bug#29463: 27.0.50; Backtraces with large strings is very slow

[Lars Ingebrigtsen <larsi@gnus.org>]

I was trying to debug some code that dealt with very large SVG images,
and getting a backtrace buffer proved impossible -- Emacs would just
hang.

Here's a trivial test case:

(let ((a (make-string 100000000 ?a)))
 (debug a))

On my machine, this takes about half a minute to display, but if you
have deeply nested code that the debugger is trying to display, you'll
get that additional time per frame, I think, so it can take arbitrarily
long to render the buffer.

It would be nice if the backtrace buffer would just chop off
ridiculously long strings instead of printing them.  Perhaps controlled
by a -print-length variable?


In GNU Emacs 27.0.50 (build 4, x86_64-pc-linux-gnu, GTK+ Version 3.22.11)
 of 2017-10-26 built on stories
Repository revision: 1ad56d0a6bdd07c722a3c161096a16f70d1cbb35
Windowing system distributor 'The X.Org Foundation', version 11.0.11902000
System Description:	Debian GNU/Linux 9.2 (stretch)


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