bug#69598: 29.2; colour support based on $TERM value not terminfo database

[Eli Zaretskii]

> From: chohag@jtan.com
> cc: chohag@jtan.com, 69598@debbugs.gnu.org
> Comments: In-reply-to Eli Zaretskii <eliz@gnu.org>
>    message dated "Thu, 07 Mar 2024 22:10:30 +0200."
> Date: Thu, 07 Mar 2024 21:45:44 +0000
> 
> I have (eventually, see below) followed this sequence of steps:
> 
>         $ cd ~/src/emacs-29.2
>         $ ./configure
>         $ gmake
> 
> launch xterm, and then within it:
> 
>         $ rm -fr ~/.terminfo
>         $ echo 'notworking|Non-working clone,use=xterm-direct,' > new.info
>         $ echo 'xterm-working|Working clone,use=xterm-direct,' >> new.info
>         $ tic new.info
> 
> # pre-patch compiled binary:
> 
>         $ export TERM=xterm-direct
>         $ ~/src/emacs-29.2/src/emacs -nw
>         $ M-x list-colors-display # confirm 256 named colours
>         $ export TERM=notworking
>         $ ~/src/emacs-29.2/src/emacs -nw
>         $ M-x list-colors-display # confirm only 8 colours
>         $ export TERM=xterm-working
>         $ emacs -nw               # system xterm
>         $ M-x list-colors-display # confirm 256 named colours
> 
> # patch:
> --- src/term.c~       Sat Jan  6 12:56:31 2024
> +++ src/term.c        Thu Mar  7 20:56:36 2024
> @@ -4235,6 +4235,15 @@
>          tty->TN_no_color_video = 0;
>      }
>  
> +  {
> +    int bug = open("/tmp/debug", O_WRONLY | O_CREAT);
> +    dprintf(bug, "pair: %s\n", tty->TS_orig_pair);
> +    dprintf(bug, "bg: %s\n", tty->TS_set_background);
> +    dprintf(bug, "fg: %s\n", tty->TS_set_foreground);
> +    dprintf(bug, "max: %zu\n", tty->TN_max_colors);
> +    close(bug);
> +  }
> +
>    tty_default_color_capabilities (tty, 1);
>  
>    MagicWrap (tty) = tgetflag ("xn");
> 
>         $ gmake -C ~/src/emacs-29.2
>         $ export TERM=xterm-direct
>         $ ~/src/emacs-29.2/src/emacs -nw
>         $ M-x list-colors-display $ # confirm 256 named colours
>         $ doas cat /tmp/debug # doas because umask 0
>         pair: 
>         bg: 
> 1%{8}%<%t4%p1%d%e48:2::%p1%{65536}%/%d:%p1%{256}%/%{255}%&%d:%p1%{255}%&%d%;m
>         fg: 
> 1%{8}%<%t3%p1%d%e38:2::%p1%{65536}%/%d:%p1%{256}%/%{255}%&%d:%p1%{255}%&%d%;m
>         max: 16777216
>         $ rm -f /tmp/debug # just in case
>         $ export TERM=notworking
>         $ ~/src/emacs-29.2/src/emacs -nw
>         $ M-x list-colors-display # confirm only 8 colours
>         $ doas cat /tmp/debug
>         pair: 
>         bg: 
> 1%{8}%<%t4%p1%d%e48:2::%p1%{65536}%/%d:%p1%{256}%/%{255}%&%d:%p1%{255}%&%d%;m
>         fg: 
> 1%{8}%<%t3%p1%d%e38:2::%p1%{65536}%/%d:%p1%{256}%/%{255}%&%d:%p1%{255}%&%d%;m
>         max: 16777216
>         $ rm -f /tmp/debug
>         $ export TERM=xterm-working
>         $ ~/src/emacs-29.2/src/emacs -nw
>         $ M-x list-colors-display # confirm 256 named colours
>         $ doas cat /tmp/debug
>         pair: 
>         bg: 
> 1%{8}%<%t4%p1%d%e48:2::%p1%{65536}%/%d:%p1%{256}%/%{255}%&%d:%p1%{255}%&%d%;m
>         fg: 
> 1%{8}%<%t3%p1%d%e38:2::%p1%{65536}%/%d:%p1%{256}%/%{255}%&%d:%p1%{255}%&%d%;m
>         max: 16777216
> 

Thanks.  These results AFAIU indicate that there's no problem with the
terminfo level: the number of colors known to term.c (printed as max:)
is the same no matter how you start Emacs.  Do you agree with this
conclusion?

I think the problem is that if the terminal's name doesn't begin with
"xterm", Emacs does not load lisp/term/xterm.el, which defines the
colors that will be available.  So please copy lisp/term/xterm.el to a
file named lisp/term/notworking.el, and then start Emacs with
TERM=notworking and see if that solves the problem.  That is, arrange
for a lisp/term/NAME.el file to be a copy of xterm.el when you invoke
Emacs with TERM=NAME.

> Notably TS_orig_pair is printed here as an empty string in all cases
> (NULL would print as "(null)"? and there there are no extra spaces),
> created by requesting the op capability which *is* defined in
> xterm-direct.
> 
>         $ infocmp dumb xterm-direct | grep op: 
>                 op: NULL, '\E[39;49m'.
> 
> In fact before getting the patch to this stage I included the print
> statements within the block you suggested but of course this was
> not reached because of op not being detected.
> 
> It didn't make any sense to me why tgetstr would return an empty
> string in that instance. address where tgetstr gets its data from
> is another name for tty->termcap_strings_buffer so changing the
> print statement to print that and setting $TERM to xterm-direct
> reveals:
> 
>         $ doas hexdump -C /tmp/debug
>         00000000  60 60 60 1b 5b 4c 27 27  27 0a                    
> |```.[L'''.|
>         0000000a

I don't think I see why TS_orig_pair's value is relevant to the issue
at hand.  Can you explain why you are interested in it?