add ContentSource type, for remotes that act on files rather than ByteStrings

Note that currently nothing cleans up a ContentSource's file, when eg,
retrieving chunks.

Types/StoreRetrieve.hs

@@ -0,0 +1,54 @@
+{- Types for Storer and Retriever
+ -
+ - Copyright 2014 Joey Hess <joey@kitenet.net>
+ -
+ - Licensed under the GNU GPL version 3 or higher.
+ -}
+
+{-# LANGUAGE Rank2Types #-}
+
+module Types.StoreRetrieve where
+
+import Common.Annex
+import Utility.Metered
+import Utility.Tmp
+
+import qualified Data.ByteString.Lazy as L
+
+-- Prepares for and then runs an action that will act on a Key's
+-- content, passing it a helper when the preparation is successful.
+type Preparer helper = forall a. Key -> (Maybe helper -> Annex a) -> Annex a
+
+-- A source of a Key's content.
+data ContentSource
+	= FileContent FilePath
+	| ByteContent L.ByteString
+
+-- Action that stores a Key's content on a remote.
+-- Can throw exceptions.
+type Storer = Key -> ContentSource -> MeterUpdate -> IO Bool
+
+-- Action that retrieves a Key's content from a remote.
+-- Throws exception if key is not present, or remote is not accessible.
+type Retriever = Key -> IO ContentSource
+
+fileStorer :: (Key -> FilePath -> MeterUpdate -> IO Bool) -> Storer
+fileStorer a k (FileContent f) m = a k f m
+fileStorer a k (ByteContent b) m = do
+	 withTmpFile "tmpXXXXXX" $ \f h -> do
+		L.hPut h b
+		hClose h
+		a k f m
+
+byteStorer :: (Key -> L.ByteString -> MeterUpdate -> IO Bool) -> Storer
+byteStorer a k c m = withBytes c $ \b -> a k b m
+
+withBytes :: ContentSource -> (L.ByteString -> IO a) -> IO a
+withBytes (ByteContent b) a = a b
+withBytes (FileContent f) a = a =<< L.readFile f
+
+fileRetriever :: (Key -> IO FilePath) -> Retriever
+fileRetriever a k = FileContent <$> a k
+
+byteRetriever :: (Key -> IO L.ByteString) -> Retriever
+byteRetriever a k = ByteContent <$> a k