bug#39733: 26.3; Unclear direct color mode documentation can cause loading delays

[Daniel Long Sockwell]

The Emacs manual describes the process for enabling direct color mode
for terminal Emacs with the following:

> Emacs can be invoked with a custom definition as shown below
>>     $ cat terminfo-24bit.src
>>     
>>     # Use colon separators.
>>     xterm-24bit|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,
>>     # 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,
>>     
>>     $ tic -x -o ~/.terminfo terminfo-24bit.src
>>     
>>     $ TERM=xterm-24bit emacs -nw
Source: Emacs FAQ, 5.3

I followed these directions literally, and enabled direct color support
in Emacs, apparently without issue.  However, what I did not realize was
that I ought *not* to have followed the example literally – instead, I
should have replaced "xterm" with my actual $TERM value.  (In
retrospect, that is fairly obvious, but it wasn't at the time, in part
because I've come across other areas where setting a $TERM value of
xterm was a valid workaround.  And, as I said, following the directions
literally appeared to work as shown by the output of `M-x
list-colors-display`.)

Months later, I realized that I was experiencing a very odd bug: my
Emacs instance was taking well over 2 seconds to start in a terminal,
even when invoked with `emacs -Q -nw`.  Even more oddly, I determined
`M-x emacs-init-time` showed a very low time – whatever was causing the
slowdown was outside of the normal init process.  After fairly extensive
testing, I discovered that the slowdown was caused by a pselect
timeout.  This one, according to strace:

> pselect6(5, [4], [], NULL, {tv_sec=1, tv_nsec=998526670}, {NULL, 8}) =
> 0 (Timeout) <2.000940>

After even more testing, I determined that the issue was caused by the
terminfo/$TERM settings: apparently, if the user has a $TERM value that
does not match their terminal, Emacs will have a 2-second delay before
launching, waiting for whatever that pselect call is looking for.

Once I finally figured the above out, I decided to file this bug in the
hope that the documentation can be updated and other can avoid spending
as long with this issue as I did.  Specifically, I believe that Section
5.3 should be updated to indicate that users should modify the custom
definition provided to use the appropriate $TERM value for their own
terminal.

Beyond that, the fact that an incorrect $TERM setting results in a
2-second delay in launching Emacs may, itself, be an Emacs bug – after
all, the incorrect $TERM/terminfo settings *worked* to display correct
colors in Emacs.  It may be that there is a way to avoid this delay, or
at least to signal the error to the user via *Messages*.  But I don't
know enough about Emacs' internals to be sure of that.

Either way, it was certainly an interesting process to debug!

Best regards,
Daniel




In GNU Emacs 26.3 (build 1, x86_64-pc-linux-gnu)
 of 2019-09-18 built on svetlemodry
System Description:     Antergos Linux

Recent messages:
For information about GNU Emacs and the GNU system, type C-h C-a.
Making completion list...

Configured using:
 'configure --prefix=/usr --sysconfdir=/etc --libexecdir=/usr/lib
 --localstatedir=/var --without-x --without-sound --with-modules
 'CFLAGS=-march=x86-64 -mtune=generic -O2 -pipe -fno-plt'
 CPPFLAGS=-D_FORTIFY_SOURCE=2
 LDFLAGS=-Wl,-O1,--sort-common,--as-needed,-z,relro,-z,now'

Configured features:
DBUS NOTIFY ACL GNUTLS LIBXML2 ZLIB MODULES THREADS LIBSYSTEMD

Important settings:
  value of $LC_COLLATE: en_US.UTF-8
  value of $LANG: en_US.UTF-8
  locale-coding-system: utf-8-unix

Major mode: Lisp Interaction

Minor modes in effect:
  tooltip-mode: t
  global-eldoc-mode: t
  eldoc-mode: t
  electric-indent-mode: t
  menu-bar-mode: t
  file-name-shadow-mode: t
  global-font-lock-mode: t
  font-lock-mode: t
  auto-composition-mode: t
  auto-encryption-mode: t
  auto-compression-mode: t
  line-number-mode: t
  transient-mark-mode: t

Load-path shadows:
None found.

Features:
(shadow sort mail-extr emacsbug message rmc puny seq byte-opt gv
bytecomp byte-compile cconv cl-loaddefs cl-lib dired dired-loaddefs
format-spec rfc822 mml easymenu mml-sec password-cache epa derived epg
epg-config gnus-util rmail tool-bar rmail-loaddefs mm-decode mm-bodies
mm-encode mail-parse rfc2231 mailabbrev gmm-utils mailheader sendmail
regexp-opt rfc2047 rfc2045 ietf-drums mm-util mail-prsvr mail-utils
term/xterm xterm time-date elec-pair mule-util tooltip eldoc electric
uniquify ediff-hook vc-hooks lisp-float-type tabulated-list replace
newcomment text-mode elisp-mode lisp-mode prog-mode register page
menu-bar rfn-eshadow isearch timer select mouse jit-lock font-lock
syntax facemenu font-core term/tty-colors frame cl-generic cham georgian
utf-8-lang misc-lang vietnamese tibetan thai tai-viet lao korean
japanese eucjp-ms cp51932 hebrew greek romanian slovak czech european
ethiopic indian cyrillic chinese composite charscript charprop
case-table epa-hook jka-cmpr-hook help simple abbrev obarray minibuffer
cl-preloaded nadvice loaddefs button faces cus-face macroexp files
text-properties overlay sha1 md5 base64 format env code-pages mule
custom widget hashtable-print-readable backquote threads dbusbind
inotify multi-tty make-network-process emacs)

Memory information:
((conses 16 97828 8897)
 (symbols 48 19448 1)
 (miscs 40 40 96)
 (strings 32 27649 1398)
 (string-bytes 1 739458)
 (vectors 16 11875)
 (vector-slots 8 449294 5376)
 (floats 8 49 66)
 (intervals 56 208 0)
 (buffers 992 12))