bug#44950: 28.0.50; 24-bit colors not used in terminal with emacsclient

[Tim Ruffing <crypto@timruffing.de>]

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I think what tastytea is saying is that when emacs checks the env
variable COLORTERM, it uses the environment of the server and not the
one of emacsclient. And yes, that's just a bug. emacsclient should read
that variable and pass it to server. But this requires new code because
it breaks with the pattern of using terminfo to detect term support.

So the terminfo detection is currently more reliable. Would you be
willing to accept something like the attached patch? This will improve
detection without relying on COLORTERM, which should make the situation
already much better. Tc is in the terminfo of many terminals, see
https://gist.github.com/XVilka/8346728 .

If yes, I can send an improved patch (with added explanations to
doc/misc/efaq.texi).

Tim

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From 8c5851a96447cd06f41d2983390e699461870072 Mon Sep 17 00:00:00 2001
From: Tim Ruffing <crypto@timruffing.de>
Date: Mon, 31 May 2021 15:38:19 +0200
Subject: [PATCH] Support Tc terminfo flag forg 24-bit color support in
 terminal

---
 src/term.c | 10 ++++++----
 1 file changed, 6 insertions(+), 4 deletions(-)

diff --git a/src/term.c b/src/term.c
index c995a4499c..fdcc116b69 100644
--- a/src/term.c
+++ b/src/term.c
@@ -4157,10 +4157,12 @@ init_tty (const char *name, const char *terminal_type, bool must_succeed)
 	       could return 32767.  */
 	    tty->TN_max_colors = 16777216;
 	  }
-	/* Fall back to xterm+direct (semicolon version) if requested
-	   by the COLORTERM environment variable.  */
-	else if ((bg = getenv("COLORTERM")) != NULL
-		 && strcasecmp(bg, "truecolor") == 0)
+	/* Fall back to xterm+direct (semicolon version) if Tc is set
+	   (de-facto standard introduced by tmux) or if	requested by
+	   the COLORTERM environment variable.  */
+	else if (tigetflag("Tc")
+		 || ((bg = getenv("COLORTERM")) != NULL
+		     && strcasecmp(bg, "truecolor") == 0))
 	  {
 	    tty->TS_set_foreground = "\033[%?%p1%{8}%<%t3%p1%d%e38;2;%p1%{65536}%/%d;%p1%{256}%/%{255}%&%d;%p1%{255}%&%d%;m";
 	    tty->TS_set_background = "\033[%?%p1%{8}%<%t4%p1%d%e48;2;%p1%{65536}%/%d;%p1%{256}%/%{255}%&%d;%p1%{255}%&%d%;m";
-- 
2.31.1