add a small cache of the most recently accessed item from the git-annex branch

This will speed up typical cases like git-annex get, which currently
has to read the location log once, then read it a second time in order to
add a line to it. Since these reads now involve more than just reading
in a file, it seemed good to add a cache layer.

Only the most recent thing needs to be cached, because git-annex has
good locality; it operates on one file at a time, and only cares
about one item from the branch per file.

Annex.hs

@@ -23,6 +23,7 @@ import GitQueue
 import Types.Backend
 import Types.Remote
 import Types.Crypto
+import Types.Branch
 import TrustLevel
 import Types.UUID
 
@@ -39,7 +40,8 @@ data AnnexState = AnnexState
 	, quiet :: Bool
 	, force :: Bool
 	, fast :: Bool
-	, updated :: Bool
+	, branchupdated :: Bool
+	, branchcache :: BranchCache
 	, forcebackend :: Maybe String
 	, forcenumcopies :: Maybe Int
 	, defaultkey :: Maybe String
@@ -60,7 +62,8 @@ newState allbackends gitrepo = AnnexState
 	, quiet = False
 	, force = False
 	, fast = False
-	, updated = False
+	, branchupdated = False
+	, branchcache = emptyBranchCache
 	, forcebackend = Nothing
 	, forcenumcopies = Nothing
 	, defaultkey = Nothing

Branch.hs

@@ -20,6 +20,7 @@ import Data.String.Utils
 import System.Cmd.Utils
 import Data.Maybe
 
+import Types.Branch
 import qualified GitRepo as Git
 import qualified GitUnionMerge
 import qualified Annex
@@ -66,11 +67,19 @@ withIndex a = do
 	liftIO $ Git.useDefaultIndex
 	return r
 
+{- There is a small cache of the most recently accessed item from the
+ - branch. git-annex has good locality, so that is enough. -}
+setCache :: FilePath -> String -> Annex ()
+setCache file content = Annex.changeState $ \s -> s { Annex.branchcache = BranchCache (Just file) content }
+
+invalidateCache :: Annex ()
+invalidateCache = Annex.changeState $ \s -> s { Annex.branchcache = emptyBranchCache }
+
 {- Ensures that the branch is up-to-date; should be called before
  - data is read from it. Runs only once per git-annex run. -}
 update :: Annex ()
 update = do
-	updated <- Annex.getState Annex.updated
+	updated <- Annex.getState Annex.branchupdated
 	unless updated $ withIndex $ do
 		g <- Annex.gitRepo
 		r <- liftIO $ Git.pipeRead g [Param "show-ref", Param name]
@@ -79,7 +88,8 @@ update = do
 		unless (null updated) $ liftIO $
 			GitUnionMerge.commit g "update" fullname
 				(fullname:updated)
-		Annex.changeState $ \s -> s { Annex.updated = True }
+		Annex.changeState $ \s -> s { Annex.branchupdated = True }
+		invalidateCache
 
 {- Ensures that a given ref has been merged into the index. -}
 updateRef :: String -> Annex (Maybe String)
@@ -108,8 +118,9 @@ change file content = do
 	g <- Annex.gitRepo
 	sha <- liftIO $ Git.hashObject g content
 	withIndex $ liftIO $ Git.run g "update-index"
-		[ Params "--add --cacheinfo 100644 ",
+		[ Param "--add", Param "--cacheinfo", Param "100644",
 		  Param sha, File file]
+	setCache file content
 
 {- Commits staged changes to the branch. -}
 commit :: String -> Annex ()
@@ -123,7 +134,9 @@ get :: FilePath -> Annex String
 get file = update >> do
 	withIndex $ do
 		g <- Annex.gitRepo
-		liftIO $ catch (cat g) (const $ return "")
+		content <- liftIO $ catch (cat g) (const $ return "")
+		setCache file content
+		return content
 	where
 		-- To avoid stderr from cat-file when file does not exist,
 		-- first run it with -e to check that it exists.

Types/Branch.hs

@@ -0,0 +1,16 @@
+{- git-annex branch data types
+ -
+ - Copyright 2011 Joey Hess <joey@kitenet.net>
+ -
+ - Licensed under the GNU GPL version 3 or higher.
+ -}
+
+module Types.Branch where
+
+data BranchCache = BranchCache {
+	cachedFile :: Maybe FilePath,
+	cachedContent :: String
+}
+
+emptyBranchCache :: BranchCache
+emptyBranchCache = BranchCache Nothing ""