🚀 go-pugleaf

In GNU Emacs 18.54.57 of Wed Nov 29 1989 on PTERO.SOAR.CS.CMU.EDU
(berkeley-unix), even with backup-by-copying-with-linked, it is
possible to edit a file that has two different names in two different
buffers, edit the buffers independently, and receive no warning of the
danger until attempting to save the buffers.  There should be a
warning given when the file is visited by its second name, there
should be a variable such as find-linked-files-carefully to control
the warning, and the documentation for the new variable, and for
backup-by-copying-when-linked, should refer to each other.

This illustrates a simple occurrence of the problem:

% echo hi >foo
% ln foo bar
% gnu-emacs -q
( s e t q SPC b a c k u p - b y - c o p y i n g - w 
h e n ' DEL - l i n k e d SPC ' t ) LFD C-x C-f f o 
o RET C-x 4 f b a r RET C-e SPC t h e r e C-x o C-e 
SPC h e r e C-x s y y C-g C-h l