Re: shutdown not affecting connection?

[Pavel Begunkov <asml.silence@gmail.com>]

Hi

On 2/8/2020 4:55 PM, Glauber Costa wrote:
> Hi
> 
> I've been trying to make sense of some weird behavior with the seastar
> implementation of io_uring, and started to suspect a bug in io_uring's
> connect.
> 
> The situation is as follows:
> 
> - A connect() call is issued (and in the backend I can choose if I use
> uring or not)
> - The connection is supposed to take a while to establish.
> - I call shutdown on the file descriptor
> 
> If io_uring is not used:
> - connect() starts by  returning EINPROGRESS as expected, and after
> the shutdown the file descriptor is finally made ready for epoll. I
> call getsockopt(SOL_SOCKET, SO_ERROR), and see the error (104)
> 
> if io_uring is used:
> - if the SQE has the IOSQE_ASYNC flag on, connect() never returns.
> - if the SQE *does not* have the IOSQE_ASYNC flag on, then most of the
> time the test works as intended and connect() returns 104, but
> occasionally it hangs too. Note that, seastar may choose not to call
> io_uring_enter immediately and batch sqes.
> 
> Sounds like some kind of race?
> 
> I know C++ probably stinks like the devil for you guys, but if you are
> curious to see the code, this fails one of our unit tests:
> 
> https://github.com/scylladb/seastar/blob/master/tests/unit/connect_test.cc
> See test_connection_attempt_is_shutdown
> (above is the master seastar tree, not including the io_uring implementation)
> 
Is this chaining with connect().then_wrapped() asynchronous? Like kind
of future/promise stuff? I wonder, if connect() and shutdown() there may
be executed in the reverse order.

The hung with IOSQE_ASYNC sounds strange anyway.


> Please let me know if this rings a bell and if there is anything I
> should be verifying here
> 

-- 
Pavel Begunkov