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Re: CVS problem with ssh


From: Richard M. Stallman
Subject: Re: CVS problem with ssh
Date: Thu, 23 Jun 2005 12:54:13 -0400

        * client.c (handle_m): Workaround to deal with stdio getting put
        into non-blocking via redirection of stderr and interaction with
        ssh on some platforms. On those boxes, stdio can put stdout
        unexpectedly into non-blocking mode which may lead to fwrite() or
        fflush() failing with EAGAIN, but cvs not checking for the error.
        (Patch suggested by Frank Hemer <address@hidden>.)

I'm glad to hear that the new CVS version has a fix that may prevent
the problem.  Maybe this problem is fixed now.  But we should verify that.

It's possible that this fix is a full and correct solution.  If the
descriptor has room for at least one byte, `write' won't return
EAGAIN; it will write at least one byte.  It may write less than the
whole buffer that was specified.  If the stdio code handles that case
correctly, the results should be correct.  Roland, does GNU libc
handle that case correctly?  I expect it does, since that case can
arise in many situations.  If it were not handled correctly, we would
hear lots of screaming--I think.

If the current fix is correct, great!

If the current fix does not make it correct, please do make the larger
fix--do your own buffering.  That is surely not really a large job.
It is worth that much effort to fix such a serious bug.

    I agree with Larry Jones, that the problem is deeper.  It seems to me
    that any program writing to stderr or stdout via stdio should have the
    reasonable expectation that the stream will remain blocking, or perhaps
    remain non-blocking, as the case may be.

That would clearly be "the right thing", but it cannot be achieved
without a basic redesign of POSIX.  As this case shows, other
processes that share the descriptor have the reasonable expectation
that they can take an inherited descriptor and make it non-blocking.
These two expectations, though both reasonable, conflict, and that is
what made this problem so hard to fix.  People argued for years about
which program is wrong, and meanwhile we remained in an impasse.

Rather that deciding what to fix by first placing blame, we can do
better if we look at how we can contribute to making the system work.
The right program to fix, in a case like this, is whichever one we
actually can get fixed.

I am grateful to Mark Baushke for approaching the situation that way.

      The
    same would need to be done in any program piping output through ssh,
    some of which don't have CVS's infrastructure, and this seems unreasonable.

This scenario concerns output from ssh that is read by CVS.  It is
true that any program that forks a remote-login program and reads that
program's output thru a pipe is liable to encounter this issue, just
as CVS does.  It would be nice if that were fixed in a more global
way, if correct use of ssh were simpler.  Perhaps someday that will
happen--but in the mean time, let's fix this bug for sure.





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