bug#27571: Crashing when printing a lisp object.

[Keith David Bershatsky <esq@lawlist.com>]

Thank you, @npostavs, for taking a look at this issue.  The 26-count example (which does *not* crash) is a mini-version of the 6651-count example that I am using to replicate the crash.  It may not possible to create a copy of the 6652-count example that crashes, because Emacs crashes before `prin1` can finish.  The 6651-count example does *not* crash.  I'll continue reading about how to increase the ulimit on OSX and try to find out whether this is a system-wide setting, or whether it can be done inside Emacs without modifying the system settings.

Last login: Mon Jul  3 17:31:55 on ttys000
server:~ HOME$ ulimit -a
core file size          (blocks, -c) 0
data seg size           (kbytes, -d) unlimited
file size               (blocks, -f) unlimited
max locked memory       (kbytes, -l) unlimited
max memory size         (kbytes, -m) unlimited
open files                      (-n) 256
pipe size            (512 bytes, -p) 1
stack size              (kbytes, -s) 8192
cpu time               (seconds, -t) unlimited
max user processes              (-u) 1250
virtual memory          (kbytes, -v) unlimited
server:~ HOME$ 

I load the 6651-count example using `read`, then I type a character, then I transfer the most recent entry from the buffer-undo-list to the undo tree history (in this case just one character breaks the camel's back) and then Emacs will crash if I call `prin1` on the history variable with `print-circle` set to `t`.

I will work on trying to come up with a way to create the 6652-count example that doesn't involve using the custom version of undo-tree to go from a count of 6651 to a count of 6652.

In terms of the reference to line 1350, I'm not sure what that refers to.  The undo-tree history variable is buffer local, and I call `(prin1 my-history-variable (current-buffer))` and the current buffer is just a temp buffer that never gets populated because Emacs crashes.  Or, at least, I never get to see the temporary buffer before Emacs crashes.

Keith