remove unused backend machinery

The only remaining vestiage of backends is different types of keys. These
are still called "backends", mostly to avoid needing to change user interface
and configuration. But everything to do with storing keys in different
backends was gone; instead different types of remotes are used.

In the refactoring, lots of code was moved out of odd corners like
Backend.File, to closer to where it's used, like Command.Drop and
Command.Fsck. Quite a lot of dead code was removed. Several data structures
became simpler, which may result in better runtime efficiency. There should
be no user-visible changes.

Command/Migrate.hs

@@ -15,6 +15,7 @@ import System.FilePath
 import Command
 import qualified Annex
 import qualified Backend
+import qualified Types.Key
 import Locations
 import Types
 import Content
@@ -32,18 +33,20 @@ start :: CommandStartBackendFile
 start (file, b) = isAnnexed file $ \(key, oldbackend) -> do
 	exists <- inAnnex key
 	newbackend <- choosebackend b
-	upgradable <- Backend.upgradableKey oldbackend key
-	if (newbackend /= oldbackend || upgradable) && exists
+	if (newbackend /= oldbackend || upgradableKey key) && exists
 		then do
 			showStart "migrate" file
 			next $ perform file key newbackend
 		else stop
 	where
-		choosebackend Nothing = do
-			backends <- Backend.list
-			return $ head backends
+		choosebackend Nothing = return . head =<< Backend.orderedList
 		choosebackend (Just backend) = return backend
 
+{- Checks if a key is upgradable to a newer representation. -}
+{- Ideally, all keys have file size metadata. Old keys may not. -}
+upgradableKey :: Key -> Bool
+upgradableKey key = Types.Key.keySize key == Nothing
+
 perform :: FilePath -> Key -> Backend Annex -> CommandPerform
 perform file oldkey newbackend = do
 	g <- Annex.gitRepo
@@ -55,9 +58,9 @@ perform file oldkey newbackend = do
 	let src = gitAnnexLocation g oldkey
 	let tmpfile = gitAnnexTmpDir g </> takeFileName file
 	liftIO $ createLink src tmpfile
-	stored <- Backend.storeFileKey tmpfile $ Just newbackend
+	k <- Backend.genKey tmpfile $ Just newbackend
 	liftIO $ cleantmp tmpfile
-	case stored of
+	case k of
 		Nothing -> stop
 		Just (newkey, _) -> do
 			ok <- getViaTmpUnchecked newkey $ \t -> do