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Date: Fri, 7 Jul 2006 01:38:40 -0400
From: "Lev Bishop" <lev DOT bishop AT gmail DOT com>
To: cygwin AT cygwin DOT com
Subject: Re: rsync over ssh hang issue understood
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On 7/7/06, Darryl Miles wrote:
> Dave Korn wrote:
> > On 07 July 2006 01:31, Darryl Miles wrote:
> >> Maybe you are in a better position to share more insight into the
> >> situation (specifically about the use of NtQueryInformationFile in
> >> addressing the writability semantics of the POSIX select/poll/write
> >> event system).
> >
> >   I think you should just use it and it should just work.
>
> Okay the immediate red flag I can see with it:
>
> else if (fpli.WriteQuotaAvailable >= PIPE_BUF)
>  gotone += s->write_ready = true;
>
>
> The underlying socket is still being used in blocking mode.

Socket?? What is this socket?

> Which means
> when we write write 1024 bytes of data but only one PIPE buffer is free
> (ulimit -p) a POSIX kernel would allow:

What is "one PIPE buffer (ulimit -p)"?

> Non-blocking:
>
> write(fd, buffer, 1024) = 512
> write(fd, &buffer[512], 512) = -1 (EAGAIN)
>
> Blocking:
>
> write(fd, buffer, 1024) = 512
> write(fd, &buffer[512], 512) = [blocking occurs until] 512

SUSv3 does not permit this behaviour. It is quite clear on this point:
"If the O_NONBLOCK flag is clear, a write request may cause the thread
to block, but on normal completion it shall return nbyte."

> I would guess under WIN32 we end up with fhandler.cc:raw_write() doing:
>
> WriteFile(hPipe, buffer, 1024, &writtenLen, 0) = [blocking occurs until]
> TRUE (writtenLen=1024)

This gives the SUSv3-mandated behaviour for blocking pipes (modulo
signal-interruptibility).

> Maybe PIPE_NOWAIT would be good for us (to setup when the fd has
> O_NONBLOCK set).
>
>
> If PIPE_NOWAIT isn't good for us.  We can get around this by using
> Overlapping I/O and controlling the outstanding pipe buffers.  This
> makes cygwin simulate 'ulimit -p' to some degree.  Which is back towards
> my proposal.

What does "simulate 'ulimit -p'" mean? ulimit -p works just fine on cygwin:
$ ulimit -p
8

Lev

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