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Date: Thu, 5 Mar 2015 14:31:00 +0100
From: Corinna Vinschen <corinna-cygwin@cygwin.com>
To: cygwin@cygwin.com
Subject: Re: PTY dies when master in parent process is closed.
Message-ID: <20150305133100.GY3213@calimero.vinschen.de>
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Hi Takashi,

On Mar  5 21:53, Takashi Yano wrote:
> Package: cygwin
> Version: 1.7.35-1
>=20
> PTY of cygwin dies when master fd in parent process
> is closed while a child process keeps valid master fd.
>=20
> To reproduce this problem:
> 1) Compile following C sources (Test Case 1, 2)
> 2) Execute them.
>=20
> Expected results are both:
> 01234567890
>=20
> However, result of Test Case 2 is:
> Hungup
> while Test Case 1 is as expected.
>=20
> Difference between Test Case 1 and Test Case 2 is
> which process is worked in parent. In other words,
> master is in parent process in Test Case 1, while
> master is in child process in Test Case 2.
>=20
> I looked into cygwin source code, and found the cause.
> fhandler_pty_master::close() calls get_ttyp()->set_master_closed()
> if it is called from the process which opend the master.
> It is regardless whether other processes have the valid
> master handle.
>=20
> get_ttyp()->set_master_closed() should be called only
> when the last valid master handle among all processes
> is closed.

No, I don't think so.  The name of the method, set_master_closed() is
probably a bit misleading, as is the name of the test method
is_master_closed().  What it really means is this:

The process creating the pty will start a master control thread (MCT).
This thread is used for duplicating handles from the slave side, if the
slave has no permissions to duplicate the from_master/to_master handles
by itself.  The tty member master_pid points to the process running the
MCT.

If the master fd is closed in the process having created the master, the
MCT is stopped as well.  set_master_closed() sets master_pid to -1 to
reflect the fact that no MCT is running anymore.  So, calling
set_master_closed() from close is not wrong, AFAICS.

What's wrong is to test for is_master_closed() in fhandler_pty_slave::read,
and to generate SIGHUP in case the MCT isn't running.  I think this code
snippet was a thinko, perhaps induced by the misleading name of the method.

While looking through the code, I think that the second usage of
is_master_closed() (in fhandler_pty_slave::open()) is just as wrong...


Corinna

--=20
Corinna Vinschen                  Please, send mails regarding Cygwin to
Cygwin Maintainer                 cygwin AT cygwin DOT com
Red Hat

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