bug#39399: tramp depends on unstable details of shell command line processing

[John F Carr]

I use emacs tramp mode for remote editing.  It stopped working on my FreeBSD 
systems when I upgraded to a development version.  The cause appears to be due 
to /bin/sh reprinting the entire line when a backspace is sent rather than 
sending only a backspace-space-backspace sequence.  The failure follows a 
change in the line editing library (shared with NetBSD) rather than the shell 
itself.

Environment is emacs 24.5 or 26.3 running on Mac OS Catalina.  The remote 
system is FreeBSD 13.0 development.  Tramp works with Linux or FreeBSD 12.1 as 
the remote system.

Here's what happens.

I load tramp and open a file like /ssh:user@host:/.  Emacs spawns ssh to 
connect.  The first thing tramp does is send

stty -inlcr -onlcr -echo kill '^U' erase '^H'

But this doesn't do much because the tty is in raw mode rather than cooked due 
to the shell's line editor.  So tramp falls back to a hack to detect echoed 
input.  It sends "_echo" followed by a string of backspace characters.  "_echo" 
is unlikely to appear in program output.

Here is the next command after the initial stty:

_echo^H^H^H^H^Hstty icanon erase ^H cols 32767_echo^H^H^H^H^H

The groups of 5 ^H represent 5 backspace characters and the lone ^H in the 
middle is a two character sequence for stty.

The terminal output from a 12.1 system is

_echo^H ^H^H ^H^H ^H^H ^H^H ^Hstty icanon erase ^H cols 32767_echo^H ^H^H ^H^H 
^H^H ^H^H ^H
#$ 

where again the middle ^H is a two character sequence and the others are 
backspace characters. There is a carriage return between the two lines.  "#$ " 
is the shell prompt set by tramp.

The terminal output from a FreeBSD 13.0 development branch system is

_echo
#$ _ech ^H
#$ _ec ^H
#$ _e ^H
#$ _ ^H
#$  ^Hstty icanon erase ^H cols 32767_echo
#$ stty icanon erase ^H cols 32767_ech ^H
#$ stty icanon erase ^H cols 32767_ec ^H
#$ stty icanon erase ^H cols 32767_e ^H
#$ stty icanon erase ^H cols 32767_ ^H
#$ stty icanon erase ^H cols 32767 ^H
#$ 

with carriage returns between lines.  This does not make sense to emacs, which 
hangs waiting for something it recognizes.  I can recover by hitting control-G 
which aborts the tramp connection.

This issue is also reported to FreeBSD at 
https://bugs.freebsd.org/bugzilla/show_bug.cgi?id=243807.