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/Add.hs

@@ -12,7 +12,7 @@ import Annex.Exception
 import Command
 import qualified Annex
 import qualified Annex.Queue
-import qualified Backend
+import Backend
 import Logs.Location
 import Annex.Content
 import Utility.Touch
@@ -46,8 +46,9 @@ start file = notBareRepo $ ifAnnexed file fixup add
 
 perform :: FilePath -> CommandPerform
 perform file = do
-	backend <- Backend.chooseBackend file
-	Backend.genKey file backend >>= go
+	let source = KeySource { keyFilename = file, contentLocation = file}
+	backend <- chooseBackend file
+	genKey source backend >>= go
 	where
 		go Nothing = stop
 		go (Just (key, _)) = do

Command/AddUrl.hs

@@ -11,7 +11,7 @@ import Network.URI
 
 import Common.Annex
 import Command
-import qualified Backend
+import Backend
 import qualified Command.Add
 import qualified Annex
 import qualified Backend.URL
@@ -72,8 +72,9 @@ download url file = do
 	tmp <- fromRepo $ gitAnnexTmpLocation dummykey
 	liftIO $ createDirectoryIfMissing True (parentDir tmp)
 	stopUnless (downloadUrl [url] tmp) $ do
-		backend <- Backend.chooseBackend file
-		k <- Backend.genKey tmp backend
+		backend <- chooseBackend file
+		let source = KeySource { keyFilename = file, contentLocation = file}
+		k <- genKey source backend
 		case k of
 			Nothing -> stop
 			Just (key, _) -> do

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)