Re: Unix Domain Socket Limitation?

[Ken Brown <kbrown@cornell.edu>]

On 12/2/2020 12:30 PM, Norton Allen wrote:
> 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.

I'm traveling at the moment and unable to do any testing, but I wonder if you're 
bumping into an issue that was just discussed on the cygwin-developers list:

   https://cygwin.com/pipermail/cygwin-developers/2020-December/012015.html

A different workaround is described there.

If it's the same issue, then I don't think it will happen with the new AF_UNIX 
implementation.  More in a few days.

Ken