Subject: bug#46586: 26.3, 27.1.50; Emacs crash in a backtrace (core) dump (a long standing issue)

From: 路客 <[email-masked]>
Date: Wed, 17 Feb 2021 18:13:56 +0800

Hi all,

I found a short data clip can always crash Emacs 26.3 or 27.1.50 with
a terminal backtrace (core) dump in the "read()" function. It took me
a few hours to narrow it down and finally reach this minimal crashing
data set:

----- code begin -----
(#1=(#("000008964 .gnus.el" 0 18 (r #1#))
(def #2=#("000008964 .gnus.el" 0 18
(r
(#2#
(def #3=#("000006393 .gnus.el" 0 18
(r #4=(#3#
(def
#("000006393 .gnus.el" 0 18 (r #4#)) "x"))))"x"))))"x")))
----- code end -----

Try to `read' or `eval' this block of code (C-x C-e) will immediately
crash Emacs 26.3 or 27.1.50; however, older Emacs 26.0.50 works well
by entering the debugger with an error like:

----- elisp debugger message begin -----
Debugger entered--Lisp error: (invalid-function (#("000008964
.gnus.el" 0 18 (r #1)) (def #("000008964 .gnus.el" 0 18 (r (#3 (def
#("000006393 .gnus.el" 0 18 (r (#7 (def #("000006393 .gnus.el" 0 18 (r
#9)) "x")))) "x")))) "x")))
  ((#("000008964 .gnus.el" 0 18 (r #1)) (def #("000008964 .gnus.el" 0
18 (r (#3 (def #("000006393 .gnus.el" 0 18 ...) "x")))) "x")))
  eval(((#("000008964 .gnus.el" 0 18 (r #2)) (def #("000008964
.gnus.el" 0 18 (r (#4 (def #("000006393 .gnus.el" 0 18 ...) "x"))))
"x"))) nil)
  elisp--eval-last-sexp(nil)
  eval-last-sexp(nil)
  funcall-interactively(eval-last-sexp nil)
  call-interactively(eval-last-sexp nil nil)
  command-execute(eval-last-sexp)
----- elisp debugger message end -----

Funny thing is that if I change any of the above ".gnus.el" to another
file name, Emacs won't crash and is able to enter the debugger like
26.0.50. Is there anything special about the ".gnus.el" ?

This short crashing code block was actually a lot longer as a part of
a big bookmark file, and has long ago started to crash newer Emacs
than 26.0.50. I didn't know the root cause till these days I decided
to figure that out and finally extracted that block out and simplified
it to this simple form.

Could anyone help fix this long standing issue? Thanks.

-- 
Best regards,
Luke Lee