improved git-annex branch changing

All changes to files in the branch are now made via pure functions that
transform the old file into the new. This will allow adding locking
to prevent read/write races. It also makes the code nicer, and purer.

I noticed a behavior change, really a sort of bug fix. Before,
'git annex untrust foo --trust bar' would change both trust levels
permanantly, now the --trust doesn't get stored.

1 files changed, 13 insertions, 3 deletions

diff --git a/Branch.hs b/Branch.hs
index e4caeece7..9340259c7 100644
--- a/Branch.hs
+++ b/Branch.hs
@@ -213,9 +213,19 @@ updateRef ref
 				liftIO $ Git.UnionMerge.merge g [ref]
 				return $ Just ref
 
-{- Records changed content of a file into the journal. -}
-change :: FilePath -> String -> Annex ()
-change file content = do
+{- Applies a function to modifiy the content of a file. -}
+change :: FilePath -> (String -> String) -> Annex ()
+change file a = do
+	lock
+	get file >>= return . a >>= set file
+	unlock
+	where
+		lock = return ()
+		unlock = return ()
+
+{- Records new content of a file into the journal. -}
+set :: FilePath -> String -> Annex ()
+set file content = do
 	setJournalFile file content
 	setCache file content