On Wed, Jul 11, 2018 at 10:26 PM Ben Pfaff <b...@ovn.org> wrote:

> On Wed, Jul 11, 2018 at 12:37:28PM +0530, Numan Siddique wrote:
> > On Wed, Jul 11, 2018 at 2:08 AM Ben Pfaff <b...@ovn.org> wrote:
> >
> > > On Wed, Jul 11, 2018 at 12:56:39AM +0530, nusid...@redhat.com wrote:
> > > > From: Numan Siddique <nusid...@redhat.com>
> > > >
> > > > The python function ovs.socket_util.check_connection_completion()
> uses
> > > select()
> > > > (provided by python) to monitor the socket file descriptor. The
> select()
> > > > returns 1 when the file descriptor becomes ready. For error cases
> like -
> > > > 111 (Connection refused) and 113 (No route to host) (POLLERR),
> > > ovs.poller._SelectSelect.poll()
> > > > expects the exceptfds list to be set by select(). But that is not the
> > > case.
> > > > As per the select() man page, writefds list will be set for POLLERR.
> > > > Please see "Correspondence between select() and poll() notifications"
> > > section of select(2)
> > > > man page.
> > > >
> > > > Because of this behavior,
> ovs.socket_util.check_connection_completion()
> > > returns success
> > > > even if the remote is unreachable or not listening on the port.
> > > >
> > > > This patch fixes this issue by adding a wrapper function -
> > > check_connection_completion_status()
> > > > which calls sock.connect_ex() to get the status of the connection if
> > > > ovs.socket_util.check_connection_completion() returns success.
> > > >
> > > > The test cases added fails without the fix in this patch.
> > > >
> > > > Signed-off-by: Numan Siddique <nusid...@redhat.com>
> > >
> > > Can we just code check_connection_completion() like we do in C, by
> > > directly using select() on Windows and poll() everywhere else?  The
> > > approach in this patch seems a little indirect to me.
> > >
> > >
> > Thanks for the review. As per the comments here -
> > https://github.com/openvswitch/ovs/blob/master/python/ovs/poller.py#L51
> > *****
> > # eventlet/gevent doesn't support select.poll. If select.poll is used,
> > # python interpreter is blocked as a whole instead of switching from the
> > # current thread that is about to block to other runnable thread.
> > # So emulate select.poll by select.select because using python means that
> > # performance isn't so important.
> > *******
> >
> > we cannot use poll() in python. I tried with this patch to test it out -
> > https://paste.fedoraproject.org/paste/YUhgVte-BOjgid-ojmHZJw
> > All the ovs python idl tests pass. But it doesn't work with openstack
> > networking-ovn. The whole neutron-server process just blocks.
> >
> > I don't see any other way for python. Once select() returns we have to
> use
> > some mechanism to get the error code.
> > Does calling sock.connect_ex() bother  you ?
>
> I don't mean to change what poller.py does.  I mean to implement
> ovs.socket_util.check_connection_completion in terms of select.poll and
> select.select directly.  The comment in python/ovs/poller.py should not
> be relevant because the use of select.poll in
> check_connection_completion would never block (it would use a timeout of
> 0).
>
>
Hi Ben,

For python applications which use eventlet, select.poll/epoll is not
available. 'select ' python module
is monkey patched and poll and few other functions are deleted.

https://github.com/eventlet/eventlet/blob/master/NEWS#L155
https://github.com/eventlet/eventlet/blob/master/eventlet/green/select.py#L9
https://github.com/eventlet/eventlet/blob/master/eventlet/patcher.py#L305

Thanks
Numan


Thanks,
>
> Ben.
>
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