 19, 2016
> with commit c61ee94959ba96b2a327df0684593f7e569e30be.  The following
> patch to the Emacs 26 branch as of today (01/08/2018) reverses the
> commit and enables the test below to be completed successfully.
>
> [FYI: I am on OSX 10.6.8 and am manually increasing the stack limit
> with `ulimit -S -s unlimited` so that I can have rather large custom
> undo-tree histories.]

By "broken" you mean that it prevents the `ulimit -S -s unlimited` trick
from working?  Perhaps it's just a matter of detecting this "unlimited"
case explicitly.