started on git-annex-shell cluster support

Works down to P2P protocol.

The question now is, how to handle protocol version negotiation for
clusters? Connecting to each node to find their protocol versions and
using the lowest would be too expensive with a lot of nodes. So it seems
that the cluster needs to pick its own protocol version to use with the
client.

Then it can either negotiate that same version with the nodes when
it comes time to use them, or it can translate between multiple protocol
versions. That seems complicated. Thinking it would be ok to refuse to
use a node if it is not able to negotiate the same protocol version with
it as with the client. That will mean that sometimes need nodes to be
upgraded when upgrading the cluster's proxy. But protocol versions
rarely change.

CmdLine/GitAnnexShell.hs

@@ -21,6 +21,7 @@ import Remote.GCrypt (getGCryptUUID)
 import P2P.Protocol (ServerMode(..))
 import Git.Types
 import Logs.Proxy
+import Logs.Cluster
 import Logs.UUID
 import Remote
 
@@ -193,21 +194,33 @@ checkProxy remoteuuid ouruuid = M.lookup ouruuid <$> getProxies >>= \case
 	Just proxies ->
 		case filter (\p -> proxyRemoteUUID p == remoteuuid) (S.toList proxies) of
 			[] -> notconfigured
-			ps -> do
-				-- This repository may have multiple
-				-- remotes that access the same repository.
-				-- Proxy for the lowest cost one that
-				-- is configured to be used as a proxy.
-				rs <- concat . byCost <$> remoteList
-				let sameuuid r = uuid r == remoteuuid
-				let samename r p = name r == proxyRemoteName p
-				case headMaybe (filter (\r -> sameuuid r && any (samename r) ps) rs) of
-					Nothing -> notconfigured
-					Just r -> do
-						Annex.changeState $ \st ->
-							st { Annex.proxyremote = Just r }
-						return True
+			ps -> case mkClusterUUID remoteuuid of
+				Just cu -> proxyforcluster cu
+				Nothing -> proxyfor ps
   where
+	-- This repository may have multiple remotes that access the same
+	-- repository. Proxy for the lowest cost one that is configured to
+	-- be used as a proxy.
+	proxyfor ps = do
+		rs <- concat . byCost <$> remoteList
+		let sameuuid r = uuid r == remoteuuid
+		let samename r p = name r == proxyRemoteName p
+		case headMaybe (filter (\r -> sameuuid r && any (samename r) ps) rs) of
+			Nothing -> notconfigured
+			Just r -> do
+				Annex.changeState $ \st ->
+					st { Annex.proxyremote = Just (Right r) }
+				return True
+
+	proxyforcluster cu = do
+		clusters <- getClusters
+		if M.member cu (clusterUUIDs clusters)
+			then do
+				Annex.changeState $ \st ->
+					st { Annex.proxyremote = Just (Left cu) }
+				return True
+			else notconfigured
+
 	notconfigured = M.lookup remoteuuid <$> uuidDescMap >>= \case
-		Just desc -> giveup $ "not configured to proxy for repository " ++ (fromUUIDDesc desc)
+		Just desc -> giveup $ "not configured to proxy for repository " ++ fromUUIDDesc desc
 		Nothing -> return False