convert fsckKey to a Maybe

This way it's clear when a backend does not implement its own fsck checks.

Backend/SHA.hs

@@ -32,7 +32,7 @@ genBackend size
 		b = Backend
 			{ name = shaName size
 			, getKey = keyValue size
-			, fsckKey = checkKeyChecksum size
+			, fsckKey = Just $ checkKeyChecksum size
 			}
 
 genBackendE :: SHASize -> Maybe Backend

Backend/URL.hs

@@ -21,7 +21,7 @@ backend :: Backend
 backend = Backend {
 	name = "URL",
 	getKey = const (return Nothing),
-	fsckKey = const (return True)
+	fsckKey = Nothing
 }
 
 fromUrl :: String -> Key

Backend/WORM.hs

@@ -18,7 +18,7 @@ backend :: Backend
 backend = Backend {
 	name = "WORM",
 	getKey = keyValue,
-	fsckKey = const (return True)
+	fsckKey = Nothing
 }
 
 {- The key includes the file size, modification time, and the

Command/Fsck.hs

@@ -137,7 +137,9 @@ checkKeySize key = do
 
 
 checkBackend :: Backend -> Key -> Annex Bool
-checkBackend = Types.Backend.fsckKey
+checkBackend backend key = case Types.Backend.fsckKey backend of
+	Nothing -> return True
+	Just a -> a key
 
 checkKeyNumCopies :: Key -> FilePath -> Maybe Int -> Annex Bool
 checkKeyNumCopies key file numcopies = do

Types/Backend.hs

@@ -16,8 +16,8 @@ data BackendA a = Backend {
 	name :: String,
 	-- converts a filename to a key
 	getKey :: FilePath -> a (Maybe Key),
-	-- called during fsck to check a key
-	fsckKey :: Key -> a Bool
+	-- called during fsck to check a key, if the backend has its own checks
+	fsckKey :: Maybe (Key -> a Bool)
 }
 
 instance Show (BackendA a) where