tighten file2key to not produce invalid keys with no keyName

A file named "foo-" or "foo-bar" was taken as a key's file, with a backend
of "foo", and an empty keyName. This led to various problems, especially
because converting that key back to a file did not yeild the same filename.

Types/Key.hs

@@ -14,7 +14,8 @@ module Types.Key (
 	key2file,
 	file2key,
 
-	prop_idempotent_key_encode
+	prop_idempotent_key_encode,
+	prop_idempotent_key_decode
 ) where
 
 import System.Posix.Types
@@ -59,7 +60,7 @@ key2file Key { keyBackendName = b, keySize = s, keyMtime = m, keyName = n } =
 	_ ?: _ = ""
 
 file2key :: FilePath -> Maybe Key
-file2key s = if key == Just stubKey then Nothing else key
+file2key s = if key == Just stubKey || (keyName <$> key) == Just "" then Nothing else key
   where
 	key = startbackend stubKey s
 
@@ -88,3 +89,8 @@ instance Arbitrary Key where
 
 prop_idempotent_key_encode :: Key -> Bool
 prop_idempotent_key_encode k = Just k == (file2key . key2file) k
+
+prop_idempotent_key_decode :: FilePath -> Bool
+prop_idempotent_key_decode f
+	| null f = True -- skip illegal empty filename
+	| otherwise = maybe True (\k -> key2file k == f) (file2key f)