Re: PTY dies when master in parent process is closed.

[Corinna Vinschen <corinna-cygwin at cygwin dot com>]

On Mar  5 14:31, Corinna Vinschen wrote:
> Hi Takashi,
> 
> On Mar  5 21:53, Takashi Yano wrote:
> > Package: cygwin
> > Version: 1.7.35-1
> > 
> > PTY of cygwin dies when master fd in parent process
> > is closed while a child process keeps valid master fd.
> > 
> > To reproduce this problem:
> > 1) Compile following C sources (Test Case 1, 2)
> > 2) Execute them.
> > 
> > Expected results are both:
> > 01234567890
> > 
> > However, result of Test Case 2 is:
> > Hungup
> > while Test Case 1 is as expected.
> > 
> > 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.
> > 
> > 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.
> > 
> > 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...

I applied a patch.  Please have a look.


Thanks.
Corinna

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

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