Optimisations to git-annex branch query and setting, avoiding repeated copies of the environment.

Speeds up commands like  "git-annex find --in remote" by over 50%.

Profiling showed that adjustGitEnv was 21% of the time and 37% of the
allocations of that command. It copied the environment each time with
getEnvironment.

The only repeated use of adjustGitEnv is in withIndexFile, which tends to
be run at least once per file. So, it was optimised by keeping a cache of
the environment, which can be reused.

There could be other better ways to optimise this. Maybe get the while
environment once at startup. But, then it would have to be serialized back
out each time running a child process, so I doubt that would be a net win.

It might be better to cache a version of the environment that is
pre-modified to use .git-annex/index. But, profiling doesn't show that
modifying the enviroment is taking any significant time.

5 files changed, 46 insertions, 18 deletions

diff --git a/Annex.hs b/Annex.hs
index 8f46f112c..1ee6e837f 100644
--- a/Annex.hs
+++ b/Annex.hs
@@ -139,6 +139,7 @@ data AnnexState = AnnexState
 	, activeremotes :: MVar (S.Set (Types.Remote.RemoteA Annex))
 	, keysdbhandle :: Maybe Keys.DbHandle
 	, cachedcurrentbranch :: Maybe Git.Branch
+	, cachedgitenv :: Maybe [(String, String)]
 	}
 
 newState :: GitConfig -> Git.Repo -> IO AnnexState
@@ -189,6 +190,7 @@ newState c r = do
 		, activeremotes = emptyactiveremotes
 		, keysdbhandle = Nothing
 		, cachedcurrentbranch = Nothing
+		, cachedgitenv = Nothing
 		}
 
 {- Makes an Annex state object for the specified git repo.
@@ -241,10 +243,10 @@ changeState modifier = do
 	mvar <- ask
 	liftIO $ modifyMVar_ mvar $ return . modifier
 
-withState :: (AnnexState -> (AnnexState, b)) -> Annex b
+withState :: (AnnexState -> IO (AnnexState, b)) -> Annex b
 withState modifier = do
 	mvar <- ask
-	liftIO $ modifyMVar mvar $ return . modifier
+	liftIO $ modifyMVar mvar modifier
 
 {- Sets a flag to True -}
 setFlag :: String -> Annex ()
diff --git a/Annex/CatFile.hs b/Annex/CatFile.hs
index 8d6237bfe..99d301b4b 100644
--- a/Annex/CatFile.hs
+++ b/Annex/CatFile.hs
@@ -83,7 +83,7 @@ catFileHandle = do
  - nothing is using the handles, eg at shutdown. -}
 catFileStop :: Annex ()
 catFileStop = do
-	m <- Annex.withState $ \s ->
+	m <- Annex.withState $ pure . \s ->
 		(s { Annex.catfilehandles = M.empty }, Annex.catfilehandles s)
 	liftIO $ mapM_ Git.CatFile.catFileStop (M.elems m)
 
diff --git a/Annex/GitOverlay.hs b/Annex/GitOverlay.hs
index 1b7ceb2e2..d809e0b23 100644
--- a/Annex/GitOverlay.hs
+++ b/Annex/GitOverlay.hs
@@ -22,9 +22,28 @@ withIndexFile :: FilePath -> Annex a -> Annex a
 withIndexFile f a = do
 	f' <- liftIO $ indexEnvVal f
 	withAltRepo
-		(\g -> addGitEnv g indexEnv f')
+		(usecachedgitenv $ \g -> liftIO $ addGitEnv g indexEnv f')
 		(\g g' -> g' { gitEnv = gitEnv g })
 		a
+  where
+	-- This is an optimisation. Since withIndexFile is run repeatedly,
+	-- and addGitEnv uses the slow copyGitEnv when gitEnv is Nothing, 
+	-- we cache the copied environment the first time, and reuse it in
+	-- subsequent calls.
+	--
+	-- (This could be done at another level; eg when creating the
+	-- Git object in the first place, but it's more efficient to let
+	-- the enviroment be inherited in all calls to git where it
+	-- does not need to be modified.)
+	usecachedgitenv m g = case gitEnv g of
+		Just _ -> m g
+		Nothing -> do
+			e <- Annex.withState $ \s -> case Annex.cachedgitenv s of
+				Nothing -> do
+					e <- copyGitEnv
+					return (s { Annex.cachedgitenv = Just e }, e)
+				Just e -> return (s, e)
+			m (g { gitEnv = Just e })
 
