[ovs-dev] [PATCH 1/2] ovs python: ovs.stream.open_block() returns success even if the remote is unreachable

[Ben Pfaff]

On Wed, Jul 11, 2018 at 12:37:28PM +0530, Numan Siddique wrote:
> On Wed, Jul 11, 2018 at 2:08 AM Ben Pfaff <blp at ovn.org> wrote:
> 
> > On Wed, Jul 11, 2018 at 12:56:39AM +0530, nusiddiq at redhat.com wrote:
> > > From: Numan Siddique <nusiddiq at 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 <nusiddiq at 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).

Thanks,

Ben.