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.

7 files changed, 34 insertions, 30 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
 
diff --git a/LocationLog.hs b/LocationLog.hs
index 7e5e81d7a..0cdf88bc6 100644
--- a/LocationLog.hs
+++ b/LocationLog.hs
@@ -15,7 +15,6 @@ module LocationLog (
 	LogStatus(..),
 	logChange,
 	readLog,
-	writeLog,
 	keyLocations,
 	loggedKeys,
 	logFile,
diff --git a/PresenceLog.hs b/PresenceLog.hs
index 7742651b8..2db1ee59b 100644
--- a/PresenceLog.hs
+++ b/PresenceLog.hs
@@ -16,7 +16,6 @@ module PresenceLog (
 	addLog,
 	readLog,
 	parseLog,
-	writeLog,
 	logNow,
 	compactLog,
 	currentLog,
@@ -75,9 +74,8 @@ instance Read LogLine where
 			ret v = [(v, "")]
 
 addLog :: FilePath -> LogLine -> Annex ()
-addLog file line = do
-	ls <- readLog file
-	writeLog file (compactLog $ line:ls)
+addLog file line = Branch.change file $ \s -> 
+	showLog $ compactLog (line : parseLog s)
 
 {- Reads a log file.
  - Note that the LogLines returned may be in any order. -}
@@ -90,9 +88,9 @@ parseLog = filter parsable . map read . lines
 		-- some lines may be unparseable, avoid them
 		parsable l = status l /= Undefined
 
-{- Stores a set of lines in a log file -}
-writeLog :: FilePath -> [LogLine] -> Annex ()
-writeLog file ls = Branch.change file (unlines $ map show ls)
+{- Generates a log file. -}
+showLog :: [LogLine] -> String
+showLog = unlines . map show
 
 {- Generates a new LogLine with the current date. -}
 logNow :: LogStatus -> String -> Annex LogLine
diff --git a/RemoteLog.hs b/RemoteLog.hs
index 620c0d875..f9c7997e4 100644
--- a/RemoteLog.hs
+++ b/RemoteLog.hs
@@ -32,11 +32,10 @@ remoteLog = "remote.log"
 
 {- Adds or updates a remote's config in the log. -}
 configSet :: UUID -> RemoteConfig -> Annex ()
-configSet u c = do
-	m <- readRemoteLog
-	Branch.change remoteLog $ unlines $ sort $
-		map toline $ M.toList $ M.insert u c m
+configSet u c = Branch.change remoteLog $
+	serialize . M.insert u c . remoteLogParse
 	where
+		serialize = unlines . sort . map toline . M.toList
 		toline (u', c') = u' ++ " " ++ unwords (configToKeyVal c')
 
 {- Map of remotes by uuid containing key/value config maps. -}
diff --git a/Trust.hs b/Trust.hs
index 232eea6a5..0c8836c85 100644
--- a/Trust.hs
+++ b/Trust.hs
@@ -64,11 +64,9 @@ trustSet :: UUID -> TrustLevel -> Annex ()
 trustSet uuid level = do
 	when (null uuid) $
 		error "unknown UUID; cannot modify trust level"
-        m <- trustMap
-	when (M.lookup uuid m /= Just level) $ do
-		let m' = M.insert uuid level m
-		Branch.change trustLog (serialize m')
-		Annex.changeState $ \s -> s { Annex.trustmap = Just m' }
+	Branch.change trustLog $
+		serialize . M.insert uuid level . M.fromList . trustMapParse
+	Annex.changeState $ \s -> s { Annex.trustmap = Nothing }
         where
                 serialize m = unlines $ map showpair $ M.toList m
 		showpair (u, t) = u ++ " " ++ show t
diff --git a/UUID.hs b/UUID.hs
index b1ccbb250..eab6bd4df 100644
--- a/UUID.hs
+++ b/UUID.hs
@@ -23,6 +23,7 @@ module UUID (
 ) where
 
 import Control.Monad.State
+import Control.Applicative
 import System.Cmd.Utils
 import System.IO
 import qualified Data.Map as M
@@ -87,18 +88,17 @@ prepUUID = do
 
 {- Records a description for a uuid in the uuidLog. -}
 describeUUID :: UUID -> String -> Annex ()
-describeUUID uuid desc = do
-	m <- uuidMap
-	let m' = M.insert uuid desc m
-	Branch.change uuidLog (serialize m')
+describeUUID uuid desc = Branch.change uuidLog $
+	serialize . M.insert uuid desc . parse
 	where
 		serialize m = unlines $ map (\(u, d) -> u++" "++d) $ M.toList m
 
-{- Read and parse the uuidLog into a Map -}
+{- Read the uuidLog into a Map -}
 uuidMap :: Annex (M.Map UUID String)
-uuidMap = do
-	s <- Branch.get uuidLog
-	return $ M.fromList $ map pair $ lines s
+uuidMap = parse <$> Branch.get uuidLog
+
+parse :: String -> M.Map UUID String
+parse = M.fromList . map pair . lines
 	where
 		pair l
 			| null ws = ("", "")
diff --git a/Upgrade/V2.hs b/Upgrade/V2.hs
index e99a7cf81..4e686288f 100644
--- a/Upgrade/V2.hs
+++ b/Upgrade/V2.hs
@@ -87,8 +87,8 @@ inject :: FilePath -> FilePath -> Annex ()
 inject source dest = do
 	g <- Annex.gitRepo
 	new <- liftIO (readFile $ olddir g </> source)
-	prev <- Branch.get dest
-	Branch.change dest $ unlines $ nub $ lines prev ++ lines new
+	Branch.change dest $ \prev -> 
+		unlines $ nub $ lines prev ++ lines new
 	showProgress
 
 logFiles :: FilePath -> Annex [FilePath]