Avoid re-checksumming when migrating from hash to hashE backend. Closes: #774494

Backend/Hash.hs

@@ -1,4 +1,4 @@
-{- git-annex hashing backends
+{- git-tnnex hashing backends
  -
  - Copyright 2011-2013 Joey Hess <joey@kitenet.net>
  -
@@ -132,13 +132,20 @@ needsUpgrade :: Key -> Bool
 needsUpgrade key = "\\" `isPrefixOf` keyHash key ||
 	any (not . validExtension) (takeExtensions $ keyName key)
 
-{- Fast migration from hashE to hash backend. (Optimisation) -}
-trivialMigrate :: Key -> Backend -> Maybe Key
-trivialMigrate oldkey newbackend
+trivialMigrate :: Key -> Backend -> AssociatedFile -> Maybe Key
+trivialMigrate oldkey newbackend afile
+	{- Fast migration from hashE to hash backend. -}
 	| keyBackendName oldkey == name newbackend ++ "E" = Just $ oldkey
 		{ keyName = keyHash oldkey
 		, keyBackendName = name newbackend
 		}
+	{- Fast migration from hash to hashE backend. -}
+	| keyBackendName oldkey ++"E" == name newbackend = case afile of
+		Nothing -> Nothing
+		Just file -> Just $ oldkey
+			{ keyName = keyHash oldkey ++ selectExtension file
+			, keyBackendName = name newbackend
+			}
 	| otherwise = Nothing
 
 hashFile :: Hash -> FilePath -> Integer -> Annex String

Command/Migrate.hs

@@ -72,7 +72,7 @@ perform file oldkey oldbackend newbackend = go =<< genkey
 	checkcontent = Command.Fsck.checkBackend oldbackend oldkey $ Just file
 	finish newkey = stopUnless (Command.ReKey.linkKey oldkey newkey) $
 		next $ Command.ReKey.cleanup file oldkey newkey
-	genkey = case maybe Nothing (\fm -> fm oldkey newbackend) (fastMigrate oldbackend) of
+	genkey = case maybe Nothing (\fm -> fm oldkey newbackend (Just file)) (fastMigrate oldbackend) of
 		Just newkey -> return $ Just (newkey, True)
 		Nothing -> do
 			content <- calcRepo $ gitAnnexLocation oldkey

Types/Backend.hs

@@ -21,7 +21,7 @@ data BackendA a = Backend
 	, canUpgradeKey :: Maybe (Key -> Bool)
 	-- Checks if there is a fast way to migrate a key to a different
 	-- backend (ie, without re-hashing).
-	, fastMigrate :: Maybe (Key -> BackendA a -> Maybe Key)
+	, fastMigrate :: Maybe (Key -> BackendA a -> AssociatedFile -> Maybe Key)
 	-- Checks if a key is known (or assumed) to always refer to the
 	-- same data.
 	, isStableKey :: Key -> Bool

debian/changelog

@@ -3,6 +3,8 @@ git-annex (5.20141232) UNRELEASED; urgency=medium
   * unlock: Don't allow unlocking files that have never been committed to git
     before, to avoid an intractable problem that prevents the pre-commit
     hook from telling if such a file is intended to be an annexed file or not.
+  « Avoid re-checksumming when migrating from hash to hashE backend.
+    Closes: #774494
 
  -- Joey Hess <id@joeyh.name>  Fri, 02 Jan 2015 13:35:13 -0400