On 12/4/2020 8:51 AM, Norton Allen wrote:
On 12/3/2020 8:11 PM, Ken Brown wrote:
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.
It does seem related.
A work around that is working for me is to do a blocking connect() and switch to
non-blocking when that completes. In my application, the connect() generally
occurs once at the beginning of a run, so blocking for a few milliseconds does
not impact responsiveness.
For the record, I can confirm that (a) the problem occurs with the current
AF_UNIX implementation and (b) it does not occur with the new implementation (on
the topic/af_unix branch). With both client1 and client2, I see "connect()
apparently succeeded immediately" using the new implementation.
The new implementation is not yet ready for prime time, but with any luck it
might be ready within a few months.
Ken
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