wsl.exe periodically sends control characters making impossible "piping" via interactive/terminal mode

[gavenkoa]

I tried to set WSL 1 connection type for Emacs TRAMP:

https://www.reddit.com/r/emacs/comments/jrkgmy/tramp_for_wsl_1_from_cygwin_emacsw32/

and debugging led me to code:

(setq xxx (make-process :name "xxx" :buffer "xxx" :command '("wsl" "echo" "OK") :connection-type 'pty))
(stop-process xxx)

which shows following (I replaced occurrences of ESC byte, or #x1b, with \e ):

\e[25l\e[K
\e[K
\e[K
\e[K
\e[K
\e[K
\e[K
\e[K
\e[K
\e[K
\e[K
\e[K
\e[K
\e[K
\e[K
\e[K
\e[K
\e[K
\e[K
\e[K
\e[K
\e[K
\e[K
\e[K
\e[K\e[H\e[?25hOK

It is not clear if it comes from conhost.exe or from wsl.exe.

pipe mode or wsl echo OK | cat disable garbage appearance but unfortunately Emacs TRAMP implementation starts wsl in the intermediate step and assumes that there is only one terminal (Cygwin's in my case), who can imagine that WSL creates intermediate "terminal"?

Also if I launch interactive program, like sh WSL (or conhost??) periodically emits similar garbage. It make impossible to pipe data as they will be corrupted.

So far:

• there is a need for clarification who is responsible for garbage emmition
• if it is wsl itself we need special flag to disable terminal "smartness" of WSL - to tell to stop to emit any control sequences to clear terminal or

Why:

• it is impossible to use external (non-WSL) expect -like programs (for example Emacs or expect from Cygwin) to automate interaction with WSL 1 via wsl cmd -opt1 -op2 like invocation.

[DHowett]

I believe this is due to the allocation of a pty. those escapes are coming out of conhost because the process was opted in for an interactive session. This is like giving a tty to a process: isatty returns true, and the application will likely assume it is in an interactive session and use screen and formatting controls.

[Biswa96]

Can you try wsltty with that same test?

[gavenkoa]

Can you try wsltty with that same test?

It is impossible. The way wsltty installs and behaves makes it unusable from Cygwin.

As I understand the goal of this project is to launch Cygwin but not be launched by Cygwin.

[gavenkoa]

@DHowett

I believe this is due to the allocation of a pty.

That's true.

I tied to trick Cygwin Emacs TRAMP mode by piping WSL over Cygwin's cat:

'((tramp-login-program "sh")
  (tramp-login-args (("-c" "'wsl -- sh | cat'")))

Emacs TRAMP mode sends to Cygwin sh interpreter:

23:02:20.075911 tramp-maybe-open-connection (3) # Sending command ‘exec sh -c 'wsl -- sh | cat'’

but still WSL generates ESC control sequences.

There is no such issue when I execute sh -c 'wsl -- sh | cat' from mintty.

Special option for activating WSL in PIPE mode will resolve this problem.

I think that some thin custom wrapper that spawns WSL via pipe also will help, my attempts to write Cygwin script with "wsl -- sh | cat" didn't help. It should be written in C or with expect trickery...

[DHowett]

I don’t know much about emacs, but couldn’t you remove :connection-type 'pty to not request a PTY?

[gavenkoa]

@DHowett

I don’t know much about emacs, but couldn’t you remove :connection-type 'pty to not request a PTY?

The problem is that Emacs TRAMP starts sh process and then communicate via pipe / terminal with SH. They send exec wsl (or whatever I configure) in an intermediate shell. At this stage I have no control over wsl executable (like to force it into a pipe mode).

When I work directly with Emacs process API and change it to a pipe mode - there is no issues:

(setq xxx (make-process :name "xxx" :buffer "xxx" :command '("wsl" "sh") :connection-type 'pipe))
(process-send-string xxx "ls\n")
(stop-process xxx)

So it is a specific design of TRAMP (designed in a way similar to Ansible) prevents me to influence how WSL is started. Because they spawn an intermediate process. To make it work one is needed:

• Rewrite TRAMP to avoid launching intermediate shell, and to start WSL directly with pipe option
• Add option --force-pipe to WSL to avoid ESC control sequences.
• Write a wrapper that starts WSL in a pipe mode (in that separate process we have full control over WSL).

[gavenkoa]

I'll ask for advice in Cygwin and/or Emacs TRAMP mail lists in order for a solution that doesn't require changes on WSL sde.

[gavenkoa]

I've got alternative solution that obsoletes need for TRAMP.

After some hints from Cygwin mailing list I managed to "mount" WSL network P9 \\wsl?\distro points in Cygwin and have idiomatic way to access files under WSL.

Details are here: https://cygwin.com/pipermail/cygwin/2020-November/246886.html

The original report with suggestion to stop WSL (or conemu??) emitting special clearing sequences still valid as it might break another Expect-like bidirectional communication programs that jump from native Windows into WSL via wsl.exe -d debian /bin/sh