rsync over ssh hang issue understood

Fri Jul 7 07:43:00 GMT 2006

Lev Bishop wrote:
> On 7/7/06, Darryl Miles wrote:
>> Dave Korn wrote:
>> > On 07 July 2006 01:31, Darryl Miles wrote:
>> The underlying socket is still being used in blocking mode.
>> Socket?? What is this socket?

Sorry s/socket/pipe/.
>> 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)"?

See below...
>> 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."

Okay this is fine too. I believe the applications which we are seeing 
problems with are using non-blocking mode.
>> 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).

For POSIX blocking mode yes of course, but for POSIX non-blocking mode 
we are relying on the information coming back from 
NtQueryInformationFile to indicate that our WriteFile() won't block 
providing our writes are <= PIPE_BUF length.
> What does "simulate 'ulimit -p'" mean? ulimit -p works just fine on cygwin:
> $ ulimit -p
> 8

I had a complete misunderstanding on that, I was thinking the 
application would tweak the kernel buffering between processes. But 
this appears to be just a reporting mechanism for PIPE_BUF size at runtime.
What was the reason the existing code in CVS for select.cc was disabled?
Maybe it would help to better understand the reasons/cases where the 
disabled code failed.
Darryl
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