Connection table: change logic for whether a new connection should be…

… used

When considering making a new outbound connection, the subscription
worker consults the connection table to see if there's an existing
connection to the remote ip:port combo. If there is, no new connection
will be established.

Change this test so that existing inbound connections are not
considered, i.e. we will now look to see if there's any existing
_outbound_ connection already, not any connection.

This should mean that we will try to connect outbound to a peer that is
already connected to us inbound (on the same remote ip:port combo).

ouroboros-network-framework/src/Ouroboros/Network/Server/ConnectionTable.hs

@@ -218,8 +218,8 @@ removeConnection
     -> m ()
 removeConnection tbl remoteAddr localAddr = atomically $ removeConnectionSTM tbl remoteAddr localAddr
 
--- | Try to see if it is possible to reference an existing connection rather
--- than creating a new one to the provied peer.
+-- | Try to see if it is possible to reference an existing /outbound/
+-- connection rather than creating a new one to the provied peer.
 --
 refConnectionSTM
     :: ( MonadSTM m
@@ -233,19 +233,28 @@ refConnectionSTM ConnectionTable{ctTable} remoteAddr refVar = do
     tbl <- readTVar ctTable
     case M.lookup remoteAddr tbl of
          Nothing -> return ConnectionTableCreate
-         Just cte ->
-             if S.member refVar $ cteRefs cte
-                 then return ConnectionTableDuplicate
-                 else do
-                     -- TODO We look up remoteAddr twice, is it possible
-                     -- to use M.alterF given that we need to be able to return
-                     -- ConnectionTableCreate or ConnectionTableExist?
-                     let refs' = S.insert refVar (cteRefs cte)
-                     mapM_ addValencyCounter $ S.toList refs'
+         Just cte
+             -- If all the existing connections are inbound we still
+             -- want to create an outbound connection.
+           | all (==ConnectionInbound) (cteLocalAddresses cte) ->
+               return ConnectionTableCreate
 
-                     writeTVar ctTable $ M.insert remoteAddr
-                         (cte { cteRefs = refs'}) tbl
-                     return ConnectionTableExist
+             -- Otherwise we know there's at least one outbound connection
+             -- but we distinguish the case of an exact duplicate.
+           | S.member refVar (cteRefs cte) ->
+               return ConnectionTableDuplicate
+
+           | otherwise -> do
+               -- TODO We look up remoteAddr twice, is it possible
+               -- to use M.alterF given that we need to be able to return
+               -- ConnectionTableCreate or ConnectionTableExist?
+               let refs' = S.insert refVar (cteRefs cte)
+               mapM_ addValencyCounter $ S.toList refs'
+
+               writeTVar ctTable $
+                 M.insert remoteAddr (cte { cteRefs = refs'}) tbl
+
+               return ConnectionTableExist
 
 refConnection
     :: ( MonadSTM m