bug#32072: 27.0.50; clear-face-cache in an X frame breaks tty colors

[Eli Zaretskii]

> From: Noam Postavsky <npostavs@gmail.com>
> Date: Wed, 18 Jul 2018 21:39:45 -0400
> Cc: 32072@debbugs.gnu.org,
>       Rami Ylimäki <rami.ylimaki@vincit.fi>
> 
> > I have done some more testing and found that 24-bit terminal frames are
> > not affected, their colors remain intact after doing (clear-face-cache)
> > in the X11 frame.
> 
> Hmm, interesting, I've bisected the problem to [1: e463e5762b].
> 
> [1: e463e5762b]: 2017-02-18 13:04:55 +0200
>   Support 24-bit direct colors on text terminals
>   
> https://git.savannah.gnu.org/cgit/emacs.git/commit/?id=e463e5762bbe628be3d15da066a90f079a8468b3

Thanks.  Just stabbing in the dark here, but does the patch below
help?

diff --git a/lisp/term/tty-colors.el b/lisp/term/tty-colors.el
index ab9149e..a776c83 100644
--- a/lisp/term/tty-colors.el
+++ b/lisp/term/tty-colors.el
@@ -824,10 +824,12 @@ tty-color-canonicalize
        (replace-regexp-in-string " +" "" (downcase color))
       color)))
 
-(defun tty-color-24bit (rgb)
-  "Return pixel value on 24-bit terminals. Return nil if RGB is
-nil or not on 24-bit terminal."
-  (when (and rgb (= (display-color-cells) 16777216))
+(defun tty-color-24bit (rgb &optional display)
+  "Return 24-bit color pixel value for RGB value on DISPLAY.
+DISPLAY can be a display name or a frame, and defaults to the
+selected frame's display.
+If DISPLAY is not on a 24-but TTY terminal, return nil."
+  (when (and rgb (= (display-color-cells display) 16777216))
     (let ((r (lsh (car rgb) -8))
          (g (lsh (cadr rgb) -8))
          (b (lsh (nth 2 rgb) -8)))
@@ -850,7 +852,7 @@ tty-color-define
       (error "Invalid specification for tty color \"%s\"" name))
   (tty-modify-color-alist
    (append (list (tty-color-canonicalize name)
-                (or (tty-color-24bit rgb) index))
+                (or (tty-color-24bit rgb frame) index))
           rgb)
    frame))
 
@@ -1026,7 +1028,7 @@ tty-color-desc
          (or (assoc color (tty-color-alist frame))
              (let ((rgb (tty-color-standard-values color)))
                (and rgb
-                    (let ((pixel (tty-color-24bit rgb)))
+                    (let ((pixel (tty-color-24bit rgb frame)))
                       (or (and pixel (cons color (cons pixel rgb)))
                           (tty-color-approximate rgb frame)))))))))