X-Recipient:	archive-cygwin AT delorie DOT com
X-Original-To:	cygwin AT cygwin DOT com
Delivered-To:	cygwin AT cygwin DOT com
DMARC-Filter:	OpenDMARC Filter v1.3.2 sourceware.org EB6C838618B2
Authentication-Results:	sourceware.org; dmarc=none (p=none dis=none)
	header.from=huarp.harvard.edu
Authentication-Results:	sourceware.org;
	spf=pass smtp.mailfrom=allen AT huarp DOT harvard DOT edu
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>
References:	<71490665-31b0-f63c-74da-461a053fac21 AT huarp DOT harvard DOT edu>
	<55ea1649-1979-6238-75ab-69100c22e069 AT cornell DOT edu>
	<4260ad1b-4ab2-fa36-fd0e-7c9644560114 AT huarp DOT harvard DOT edu>
	<38a82f82-1ef9-768e-7d3e-15f63147e188 AT cornell DOT edu>
	<a1f6e9af-7c0b-4d3f-4198-1c7bff4869dc AT huarp DOT harvard DOT edu>
	<16165727-f614-1543-70bc-36457ddbf260 AT cornell DOT edu>
	<75d1315b-5a56-a2e5-310d-6ac33a3cf17c AT huarp DOT harvard DOT edu>
	<f5373cd4-1d3e-500a-a967-1c558541e720 AT cornell DOT edu>
	<85c9c70f-c016-0f88-099e-5c772adbc648 AT huarp DOT harvard DOT edu>
	<fb523694-7775-1d7a-db55-27ccbd6d157c AT huarp DOT harvard DOT edu>
Message-ID:	<1a0944b7-5924-31ab-7198-a5c311f39e06@huarp.harvard.edu>
Date:	Wed, 2 Dec 2020 12:30:51 -0500
User-Agent:	Mozilla/5.0 (Windows NT 10.0; Win64; x64; rv:78.0) Gecko/20100101
	Thunderbird/78.5.0
MIME-Version:	1.0
In-Reply-To:	<fb523694-7775-1d7a-db55-27ccbd6d157c@huarp.harvard.edu>
X-Spam-Status:	No, score=-2.6 required=5.0 tests=BAYES_00, KAM_DMARC_STATUS,
	NICE_REPLY_A, RCVD_IN_DNSWL_LOW, SPF_HELO_NONE, SPF_PASS,
	TXREP autolearn=ham autolearn_force=no version=3.4.2
X-Spam-Checker-Version:	SpamAssassin 3.4.2 (2018-09-13) on
	server2.sourceware.org
X-BeenThere:	cygwin AT cygwin DOT com
X-Mailman-Version:	2.1.29
List-Id:	General Cygwin discussions and problem reports <cygwin.cygwin.com>
List-Unsubscribe:	<https://cygwin.com/mailman/options/cygwin>,
	<mailto:cygwin-request AT cygwin DOT com?subject=unsubscribe>
List-Archive:	<https://cygwin.com/pipermail/cygwin/>
List-Post:	<mailto:cygwin AT cygwin DOT com>
List-Help:	<mailto:cygwin-request AT cygwin DOT com?subject=help>
List-Subscribe:	<https://cygwin.com/mailman/listinfo/cygwin>,
	<mailto:cygwin-request AT cygwin DOT com?subject=subscribe>
Errors-To:	cygwin-bounces AT cygwin DOT com
Sender:	"Cygwin" <cygwin-bounces AT cygwin DOT com>
X-MIME-Autoconverted:	from quoted-printable to 8bit by delorie.com id 0B2HVKeS030470

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.


--
Problem reports:      https://cygwin.com/problems.html
FAQ:                  https://cygwin.com/faq/
Documentation:        https://cygwin.com/docs.html
Unsubscribe info:     https://cygwin.com/ml/#unsubscribe-simple

- Raw text -