implement cluster.log

Not used yet. (Or tested.)

I did consider making the log start with the uuid of the node, followed
by the cluster uuid (or uuids). That would perhaps mean a smaller write
to the git-annex branch when adding a node, but overall the log file
would be larger, and it will be read and cached near to startup on most
git-annex runs.

Types/Cluster.hs

@@ -0,0 +1,29 @@
+{- git-annex cluster types
+ -
+ - Copyright 2024 Joey Hess <id@joeyh.name>
+ -
+ - Licensed under the GNU AGPL version 3 or higher.
+ -}
+
+module Types.Cluster where
+
+import Types.UUID
+
+import qualified Data.Set as S
+import qualified Data.Map as M
+
+-- The UUID of a cluster as a whole.
+newtype ClusterUUID = ClusterUUID { fromClusterUUID :: UUID }
+	deriving (Show, Eq, Ord)
+
+-- The UUID of a node in a cluster. The UUID can be either the UUID of a
+-- repository, or of another cluster. 
+newtype ClusterNodeUUID = ClusterNodeUUID { fromClusterNodeUUID :: UUID }
+	deriving (Show, Eq, Ord)
+
+-- The same information is indexed two ways to allow fast lookups in either
+-- direction.
+data Clusters = Clusters
+	{ clusterUUIDs :: M.Map ClusterUUID (S.Set ClusterNodeUUID)
+	, clusterNodeUUIDs :: M.Map ClusterNodeUUID (S.Set ClusterUUID)
+	}