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Date: Fri, 25 Aug 2023 17:48:32 +0900
To: cygwin@cygwin.com
Subject: Re: scp stalls on uploading in cygwin 3.5 current master.
Message-Id: <20230825174832.9ebae8112667d5d5411cb8db@nifty.ne.jp>
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From: Takashi Yano via Cygwin <cygwin@cygwin.com>
Reply-To: Takashi Yano <takashi.yano@nifty.ne.jp>
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On Thu, 24 Aug 2023 10:59:33 +0200
Corinna Vinschen wrote:
> > I'm not sure why at all, however, the following patch seems to
> > solve the issue.
> > 
> > diff --git a/winsup/cygwin/select.cc b/winsup/cygwin/select.cc
> > index 7b9473849..de5794c9f 100644
> > --- a/winsup/cygwin/select.cc
> > +++ b/winsup/cygwin/select.cc
> > @@ -1790,7 +1790,7 @@ peek_socket (select_record *me, bool)
> >        if (events & FD_WRITE)
> >  	{
> >  	  wfd_set w = { 1, { fh->get_socket () } };
> > -	  TIMEVAL t = { 0 };
> > +	  TIMEVAL t = { .tv_sec = 0, .tv_usec = 1 };
> >  
> >  	  if (_win32_select (0, NULL, &w, NULL, &t) == 0)
> >  	    events &= ~FD_WRITE;
> 
> Yeah, this is weird. A TIMEVAL value of 0 indicates non-blocking,
> so why should waiting a usec make that better?  It also potentially
> slows down Cygwin's select noticably if multiple sockets are part
> of the descriptor set.
> 
> Hmmm.
> 
> Is it possible that _win32_select returns with SOCKET_ERROR for 
> some reason?
> 
> Unfortunately I'm a bit swamped ATM, but rather than setting t to 1
> usec, what if the check goes:
> 
>   if (_win32_select (0, NULL, &w, NULL, &t) != 1)
> 
> ?

This did not help. I looked into this deeper and noticed that:
1) _win32_select() sometimes returns 0.
2) If _win32_select() returns 0, WaitForMultipleObjects(..., INFINITE)
   is called in thread_socket().
3) WaitForMultipleObjects() sometimes does not return for FD_WRITE
   for unknown reason.
This causes the stall.

-- 
Takashi Yano <takashi.yano@nifty.ne.jp>

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