bin/164947: tee looses data when writing to non-blocking file descriptors

[Diomidis Spinellis]

The following reply was made to PR bin/164947; it has been noted by GNATS.

From: Diomidis Spinellis <dds at aueb.gr>
To: Martin Cracauer <cracauer at cons.org>
Cc: freebsd-gnats-submit at freebsd.org
Subject: Re: bin/164947: tee looses data when writing to non-blocking file
 descriptors
Date: Fri, 10 Feb 2012 22:32:02 +0200

 On 10/02/2012 21:17, Martin Cracauer wrote:
 > Diomidis Spinellis wrote on Fri, Feb 10, 2012 at 07:04:41AM +0000:
 >>
 >>> Number:         164947
 [...]
 
 >>> How-To-Repeat:
 >> Run the following:
 >> #!/usr/local/bin/bash
 >> # bash needed for the>(...) functionality
 >> # ssh apparently sets O_NONBLOCK
 >> # Remove the 2>/dev/null to see tee complaining
 >> dd count=100000 if=/dev/zero |
 >> tee>(ssh localhost dd of=/dev/null) 2>/dev/null |
 >> (ssh localhost dd of=/dev/null)
 >
 > I don't think it is ssh that is causing this. If you use a named pipe
 > explicitly and hook ssh up to that the error doesn't appear.  Seems to
 > be something that bash is doing there.
 
 I think the named pipe isolates the write fd from the ssh end.  If you 
 use cat or dd instead of ssh the problem goes away.
 
 > That doesn't mean I am opposed to handling EAGAIN.
 >
 > The way I normally do it is a simple retry loop, not using select.
 > I'm aware of the tradeoffs, so far I was always better off not
 > investing a second system call into every retry.
 
 I agree this can be cheaper for many cases, but it can become very 
 expensive for long waits.