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Date:	Mon, 30 Aug 2021 17:02:04 +0900
To:	cygwin AT cygwin DOT com
Subject:	Re: cygrunsrv + sshd + rsync = 20 times too slow -- throttled?
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From:	Takashi Yano via Cygwin <cygwin AT cygwin DOT com>
Reply-To:	Takashi Yano <takashi DOT yano AT nifty DOT ne DOT jp>
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On Sun, 29 Aug 2021 22:15:29 -0400
Ken Brown wrote:
> On 8/29/2021 8:22 PM, Takashi Yano via Cygwin wrote:
> > On Mon, 30 Aug 2021 09:13:14 +0900
> > Takashi Yano wrote:
> >> On Sun, 29 Aug 2021 17:04:56 -0400
> >> Ken Brown wrote:
> >>> On 8/29/2021 5:07 AM, Takashi Yano via Cygwin wrote:
> >>>> On Sat, 28 Aug 2021 18:41:02 +0900
> >>>> Takashi Yano wrote:
> >>>>> On Sat, 28 Aug 2021 10:43:27 +0200
> >>>>> Corinna Vinschen wrote:
> >>>>>> On Aug 28 02:21, Takashi Yano via Cygwin wrote:
> >>>>>>> On Fri, 27 Aug 2021 12:00:50 -0400
> >>>>>>> Ken Brown wrote:
> >>>>>>>> Two years ago I thought I needed nt_create to avoid problems when calling
> >>>>>>>> set_pipe_non_blocking.  Are you saying that's not an issue?  Is
> >>>>>>>> set_pipe_non_blocking unnecessary?  Is that the point of your modification to
> >>>>>>>> raw_read?
> >>>>>>>
> >>>>>>> Yes. Instead of making windows read function itself non-blocking,
> >>>>>>> it is possible to check if the pipe can be read before read using
> >>>>>>> PeekNamedPipe(). If the pipe cannot be read right now, EAGAIN is
> >>>>>>> returned.
> >>>>>>
> >>>>>> The problem is this:
> >>>>>>
> >>>>>>     if (PeekNamedPipe())
> >>>>>>       ReadFile(blocking);
> >>>>>>
> >>>>>> is not atomic.  I. e., if PeekNamedPipe succeeds, nothing keeps another
> >>>>>> thread from draining the pipe between the PeekNamedPipe and the ReadFile
> >>>>>> call.  And as soon as ReadFile runs, it hangs indefinitely and we can't
> >>>>>> stop it via a signal.
> >>>>>
> >>>>> Hmm, you are right. Mutex guard seems to be necessary like pty code
> >>>>> if we go this way.
> >>>>
> >>>> I have found that set_pipe_non_blocking() succeeds for both read and
> >>>> write pipes if the write pipe is created by CreateNamedPipe() and the
> >>>> read pipe is created by CreateFile() contrary to the current create()
> >>>> code. Therefore, not only nt_create() but also PeekNamedPipe() become
> >>>> unnecessary.
> >>>>
> >>>> Please see the revised patch attached.
> >>>
> >>> I haven't had a chance to test this myself yet, but occurs to me that we might
> >>> have a different problem after this patch: Does the write handle that we get
> >>> from CreateNamedPipe() have FILE_READ_ATTRIBUTES access?
> >>
> >> I have just checked this, and the answer is "No". Due to this problem,
> >> NtQueryInformationFile() call in select() fails on the write pipe.
> >>
> >> It seems that we need more consideration...
> > 
> > We have two easy options:
> > 1) Configure the pipe with PIPE_ACCESS_DUPLEX.
> > 2) Use nt_create() again and forget C# program issue.
> 
> I vote for 2), but let's see what Corinna thinks.

BTW. what's wrong if just:

static int
nt_create (LPSECURITY_ATTRIBUTES sa_ptr, PHANDLE r, PHANDLE w,
                DWORD psize, int64_t *unique_id)
{
  if (r && w)
    {
      static volatile ULONG pipe_unique_id;
      LONG id = InterlockedIncrement ((LONG *) &pipe_unique_id);
      if (unique_id)
        *unique_id = ((int64_t) id << 32 | GetCurrentProcessId ());
      if (!CreatePipe (r, w, sa_ptr, psize))
        {
          *r = *w = NULL;
          return GetLastError ();
        }
    }
  return 0;
}

?

In my environment, I cannot find any defects.
- No performance degradation.
- set_pipe_non_blocking() works for both read and write pipes.
- NtQueryInformationFile() in select() works for both r/w pipes.
- Piping C# program works.

Is naming the pipe really necessary?

-- 
Takashi Yano <takashi DOT yano AT nifty DOT ne DOT jp>

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