refactor catfile code

split into generic IO code, and a thin Annex wrapper

Git/CatFile.hs

@@ -0,0 +1,63 @@
+{- git cat-file interface
+ -
+ - Copyright 2011 Joey Hess <joey@kitenet.net>
+ -
+ - Licensed under the GNU GPL version 3 or higher.
+ -}
+
+module Git.CatFile (
+	CatFileHandle,
+	catFileStart,
+	catFileStop,
+	catFile
+) where
+
+import Control.Monad.State
+import System.Cmd.Utils
+import System.IO
+import qualified Data.ByteString.Char8 as B
+
+import Git
+import Utility.SafeCommand
+
+type CatFileHandle = (PipeHandle, Handle, Handle)
+
+{- Starts git cat-file running in batch mode in a repo and returns a handle. -}
+catFileStart :: Repo -> IO CatFileHandle
+catFileStart repo = hPipeBoth "git" $ toCommand $
+	Git.gitCommandLine repo [Param "cat-file", Param "--batch"]
+
+{- Stops git cat-file. -}
+catFileStop :: CatFileHandle -> IO ()
+catFileStop (pid, from, to) = do
+	hClose to
+	hClose from
+	forceSuccess pid
+
+{- Uses a running git cat-file read the content of a file from a branch.
+ - Files that do not exist on the branch will have "" returned. -}
+catFile :: CatFileHandle -> String -> FilePath -> IO String
+catFile (_, from, to) branch file = do
+	hPutStrLn to want
+	hFlush to
+	header <- hGetLine from
+	case words header of
+		[sha, blob, size]
+			| length sha == Git.shaSize &&
+			  blob == "blob" -> handle size
+			| otherwise -> empty
+		_
+			| header == want ++ " missing" -> empty
+			| otherwise -> error $ "unknown response from git cat-file " ++ header
+	where
+		want = branch ++ ":" ++ file
+		handle size = case reads size of
+			[(bytes, "")] -> readcontent bytes
+			_ -> empty
+		readcontent bytes = do
+			content <- B.hGet from bytes
+			c <- hGetChar from
+			when (c /= '\n') $
+				error "missing newline from git cat-file"
+			return $ B.unpack content
+		empty = return ""