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bug#54774: 28.1; ansi-color with no colors in Emacs-28

From: Jim Porter
Subject: bug#54774: 28.1; ansi-color with no colors in Emacs-28
Date: Sat, 9 Apr 2022 10:31:50 -0700

On 4/9/2022 12:25 AM, Thierry Volpiatto wrote:
Apart advicing `ansi-color-get-face-1` I couldn't find how to fix my
code as your changes are binding ansi-code 5 to `ansi-color-slow-blink`
face which create unwanted squares.

I had a chance to look into this a bit more. ansi-color.el has mis-parsed 256-color and 24-bit color escapes for a long time. For example, you can try this under Emacs 27.2:

  (ansi-color-apply "\033[38;5;2mhi\033[0m")
    -> #("hi" 0 2 (font-lock-face success))

This should actually be equivalent to the 8-color green value (ANSI SGR value 32). (Note: Under Emacs 27.2, the "slow blink" face is mapped to `success', whereas in 28.1, it's mapped to `ansi-color-slow-blink'. In Emacs 29, this properly sets the foreground to green.)

Normally this isn't an issue, since Emacs' shell/term modes indicate that they only support 8-color mode, so commands usually don't emit 256-color sequences. However, in your case this looks to be some text from a web service, so it obviously can't inspect your terminal capabilities (unless there's a way to send them in the request?).

Looking at your code in more detail, I think you just got lucky that things worked ok in Emacs 27. See here:

            (while (re-search-forward "38;5;\\([0-9]+\\)m" nil t)
               ;; ...
               (replace-match (pcase (match-string 1)
                               ("154" "32")
                               ("190" "31")
                               ("118" "32")
                               ("208" "37")
                               ("202" "34")
                               ("214" "35")
                               ("220" "36")
                               ("226" "33")
                               (r     r))
                             t t nil 1))

This replaces an ANSI escape like "\033[38;5;154m" with "\033[38;5;32m". So the 256-color ANSI escape is still there, just slightly transformed. I think you want the result to be "\033[32m" instead, so that you're not using 256-color escape sequences at all.

If you adjust the regexp/replacement to remove the "38;5;" bit, I think your existing code should work fine in both Emacs 27 and 28.

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