The problem was observed for direct modex used with recursive doubling
algorithm (used for collective ID calculation prior to d52a2d081e9598a9ac9a50fb4b013a6d2a72375b)
that has pairwise nature and counter-connections are highly likely.
The following scenario was uncovering the issue:
* ranks `x` and `y` want to communicate with each other, `x` < `y`;
* rank `x` initiates the connection and sends the ack;
* rank `y` starts to `connect()` and gets the ack from `x`;
* `y` identifies that it already started connecting and `y` > `x` so it rejects incoming connection.
* `x` sees that his connection was rejected in `mca_oob_tcp_peer_recv_connect_ack()` when trying to
read the message header using `tcp_peer_recv_blocking()` which calls `mca_oob_tcp_peer_close()`
that effectively flushes all the messages in the peer->send_queue.
* `y` send the ack to `x` and the connection is established, however all the messages for the peer
at `x` are vanished (except the front one in peer->send_msg).
This commit introduces a "nack" function that will be used at `y` side to tell `x` that `y` has the
priority and `x`'s connection should be closed. This allows to avoid "guessing" on the unexpectedly
closed connection.
Signed-off-by: Artem Polyakov <artpol84@gmail.com>
ada93e0c02