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Subject: Re: Unix Domain Socket Limitation?
From: Norton Allen <allen AT huarp DOT harvard DOT edu>
To: Ken Brown <kbrown AT cornell DOT edu>, cygwin <cygwin AT cygwin DOT com>
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Message-ID: <1a0944b7-5924-31ab-7198-a5c311f39e06@huarp.harvard.edu>
Date: Wed, 2 Dec 2020 12:30:51 -0500
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On 11/30/2020 9:22 PM, Norton Allen wrote:
> Yeah, so now the example no longer blocks for me. Unfortunately these 
> bugs are not present in my application, so I will need to keep working 
> on this.
>

After paring the main application down and back up, I finally narrowed 
in on the condition that was causing this blocking behavior. The issue 
arises when a client connect()s twice to the same server with 
non-blocking unix-domain sockets before calling select().

There are a few pieces to this. With the client configured to connect() 
just once, I can see that the server's select() returns as soon as the 
client calls connect(), but then the server's accept() blocks until the 
client calls select(). That is not proper non-blocking behavior, but it 
appears that the implementation under Cygwin does require that client 
and server both be communicating synchronously to accomplish the 
connect() operation.

I tried running this under Ubuntu 16.04 and found that connect() 
succeeded immediately, so no subsequent select() is required, and there 
does not appear to be a possibility for this collision. That proves to 
hold true even if the server is not waiting in select() to process the 
connect() with accept().

A workaround for this issue may be to keep the socket blocking until 
after connect().

I have pushed the new minimal example program,  'rapid_connects' to 
https://github.com/nthallen/cygwin_unix

The server is run like before as:

    $ ./rapid_connects server

The client can be run in two different modes. To connect with just one 
socket:

    $ ./rapid_connects client1

To connect with two:

    $ ./rapid_connects client2

My immediate strategy will be to develop a workaround for my project. 
Having spent a day inside cygwin1.dll, I can see that I have a steep 
learning curve to make much of a contribution there.


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