add git queue to Annex monad

not used anywhere just yet..

GitQueue.hs

@@ -0,0 +1,64 @@
+{- git repository command queues
+ -}
+
+module GitQueue (
+	Queue,
+	empty,
+	add,
+	run
+) where
+
+import qualified Data.Map as M
+
+import qualified GitRepo as Git
+
+{- An action to perform in a git repository. The file to act on
+ - is not included, and must be able to be appended after the params. -}
+data Action = Action {
+		subcommand :: String,
+		params :: [String]
+	} deriving (Show, Eq, Ord)
+
+{- A queue of actions to perform (in any order) on a git repository,
+ - with lists of files to perform them on. This allows coalescing 
+ - similar git commands. -}
+type Queue = M.Map Action [FilePath]
+
+{- Constructor for empty queue. -}
+empty :: Queue
+empty = M.empty
+
+{- Adds an action to a queue. -}
+add :: Queue -> String -> [String] -> FilePath -> Queue
+add queue subcommand params file = M.insertWith (++) action [file] queue
+	where
+		action = Action subcommand params
+
+{- Runs a queue on a git repository. -}
+run :: Git.Repo -> Queue -> IO ()
+run repo queue = do
+	mapM (\(k, v) -> runAction repo k v) $ M.toList queue
+	return ()
+
+{- Runs an Action on a list of files in a git repository.
+ -
+ - Complicated by commandline length limits. -}
+runAction :: Git.Repo -> Action -> [FilePath] -> IO ()
+runAction repo action files = do
+	xargs [] 0 files
+	where
+		arg_max = 2048 -- TODO get better ARG_MAX
+		maxlen = arg_max - cmdlen
+		c = (subcommand action):(params action)
+		cmdlen = (length "git") + 
+			(foldl (\a b -> a + b + 1) 1 $ map length c)
+		xargs collect _ [] = exec collect
+		xargs collect len (f:fs) = do
+			let len' = len + 1 + length f
+			if (len' >= maxlen)
+				then do
+					exec collect
+					xargs [f] (length f) fs
+				else xargs (f:collect) len' fs
+		exec [] = return ()
+		exec fs = Git.run repo $ c ++ fs