add key stability checking interface

Needed for resuming from chunks.

Url keys are considered not stable. I considered treating url keys with a
known size as stable, but just don't feel that is enough information.

Backend.hs

@@ -14,7 +14,8 @@ module Backend (
 	isAnnexLink,
 	chooseBackend,
 	lookupBackendName,
-	maybeLookupBackendName
+	maybeLookupBackendName,
+	checkStableKey,
 ) where
 
 import Common.Annex
@@ -124,3 +125,7 @@ maybeLookupBackendName s = M.lookup s nameMap
 
 nameMap :: M.Map String Backend
 nameMap = M.fromList $ zip (map B.name list) list
+
+checkStableKey :: Key -> Bool
+checkStableKey k = maybe False (`B.isStableKey` k) 
+	(maybeLookupBackendName (keyBackendName k))

Backend/Hash.hs

@@ -45,6 +45,7 @@ genBackend hash = Just Backend
 	, fsckKey = Just $ checkKeyChecksum hash
 	, canUpgradeKey = Just needsUpgrade
 	, fastMigrate = Just trivialMigrate
+	, isStableKey = const True
 	}
 
 genBackendE :: Hash -> Maybe Backend

Backend/URL.hs

@@ -25,6 +25,9 @@ backend = Backend
 	, fsckKey = Nothing
 	, canUpgradeKey = Nothing
 	, fastMigrate = Nothing
+	-- The content of an url can change at any time, so URL keys are
+	-- not stable.
+	, isStableKey = const False
 	}
 
 {- Every unique url has a corresponding key. -}

Backend/WORM.hs

@@ -23,6 +23,7 @@ backend = Backend
 	, fsckKey = Nothing
 	, canUpgradeKey = Nothing
 	, fastMigrate = Nothing
+	, isStableKey = const True
 	}
 
 {- The key includes the file size, modification time, and the

Types/Backend.hs

@@ -15,9 +15,16 @@ import Types.KeySource
 data BackendA a = Backend
 	{ name :: String
 	, getKey :: KeySource -> a (Maybe Key) 
+	-- Checks the content of a key.
 	, fsckKey :: Maybe (Key -> FilePath -> a Bool)
+	-- Checks if a key can be upgraded to a better form.
 	, canUpgradeKey :: Maybe (Key -> Bool)
+	-- Checks if there is a fast way to migrate a key to a different
+	-- backend (ie, without re-hashing).
 	, fastMigrate :: Maybe (Key -> BackendA a -> Maybe Key)
+	-- Checks if a key is known (or assumed) to always refer to the
+	-- same data.
+	, isStableKey :: Key -> Bool
 	}
 
 instance Show (BackendA a) where