bug#22154: 25.0.50; emacsclient -c "breaks" 256-color display in server

[Eli Zaretskii <eliz@gnu.org>]

> From: Dan Nicolaescu <dann@gnu.org>
> Cc: eric.hanchrow@gmail.com,  22154@debbugs.gnu.org
> Date: Mon, 14 Dec 2015 11:39:39 -0500
> 
> Eli Zaretskii <eliz@gnu.org> writes:
> 
> >> From: Dan Nicolaescu <dann@gnu.org>
> >> Cc: Eric Hanchrow <eric.hanchrow@gmail.com>,  22154@debbugs.gnu.org
> >> Gcc: nnml+archive:sent-mail-2015
> >> Date: Mon, 14 Dec 2015 01:21:26 -0500
> >> 
> >> Using different number of colors on different ttys should work.
> >> I just tried it briefly, and it works fine on my Fedora machine with
> >> 24.5.
> >> I don't have a very recent version compiled.
> >> 
> >> You can try it with
> >> $ emacs -Q -f server-start&
> >> Then from an xterm: emacsclient -t
> >> And then from a different one: env TERM=vt100 emacsclient -t
> >> 
> >> The frame in the first xterm should display some colors, the one in the
> >> second should be b&w...
> >
> > This simple use case indeed (almost) works.  (To have it work better,
> > you need the patch I posted here.)  But in general, the current
> > implementation doesn't support this, AFAICT, for 2 reasons:
> 
> What exactly is the problem that your patch fixes?

The fact that the default escape sequences for turning colors on or
off are stored in global variables that get overwritten each time
another tty is initialized.

> I don't remember all the details, but having multiple terminal frames
> running on multiple kinds of terminals, with different color depths and
> even background modes was heavily tested when the multi-tty work was
> going on.  One of the usual tests was to have rxvt with both 8 and 256
> colors and white on black and black on white (rxvt not xterm because
> rxvt sets an environment variable with the default color and emacs can
> decide if it's a light or dark background based on that).  It worked
> fine.
> Did something break meanwhile or you are dealing with some new thing
> that was not dealt with back then? 

I don't know.  It's possible that the fact that set_tty_color_mode is
now called as part of redisplay exposed some issue.

And I don't understand how could what you describe work when there's
only one global value of tty-defined-color-alist.  Can you explain how
that worked, given that each terminal's initialization overwrites that
list?