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Date: Fri, 25 Aug 2023 15:19:03 +0200
From: Corinna Vinschen <corinna-cygwin@cygwin.com>
To: cygwin@cygwin.com
Subject: Re: [EXTERNAL] Re: scp stalls on uploading in cygwin 3.5 current
 master.
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On Aug 25 14:23, Corinna Vinschen via Cygwin wrote:
> On Aug 25 12:08, Lavrentiev, Anton (NIH/NLM/NCBI) [C] via Cygwin wrote:
> > > I don't have an answer to this problem yet.
> > > 
> > > Can we use send(sock, "", 0) to reenable FD_WRITE, perhaps?
> > 
> > Can't it just be assumed that the socket is _always_ writeable _unless_ the last send() failed?
> > In other words, try to always send() if it did not fail before.  If it did, only send() after
> > FD_WRITE was returned in the event mask.
> 
> You're looking from the application perspective, but as the underlying
> library we don't have the application under control.  The application
> can rightfully expect POSIX-like behaviour from select(2), and *that*
> means, it can expect select(2) to return a socket as non-writable if the
> internal buffer is full, *before* it calls send:
> 
>   while (...)
>     {
>       /* send as long as we can, otherwise do another job in the meantime */
>       while (select (..., <zero timeout>))
> 	send (<blocking>);
>       <do something else>
>     }

No, wait.

I just realized that this isn't correct.  While select indicates that
data can be written, it doesn't indicate how much data can be written.
I. e., if select returns, and there's only buffer space for 10 bytes,
and the send call tries to send 100 bytes, it *will* block, unless the
socket is non-blocking and returns EAGAIN.

The testcase my patch was based on called a poll/write loop on a
socketpair without changing the socket to non-blocking before.  At the
time I didn't even realize that it's actually not a good test, d'oh.


Corinna