ctdb-tcp: Avoid orphaning the TCP incoming queue

ctdb-tcp: Avoid orphaning the TCP incoming queue

CTDB's incoming queue handling does not check whether an existing
queue exists, so can overwrite the pointer to the queue.  This used to
be harmless until commit c68b6f96f26664459187ab2fbd56767fb31767e0
changed the read callback to use a parent structure as the callback
data.  Instead of cleaning up an orphaned queue on disconnect, as
before, this will now free the new queue.

At first glance it doesn't seem possible that 2 incoming connections
from the same node could be processed before the intervening
disconnect.  However, the incoming connections and disconnect occur on
different file descriptors.  The queue can become orphaned on node A
when the following sequence occurs:

1. Node A comes up
2. Node A accepts an incoming connection from node B
3. Node B processes a timeout before noticing that outgoing the queue is writable
4. Node B tears down the outgoing connection to node A
5. Node B initiates a new connection to node A
6. Node A accepts an incoming connection from node B

Node A processes then the disconnect of the old incoming connection
from (2) but tears down the new incoming connection from (6).  This
then occurs until the originally affected node is restarted.

However, due to the number of outgoing connection attempts and
associated teardowns, this induces the same behaviour on the
corresponding incoming queue on all nodes that node A attempts to
connect to.  Therefore, other nodes become affected and need to be
restarted too.

As a result, the whole cluster probably needs to be restarted to
recover from this situation.

The problem can occur any time CTDB is started on a node.

The fix is to avoid accepting new incoming connections when a queue
for incoming connections is already present.  The connecting node will
simply retry establishing its outgoing connection.

BUG: https://bugzilla.samba.org/show_bug.cgi?id=14175

Signed-off-by: Martin Schwenke <martin@meltin.net>
Reviewed-by: Amitay Isaacs <amitay@gmail.com>
(cherry picked from commit d0baad257e511280ff3e5c7372c38c43df841070)