Re: Cygwin-3.1.3: Terminal contents are overwritten by less graphical ones on the start of non-Cygwin programs

[Takashi Yano <takashi.yano@nifty.ne.jp>]

On Sat, 8 Feb 2020 20:13:09 +0800
Koichi Murase wrote:
> Hi, this is another report related to the pseudo console.
> 
> Description:
> 
>   When a non-Cygwin program is started for the first time in that PTY
>   in terminal emulators such as Mintty, Xterm, Urxvt and GNU Screen,
>   the terminal contents are overwritten by less graphical ones.  I
>   guess that the contents of the internal pseudo console (which is
>   constructed in the background) is somehow output and overwrites the
>   original contents of those functional terminal emulators.
> 
>   This is reproduced with the latest snapshot and also with the latest
>   commit 3a71c4638.
> 
> Repeat-By:
> 
>   Open Mintty or Xterm and run the following commands (with Bash):
> 
>   $ x86_64-w64-mingw32-gcc -xc - -o nocyg.exe <<< 'main(){}'
>   $ printf '\e[%smhello\e[m\n' {0..9}
>   $ ./nocyg.exe
> 
>   With the first line, the simplest non-Cygwin program is compiled.
>   With the second line, "hello" is printed with various graphic
>   styles.  However, when one runs a non-Cygwin command in the third
>   line, those graphic styles are reset (i.e., overwritten by contents
>   of a less functional terminal).
>
> Details:
> 
> * If I start Mintty or Xterm with the environment variable
>   `CYGWIN=disable_pcon', the problem does not occur.
> 
> * Before the commit `e38f2dc9b' (Cygwin: pty: Revise code waiting for
>   forwarding again.), this occured every time a non-Cygwin command is
>   executed.  But after the commit, this only occurs for the first
>   execution of any non-Cygwin program in that PTY session.

Thanks for the report. However, this is the intentional behaviour.
In early stage of pseudo console support, PTY cleared screen when
PTY is opened. This is because:
https://www.cygwin.com/ml/cygwin/2019-10/msg00063.html
(Currently, behaviour change when TERM=dumb is removed.)

However, several users who dislike clear screen complains.
So, I changed the behaviour to that of the current version.
https://cygwin.com/ml/cygwin-patches/2019-q4/msg00089.html

In Win10 1809, the result of your test case is even worse.

> * The problem is not restricted in just clearing graphic styles of
>   characters.  For GNU Screen I use Screen-specific terminal sequences
>   "ESC k ... ST" in my shell prompt to set the title of each window.
>   But the overwriting contents seem to be constructed by another
>   terminal which does not recognize this terminal sequence.  As a
>   results, the layout of terminal contents after the non-Cygwin
>   program is completely broken.  The problem should occur with any
>   other functionalities which are supported by modern terminal
>   emulators but not by pseudo console.

How can I reproduce this?

> * As well as non-Cygwin programs, also Cygwin programs `ssh', `scp' or
>   `git push/pull' with SSH connections cause the same results.  I
>   guess SSH utilities internally use non-Cygwin programs.  (Actually I
>   originally faced with this problem with these "Cygwin" commands.)

I cannot reproduce this. Cygwin ssh, scp and git does not triggers
redraw screen in my environment.

Do you surely execute *cygwin* command rather that windows one?

-- 
Takashi Yano <takashi.yano@nifty.ne.jp>

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