I've got a problem in which cvs is dangling at the end of a pipe with its
stdin/stderr tied through a pipe to a program that's parsing its output (such as
emacs with its VC minor mode or CVS major mode).

cvs is using ssh to access the CVS server, and this ssh inherits its stderr file
from cvs.

Now ssh sets O_NONBLOCK on its stderr output, and this also affects both stdout
and stderr coming from cvs (since the cvs's fd1 and fd2, and ssh's fd2 all refer
to the same kernel object).

After this has happened, strace shows that cvs starts experiencing writes that
return EAGAIN on its stderr output when the pipe buffer becomes full (which it
just ignores). As far as cvs is concerned, this shouldn't happen (it should,
however,  probably check for the error message and retry the write).

ssh's stdout isn't a problem because cvs opens its own pipes for connecting to
ssh's stdin and stdout.

Note that ssh doesn't set the O_NONBLOCK flag on exit either.

As far as strace is concerned, the following two adjacent fcntl64 syscalls are
the only ones that deal with stderr's O_NONBLOCK flag.

ioctl(2, 0x5401, 0xbffff150)            = -1 EINVAL (Invalid argument)
fcntl64(0x2, 0x3, 0, 0x2)               = 1
fcntl64(0x2, 0x4, 0x801, 0x2)           = 0

Thanks,
David