update

Backend.hs

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+{- git-annex key/value storage backends
+ -
+ - git-annex uses a key/value abstraction layer to allow files contents to be
+ - stored in different ways. In theory, any key/value storage system could be
+ - used to store the file contents, and git-annex would then retrieve them
+ - as needed and put them in `.git/annex/`.
+ - 
+ - When a file is annexed, a key is generated from its content and/or metadata.
+ - This key can later be used to retrieve the file's content (its value). This
+ - key generation must be stable for a given file content, name, and size.
+ - 
+ - The mapping from filename to its key is stored in the .git-annex directory,
+ - in a file named `$filename.$backend`
+ - 
+ - Multiple pluggable backends are supported, and more than one can be used
+ - to store different files' contents in a given repository.
+ - -}
+
+module Backend where
+
+import GitRepo
+import System.Directory
+
+data Backend = Backend {
+	name :: String,				-- name of this backend
+	keyvalue :: FilePath -> Maybe String,	-- maps from key to value
+	retrievekey :: IO String -> IO (Bool)	-- retrieves value given key
+}
+
+{- Name of state file that holds the key for an annexed file,
+ - using a given backend. -}
+backendFile :: Backend -> GitRepo -> FilePath -> String
+backendFile backend repo file = gitStateDir repo ++
+	(gitRelative repo file) ++ "." ++ (name backend)
+
+{- Looks up the backend used for an already annexed file. -}
+lookupBackend :: [Backend] -> GitRepo -> FilePath -> IO (Maybe Backend)
+lookupBackend [] repo file = return Nothing
+lookupBackend (b:bs) repo file = do
+	present <- checkBackend b repo file
+	if present
+		then
+			return $ Just b
+		else
+			lookupBackend bs repo file
+
+{- Checks if a file is available via a given backend. -}
+checkBackend :: Backend -> GitRepo -> FilePath -> IO (Bool)
+checkBackend backend repo file = doesFileExist $ backendFile backend repo file
+
+{- Attempts to retrieve an annexed file from one of the backends. -}
+retrieveFile :: [Backend] -> GitRepo -> FilePath -> IO (Bool)
+retrieveFile backends repo file = do
+	result <- lookupBackend backends repo file
+	case (result) of
+		Nothing -> return False
+		Just b -> (retrievekey b) key
+			where key = readFile (backendFile b repo file)