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Subject: [Bug 209509] EAGAIN on shell pipes / O_NONBLOCK error in kernel ?
Date: Sat, 14 May 2016 23:47:06 +0000
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https://bugs.freebsd.org/bugzilla/show_bug.cgi?id=3D209509

Jilles Tjoelker <jilles@FreeBSD.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
                 CC|                            |jilles@FreeBSD.org

--- Comment #3 from Jilles Tjoelker <jilles@FreeBSD.org> ---
The problem may be caused by ssh. When it starts, ssh sets fd 0, 1 and 2 to
non-blocking mode if they are not TTYs, restoring them to their original st=
ate
on exit. This causes breakage if you use the open files (pipes or sockets) =
for
other things while ssh is running.

Unfortunately, fixing ssh requires adding threads or processes to do blocki=
ng
reads and writes. On FreeBSD, although socket receives support MSG_DONTWAIT,
socket sends do not (although their behaviour is affected by it slightly) a=
nd
pipe reads and writes do not support anything like it. Performance of common
use cases may be affected negatively.

As a workaround, try redirecting ssh's stderr through a 'cat'. For example,

{ ssh ... 2>&1 >&3 3>&- | cat >&2; } 3>&1

(Bug: this example loses ssh's exit status. Using fifos or doing pipe
manipulations from C will avoid that.)

--=20
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