Re: [PATCH v5 1/1] Cygwin: pty: add pseudo console support.

[Takashi Yano <takashi dot yano at nifty dot ne dot jp>]

On Mon, 15 Apr 2019 10:38:32 +0200 Corinna Vinschen wrote:
> This is supposed to be combined with the preceeding code to a single
> expression in the end, right?

Do you mean this code and the preceding code should be executed
exclusively? If so, the answer is no. If the stderr is pty slave,
the message should be written to both output_handle and
output_handle_cyg. The reason will be described later.

> I don't quite understand why writing
> to STD_ERROR_HANDLE doesn't work.  Is STD_ERROR_HANDLE not connected
> to the ConPty?

You are right. STD_ERROR_HANDLE (output_handle) is connected to
the ConPTY. The data written to output_handle is fowarded to
to_master_cyg in the master, then displayed. to_master_cyg is
duplicated to output_handle_cyg in the slave. Therefore, both
writing to output_handle and writing to output_handle_cyg seem
to work, at first glance.

However, in the fact, the code below stops forwarding if the
process is a cygwin process. 
+	  /* Avoid duplicating slave output which is already sent to
+	     to_master_cyg */
+	  if (!get_ttyp ()->switch_to_pcon)
+	    continue;

The role of the code above is as follows. ConPTY sometimes redraws
screen based on its own screen buffer. The screen buffer has only
the data written to output_handle. Basically, cygwin apps use
output_handle_cyg and native apps use output_handle. Terefore,
outputs from cygwin apps are disappeared at that time. To avoid
this, the data write to output_handle_cyg is pushed into the ConPTY
screen buffer by WriteFile() to output_handle. To prevent the data
from being displayed twice, forwarding is stopped by the code above
if the process is a cygwin app.

In strace.cc, writing to output_handle is necessary for pushing
the data into ConPTY screen buffer, and writing to output_handle_cyg
is necessary for displaying it.

> > +      fhandler_base *fh = ::cygheap->fdtab[2];
> > +      if (fh && fh->get_major () == DEV_PTYS_MAJOR)
> 
> For a quick test if the process is connected to a pty, you can
> use `if (istty_slave_dev (myself->ctty))'

Thanks for advice. My code is checking if the stderr is pty or
not, and write the message to output_handle_cyg if the stderr
is pty. If the stderr is redirected to another device, writing
is not done. Does istty_slave_dev() checking work same for
stderr if it is redirected? How can I get output_handle_cyg
after checking using istty_slave_dev()?

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