linux-kernel - Re: n_tty: Check the other end of pty pair before returning EAGAIN on a read()

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Message-ID: <567436DE.2020101@hurleysoftware.com>
Date:	Fri, 18 Dec 2015 08:39:58 -0800
From:	Peter Hurley <peter@...leysoftware.com>
To:	Marc Aurele La France <tsi@...oix.net>
Cc:	Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
	Jiri Slaby <jslaby@...e.com>, linux-kernel@...r.kernel.org,
	Volth <openssh@...th.com>, Damien Miller <djm@...drot.org>
Subject: Re: n_tty: Check the other end of pty pair before returning EAGAIN on
 a read()

Hi Marc,

On 12/18/2015 06:26 AM, Marc Aurele La France wrote:
> On Fri, 11 Dec 2015, Peter Hurley wrote:
>> On 12/11/2015 05:37 AM, Marc Aurele La France wrote:
> 
>>> I am not asking to read data before it has been produced.  I am puzzled
>>> that despite knowing that the data exists, I can now be lied to when I
>>> try to retrieve it, when I wasn't before.  We are talking about what is
>>> essentially a two-way pipe, not some network or serial connection with
>>> transmission delays userland has long experience in dealing with.
> 
>>> These previously internal additional delays, that are now exposed to
>>> userland, are simply an implementation detail that userland did not,
>>> and should not, need to worry about.
>>
>> Your mental model is that pseudo-terminals are a synchronous pipe, which
>> is not true.
>>
>> But this argument is pointless because the regression needs to be fixed
>> regardless of the merits.
> 
> Fair enough.
> 
> Anything new on this?

It's on my todo list.

While considering this issue further, I was curious what ssh does
regarding the entire foreground process group and its output?

If ssh only knows that the child has terminated, how does it wait
for the rest of the foreground process group's output since those
processes may not yet have received their SIGHUP/SIGCONT signals
yet?

Regards,
Peter Hurley

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