Support standard Terminfo direct mode terminals

Latest Terminfo introduces terminal definitions that support direct
color mode.  The "Co"/"colors" capability is set to 0x1000000 on these
terminals and Emacs is already compatible with them.
However, if used Terminfo library hasn't been compiled with 32-bit
value support, "Co"/"colors" is truncated to 0x7fff.  In this case
direct color mode support can be detected from the "RGB" capability
flag.
There are some minor problems if the color count isn't corrected from
0x7fff.  First eight standard colors defined in xterm-standard-colors
are shown correctly.  However, their RGB values match the terminal
settings, not the RGB values defined in xterm-standard-colors.  Bright
versions of these colors are shown incorrectly.  They are interpreted
as pixels #8 - #15, which are very dark shades of blue.

* src/term.c (init_tty): Force terminal color count to 0x1000000 if
"RGB" capability is present.
* src/tparam.h: Define prototype for tigetflag.  (Bug#30308)

* doc/misc/efaq.texi (Colors on a TTY): Add information about direct
mode terminals supported by Terminfo.

doc/misc/efaq.texi

@@ -1496,32 +1496,41 @@ direct color mode is used.  The capability strings are expected to
 take one 24-bit pixel value as argument and transform the pixel to a
 string that can be used to send 24-bit colors to the terminal.
 
-There aren't yet any standard terminal type definitions that would
-support the capabilities, but Emacs can be invoked with a custom
-definition as shown below.
+Standard terminal definitions don't support these capabilities and
+therefore custom definition is needed.
 
 @example
-$ cat terminfo-24bit.src
+$ cat terminfo-custom.src
 
-# Use colon separators.
-xterm-24bit|xterm with 24-bit direct color mode,
+xterm-emacs|xterm with 24-bit direct color mode for Emacs,
   use=xterm-256color,
-  setb24=\E[48:2:%p1%@{65536@}%/%d:%p1%@{256@}%/%@{255@}%&%d:%p1%@{255@}%&%dm,
-  setf24=\E[38:2:%p1%@{65536@}%/%d:%p1%@{256@}%/%@{255@}%&%d:%p1%@{255@}%&%dm,
-# Use semicolon separators.
-xterm-24bits|xterm with 24-bit direct color mode,
-  use=xterm-256color,
-  setb24=\E[48;2;%p1%@{65536@}%/%d;%p1%@{256@}%/%@{255@}%&%d;%p1%@{255@}%&%dm,
-  setf24=\E[38;2;%p1%@{65536@}%/%d;%p1%@{256@}%/%@{255@}%&%d;%p1%@{255@}%&%dm,
+  setb24=\E[48\:2\:\:%p1%@{65536@}%/%d\:%p1%@{256@}%/%@{255@}%&%d\:%p1%@{255@}%&%dm,
+  setf24=\E[38\:2\:\:%p1%@{65536@}%/%d\:%p1%@{256@}%/%@{255@}%&%d\:%p1%@{255@}%&%dm,
+
+$ tic -x -o ~/.terminfo terminfo-custom.src
+
+$ TERM=xterm-emacs emacs -nw
+@end example
+
+Emacs 27.1 and later support Terminfo capability @samp{RGB} for
+detecting 24-bit direct color mode.  Multiple standard terminal
+definitions support this capability.
+
+@example
+$ TERM=xterm-direct infocmp | grep seta[bf]
 
-$ tic -x -o ~/.terminfo terminfo-24bit.src
+  setab=\E[%?%p1%@{8@}%<%t4%p1%d%e48\:2\:\:%p1%@{65536@}%/%d\:%p1%@{256@}%/%@{255@}%&%d\:%p1%@{255@}%&%d%;m,
+  setaf=\E[%?%p1%@{8@}%<%t3%p1%d%e38\:2\:\:%p1%@{65536@}%/%d\:%p1%@{256@}%/%@{255@}%&%d\:%p1%@{255@}%&%d%;m,
 
-$ TERM=xterm-24bit emacs -nw
+$ TERM=xterm-direct emacs -nw
 @end example
 
-Currently there's no standard way to determine whether a terminal
-supports direct color mode.  If such standard arises later on, support
-for @samp{setb24} and @samp{setf24} may be removed.
+Terminals with @samp{RGB} capability treat pixels #000001 - #000007 as
+indexed colors to maintain backward compatibility with applications
+that are unaware of direct color mode.  Therefore the seven darkest
+blue shades may not be available.  If this is a problem, you can
+always use custom terminal definition with @samp{setb24} and
+@samp{setf24}.
 
 @node Debugging a customization file
 @section How do I debug a @file{.emacs} file?

src/term.c

@@ -4144,10 +4144,10 @@ use the Bourne shell command 'TERM=...; export TERM' (C-shell:\n\
       tty->TN_max_colors = tgetnum ("Co");
 
 #ifdef TERMINFO
-      /* Non-standard support for 24-bit colors. */
       {
 	const char *fg = tigetstr ("setf24");
 	const char *bg = tigetstr ("setb24");
+	/* Non-standard support for 24-bit colors. */
 	if (fg && bg
 	    && fg != (char *) (intptr_t) -1
 	    && bg != (char *) (intptr_t) -1)
@@ -4156,6 +4156,14 @@ use the Bourne shell command 'TERM=...; export TERM' (C-shell:\n\
 	    tty->TS_set_background = bg;
 	    tty->TN_max_colors = 16777216;
 	  }
+	/* Standard support for 24-bit colors.  */
+	else if (tigetflag ("RGB") > 0)
+	  {
+	    /* If the used Terminfo library supports only 16-bit
+	       signed values, tgetnum("Co") and tigetnum("colors")
+	       could return 32767.  */
+	    tty->TN_max_colors = 16777216;
+	  }
       }
 #endif
 

src/tparam.h

@@ -37,7 +37,8 @@ extern char *BC;
 extern char *UP;
 
 #ifdef TERMINFO
-char *tigetstr(const char *);
+int tigetflag (const char *);
+char *tigetstr (const char *);
 #endif
 
 #endif /* EMACS_TPARAM_H */