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.
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