updatecluster: support multiple gateways

Just look at the existing proxied remotes that correspond to already
existing nodes of the cluster, and keep those nodes in the cluster.
While adding any remotes of the local repo that are configured as
cluster nodes. This allows removing cluster nodes from the local repo
and updating, without it also removing nodes provided by other gateways.

Command/UpdateCluster.hs

@@ -50,23 +50,35 @@ start = startingCustomOutput (ActionItemOther Nothing) $ do
 		let mynodes = S.map (ClusterNodeUUID . R.uuid) mynodesremotes
 		let recordednodes = fromMaybe mempty $ M.lookup cu $
 			clusterUUIDs recordedclusters
-		if recordednodes == mynodes
+		proxiednodes <- findProxiedNodes recordednodes 
+		let allnodes = S.union mynodes proxiednodes
+		if recordednodes == allnodes
 			then liftIO $ putStrLn $ safeOutput $
 				"No cluster node changes for cluster: " ++ clustername
 			else do
-				describechanges descs clustername recordednodes mynodes mynodesremotes
-				recordCluster cu mynodes
+				describechanges descs clustername recordednodes allnodes mynodesremotes
+				recordCluster cu allnodes
 
 	next $ return True
   where
-	describechanges descs clustername oldnodes mynodes mynodesremotes = do
+	describechanges descs clustername oldnodes allnodes mynodesremotes = do
 		forM_ (S.toList mynodesremotes) $ \r ->
 			unless (S.member (ClusterNodeUUID (R.uuid r)) oldnodes) $
 				liftIO $ putStrLn $ safeOutput $
 					"Added node " ++ R.name r ++ " to cluster: " ++ clustername
 		forM_ (S.toList oldnodes) $ \n ->
-			unless (S.member n mynodes) $ do
+			unless (S.member n allnodes) $ do
 				let desc = maybe (fromUUID (fromClusterNodeUUID n)) fromUUIDDesc $
 					M.lookup (fromClusterNodeUUID n) descs
 				liftIO $ putStrLn $ safeOutput $
 					"Removed node " ++ desc ++ " from cluster: " ++ clustername
+
+-- Finds nodes that are proxied by other cluster gateways.
+findProxiedNodes :: S.Set ClusterNodeUUID -> Annex (S.Set ClusterNodeUUID)
+findProxiedNodes recordednodes =
+	(S.fromList . map asclusternode . filter isproxynode) <$> R.remoteList
+  where
+	isproxynode r = 
+		asclusternode r `S.member` recordednodes
+			&& remoteAnnexProxied (R.gitconfig r)
+	asclusternode = ClusterNodeUUID . R.uuid

doc/git-annex-extendcluster.mdwn

@@ -16,7 +16,7 @@ gateway to the cluster. The `gateway` parameter is the name of that remote.
 The `clustername` parameter is the name of the cluster.
 
 The next step after running this command is to configure
-any additional cluster nodes that this gateway provide to the cluster,
+any additional cluster nodes that this gateway serves to the cluster,
 then run [[git-annex-updatecluster]]. See the documentation of
 that command for details about configuring nodes.
 

doc/git-annex-updatecluster.mdwn

@@ -9,7 +9,7 @@ git-annex updatecluster
 # DESCRIPTION
 
 This command is used to record the nodes of a cluster in the git-annex
-branch. It should be run in each repository that will serve as a gateway
+branch. It should be run in the repository that will serve as a gateway
 to the cluster.
 
 It looks at the git config `remote.name.annex-cluster-node` of