improve git cat-file code

Now it reads the size specified, rather than using the sentinal hack to
determine EOF.

It still depends on error messages to handle files that are not present.

Branch.hs

@@ -17,7 +17,7 @@ module Branch (
 	name	
 ) where
 
-import Control.Monad (unless, liftM)
+import Control.Monad (when, unless, liftM)
 import Control.Monad.State (liftIO)
 import System.FilePath
 import System.Directory
@@ -26,6 +26,9 @@ import System.Cmd.Utils
 import Data.Maybe
 import Data.List
 import System.IO
+import System.IO.Unsafe
+import Foreign
+import Data.Char
 
 import Types.BranchState
 import qualified GitRepo as Git
@@ -239,32 +242,39 @@ catFile file = do
 			g <- Annex.gitRepo
 			let cmd = Git.gitCommandLine g
 				[Param "cat-file", Param "--batch"]
-			let gitcmd = join " " $ "git" : toCommand cmd
+			let gitcmd = join " " ("git" : toCommand cmd)
 			(_, from, to) <- liftIO $ hPipeBoth "sh"
-				-- want stderr on stdin for sentinal, and
-				-- to ignore other error messages
-				["-c", gitcmd ++ " 2>&1"]
+				-- want stderr on stdin to handle error messages
+				["-c", "LANG=C exec " ++ gitcmd ++ " 2>&1"]
 			setState state { catFileHandles = Just (from, to) }
 			ask (from, to)
-		ask (from, to) = do
-			_ <- liftIO $ do
-				hPutStr to $
-					fullname ++ ":" ++ file ++ "\n" ++
-					sentinal ++ "\n"
-				hFlush to
-			return . unlines =<< readContent from []
-		readContent from ls = do
-			l <- liftIO $ hGetLine from
-			if l == sentinal_line
-				-- first line is blob info,
-				-- or maybe an error message
-				then return $ drop 1 $ reverse ls
-				else readContent from (l:ls)
-		-- To find the end of a catted file, ask for a sentinal
-		-- value that is always missing, and look for the error
-		-- message. Utterly nasty, probably will break one day.
-		sentinal = ":"
-		sentinal_line = sentinal ++ " missing"
+		ask (from, to) = liftIO $ do
+			let want = fullname ++ ":" ++ file
+			hPutStrLn to want
+			hFlush to
+			header <- hGetLine from
+			if header == want ++ " missing"
+				then return ""
+				else do
+					let [_sha, _type, size] = words header
+					let bytes = read size
+					fp <- mallocForeignPtrBytes (fromIntegral bytes)
+					len <- withForeignPtr fp $ \buf -> hGetBuf from buf (fromIntegral bytes)
+					when (len /= bytes) $
+						error "short read from git cat-file"
+					content <- lazySlurp fp 0 len
+					c <- hGetChar from
+					when (c /= '\n') $
+						error "missing newline from git cat-file"
+					return content
+
+lazySlurp :: ForeignPtr Word8 -> Int -> Int -> IO String
+lazySlurp fp ix len
+	| ix == len = return []
+	| otherwise = do
+		c <- withForeignPtr fp $ \p -> peekElemOff p ix
+		cs <- unsafeInterleaveIO (lazySlurp fp (ix+1) len)
+		return $ chr (fromIntegral c) : cs
 
 {- Lists all files on the branch. There may be duplicates in the list. -}
 files :: Annex [FilePath]