Re: Emacs difficulties in linux console with ncurses-6.3 caused by kcbt=\E^I.

[Thomas Dickey]

On Thu, May 05, 2022 at 06:18:00PM +0000, Alan Mackenzie wrote:
> Hello, ncurses!
> 
> I'm writing as a member of the Emacs development team.
> 
> In the recent change from ncurses-6.2 to ncurses-6.3, the following
> change was make in the linux console terminfo:
> 
> --- Infocmp-linux-6.2   2022-05-04 20:16:01.609557894 +0000
> +++ infocmp-linux-6.3   2022-05-04 20:09:02.046581014 +0000
> @@ -14,7 +14,7 @@
>         home=\E[H, hpa=\E[%i%p1%dG, ht=^I, hts=\EH, ich=\E[%p1%d@,
>         ich1=\E[@, il=\E[%p1%dL, il1=\E[L, ind=\n,
>         
> initc=\E]P%p1%x%p2%{255}%*%{1000}%/%02x%p3%{255}%*%{1000}%/%02x%p4%{255}%*%{1000}%/%02x,
> -       kb2=\E[G, kbs=^?, kcbt=\E[Z, kcub1=\E[D, kcud1=\E[B,
> +       kb2=\E[G, kbs=^?, kcbt=\E^I, kcub1=\E[D, kcud1=\E[B,
>         kcuf1=\E[C, kcuu1=\E[A, kdch1=\E[3~, kend=\E[4~, kf1=\E[[A,
>         kf10=\E[21~, kf11=\E[23~, kf12=\E[24~, kf13=\E[25~,
>         kf14=\E[26~, kf15=\E[28~, kf16=\E[29~, kf17=\E[31~,
> 
> We now have kcbt=\E^I.

A bug report got me to verify this, and
shift-tab on the console appeared to send that sequence,
so I documented it in the terminal description.

Tracing changes to default keyboard configuration in Linux isn't
as "simple" as control-sequences, but see below (it's "kbd").

I don't see any that send \E[Z.  Having the terminal description
list a key definition that no one uses isn't very useful.
(I'll allow for some keyboard differences -- who has a "clear" key? --
but shift-tab has been used for a long time).

Just to check:
        Debian, Fedora, Mageia, OpenSUSE do this (send \E^I)
        Arch, Slackware don't do this (I get just ^I)

I have a few others that I could check, but (CentOS and Scientific Linux)
those are either obsolete or derived from the ones that I listed.

I'm aware that there are (down in the 1% range) still other Linux-based
systems, but generally speaking few/none of those have contributed
in this area, so I don't have a machine to verify bug reports.

fwiw, I noticed some comment in the usual source of misinformation
recently stating that \E[Z originated with Linux in 1995.
That was incorrect (the terminal database shows this for several
AT&T entries and an Ann Arbor Ambassador entry - roughly ten
years earlier).
 
> In Emacs on the Linux console, our use of terminfo now has the effect of
> replacing the M-tab keysequence with backtab.  This make several
> important key-bindings unusable.  (The backtab binding is also used, but
> is perhaps a little less important.)

Perhaps that original kcbt was changed to allow the \E^I to work
with Emacs.  If that's the case, then it should be something that
I can document in the terminal description.

All that I have so far is the commit message, which gives no useful
information regarding the intent of the change:

        commit 0baa7f071e79bb700b62b1f8507630387cbc4bbb
        Author: Alexey Gladkov <legion@altlinux.org>
        Date:   Tue May 8 22:55:07 2007 +0400

            Apply patchkbd-1.12-Meta-Tab.diff from SUSE
            
            Convert Shift Tab to Meta_Tab
            
            Signed-off-by: Alexey Gladkov <legion@altlinux.org>

and this equally non-informative mailing list thread:

https://lists.opensuse.org/archives/list/commit@lists.opensuse.org/message/P3DQHCZB7IWQ7F6VZD3LGXHVOIN6OFE6/

(which for example doesn't mention Emacs).

> Also, this would seem to be a difficult problem for a normal user to
> diagnose, and difficult also to fix.
> 
> May I ask why this change was made to the linux terminal?  Is there any
> possibiliy it might be reverted?

It was a bug report:

# 2021-09-04
#       + modify linux3.0 entry to reflect default mapping of shift-tab by
#         kbd 1.14 (report by Jan Engelhardt) -TD
 
> Is there perhaps some strategy we could use in Emacs (C code) which would
> work around this problem with using ugly ad-hoc code?

I suppose you could tell Emacs to ignore kcbt (termcap kB) for Linux.

non-Emacs users probably would like to use the key.

-- 
Thomas E. Dickey <dickey@invisible-island.net>
https://invisible-island.net
ftp://ftp.invisible-island.net

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