separate source of content from the filename associated with the key when generating a key

This already made migrate's code a lot simpler.

Command/Migrate.hs

@@ -9,7 +9,7 @@ module Command.Migrate where
 
 import Common.Annex
 import Command
-import qualified Backend
+import Backend
 import qualified Types.Key
 import Annex.Content
 import qualified Command.ReKey
@@ -23,14 +23,14 @@ seek = [withFilesInGit $ whenAnnexed start]
 start :: FilePath -> (Key, Backend) -> CommandStart
 start file (key, oldbackend) = do
 	exists <- inAnnex key
-	newbackend <- choosebackend =<< Backend.chooseBackend file
+	newbackend <- choosebackend =<< chooseBackend file
 	if (newbackend /= oldbackend || upgradableKey key) && exists
 		then do
 			showStart "migrate" file
 			next $ perform file key newbackend
 		else stop
 	where
-		choosebackend Nothing = Prelude.head <$> Backend.orderedList
+		choosebackend Nothing = Prelude.head <$> orderedList
 		choosebackend (Just backend) = return backend
 
 {- Checks if a key is upgradable to a newer representation. -}
@@ -40,25 +40,13 @@ upgradableKey key = isNothing $ Types.Key.keySize key
 
 {- Store the old backend's key in the new backend
  - The old backend's key is not dropped from it, because there may
- - be other files still pointing at that key.
- -
- - Use the same filename as the file for the temp file name, to support
- - backends that allow the filename to influence the keys they
- - generate.
- -}
+ - be other files still pointing at that key. -}
 perform :: FilePath -> Key -> Backend -> CommandPerform
 perform file oldkey newbackend = maybe stop go =<< genkey
 	where
 		go newkey = stopUnless (Command.ReKey.linkKey oldkey newkey) $
 			next $ Command.ReKey.cleanup file oldkey newkey
 		genkey = do
-			src <- inRepo $ gitAnnexLocation oldkey
-			tmp <- fromRepo gitAnnexTmpDir
-			let tmpfile = tmp </> takeFileName file
-			cleantmp tmpfile
-			liftIO $ createLink src tmpfile
-			newkey <- liftM fst <$>
-				Backend.genKey tmpfile (Just newbackend)
-			cleantmp tmpfile
-			return newkey
-		cleantmp t = liftIO $ whenM (doesFileExist t) $ removeFile t
+			content <- inRepo $ gitAnnexLocation oldkey
+			let source = KeySource { keyFilename = file, contentLocation = content }
+			liftM fst <$> genKey source (Just newbackend)