 {- Runs an action using a different git work tree.
  -
@@ -52,7 +71,7 @@ withWorkTree d = withAltRepo
 withWorkTreeRelated :: FilePath -> Annex a -> Annex a
 withWorkTreeRelated d = withAltRepo modrepo unmodrepo
   where
-	modrepo g = do
+	modrepo g = liftIO $ do
 		g' <- addGitEnv g "GIT_COMMON_DIR" =<< absPath (localGitDir g)
 		g'' <- addGitEnv g' "GIT_DIR" d
 		return (g'' { gitEnvOverridesGitDir = True })
@@ -62,7 +81,7 @@ withWorkTreeRelated d = withAltRepo modrepo unmodrepo
 		}
 
 withAltRepo 
-	:: (Repo -> IO Repo)
+	:: (Repo -> Annex Repo)
 	-- ^ modify Repo
 	-> (Repo -> Repo -> Repo)
 	-- ^ undo modifications; first Repo is the original and second
@@ -71,7 +90,7 @@ withAltRepo
 	-> Annex a
 withAltRepo modrepo unmodrepo a = do
 	g <- gitRepo
-	g' <- liftIO $ modrepo g
+	g' <- modrepo g
 	q <- Annex.Queue.get
 	v <- tryNonAsync $ do
 		Annex.changeState $ \s -> s
diff --git a/CHANGELOG b/CHANGELOG
index 672072597..71f53793a 100644
--- a/CHANGELOG
+++ b/CHANGELOG
@@ -2,6 +2,9 @@ git-annex (6.20160924) UNRELEASED; urgency=medium
 
   * Optimisations to time it takes git-annex to walk working tree and find
     files to work on. Sped up by around 18%.
+  * Optimisations to git-annex branch query and setting, avoiding repeated
+    copies of the environment. Speeds up commands like 
+    "git-annex find --in remote" by over 50%.
 
  -- Joey Hess <id@joeyh.name>  Mon, 26 Sep 2016 16:46:19 -0400
 
diff --git a/Git/Env.hs b/Git/Env.hs
index f41f3ad0e..7e5a2b242 100644
--- a/Git/Env.hs
+++ b/Git/Env.hs
@@ -18,22 +18,26 @@ import Utility.Env
  - does not have any gitEnv yet. -}
 adjustGitEnv :: Repo -> ([(String, String)] -> [(String, String)]) -> IO Repo
 adjustGitEnv g adj = do
-	e <- maybe copyenv return (gitEnv g)
+	e <- maybe copyGitEnv return (gitEnv g)
 	let e' = adj e
 	return $ g { gitEnv = Just e' }
   where
-	copyenv = do
+
+{- Copies the current environment, so it can be adjusted when running a git
+ - command. -}
+copyGitEnv :: IO [(String, String)]
+copyGitEnv = do
 #ifdef __ANDROID__
-		{- This should not be necessary on Android, but there is some
-		 - weird getEnvironment breakage. See
-		 - https://github.com/neurocyte/ghc-android/issues/7
-		 - Use getEnv to get some key environment variables that
-		 - git expects to have. -}
-		let keyenv = words "USER PATH GIT_EXEC_PATH HOSTNAME HOME"
-		let getEnvPair k = maybe Nothing (\v -> Just (k, v)) <$> getEnv k
-		catMaybes <$> forM keyenv getEnvPair
+	{- This should not be necessary on Android, but there is some
+	 - weird getEnvironment breakage. See
+	 - https://github.com/neurocyte/ghc-android/issues/7
+	 - Use getEnv to get some key environment variables that
+	 - git expects to have. -}
+	let keyenv = words "USER PATH GIT_EXEC_PATH HOSTNAME HOME"
+	let getEnvPair k = maybe Nothing (\v -> Just (k, v)) <$> getEnv k
+	catMaybes <$> forM keyenv getEnvPair
 #else
-		getEnvironment
+	getEnvironment
 #endif
 
 addGitEnv :: Repo -> String -> String -> IO Repo