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diff --git a/Annex/UUID.hs b/Annex/UUID.hs
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+{- git-annex uuids
+ -
+ - Each git repository used by git-annex has an annex.uuid setting that
+ - uniquely identifies that repository.
+ -
+ - UUIDs of remotes are cached in git config, using keys named
+ - remote.<name>.annex-uuid
+ -
+ - Copyright 2010-2011 Joey Hess <joey@kitenet.net>
+ -
+ - Licensed under the GNU GPL version 3 or higher.
+ -}
+
+module Annex.UUID (
+	getUUID,
+	getRepoUUID,
+	getUncachedUUID,
+	prepUUID,
+	genUUID
+) where
+
+import Common.Annex
+import qualified Git
+import qualified Build.SysConfig as SysConfig
+import Config
+
+configkey :: String
+configkey = "annex.uuid"
+
+{- Generates a UUID. There is a library for this, but it's not packaged,
+ - so use the command line tool. -}
+genUUID :: IO UUID
+genUUID = pOpen ReadFromPipe command params hGetLine
+	where
+		command = SysConfig.uuid
+		params = if command == "uuid"
+			-- request a random uuid be generated
+			then ["-m"]
+			-- uuidgen generates random uuid by default
+			else []
+
+getUUID :: Annex UUID
+getUUID = getRepoUUID =<< gitRepo
+
+{- Looks up a repo's UUID. May return "" if none is known. -}
+getRepoUUID :: Git.Repo -> Annex UUID
+getRepoUUID r = do
+	g <- gitRepo
+
+	let c = cached g
+	let u = getUncachedUUID r
+	
+	if c /= u && u /= ""
+		then do
+			updatecache g u
+			return u
+		else return c
+	where
+		cached g = Git.configGet g cachekey ""
+		updatecache g u = when (g /= r) $ setConfig cachekey u
+		cachekey = "remote." ++ fromMaybe "" (Git.repoRemoteName r) ++ ".annex-uuid"
+
+getUncachedUUID :: Git.Repo -> UUID
+getUncachedUUID r = Git.configGet r configkey ""
+
+{- Make sure that the repo has an annex.uuid setting. -}
+prepUUID :: Annex ()
+prepUUID = whenM (null <$> getUUID) $
+	setConfig configkey =<< liftIO genUUID