update

Backend.hs

@@ -0,0 +1,58 @@
+{- 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)

BackendFile.hs

@@ -0,0 +1,17 @@
+{- git-annex "file" backend
+ - -}
+
+module BackendFile (backend) where
+
+import Backend
+
+backend = Backend {
+	name = "file",
+	keyvalue = keyValue,
+	retrievekey = copyFile
+}
+
+-- direct mapping from filename to key
+keyValue k = Just $ id k
+
+copyFile f = error "unimplemented"

BackendUrl.hs

@@ -0,0 +1,17 @@
+{- git-annex "url" backend
+ - -}
+
+module BackendUrl (backend) where
+
+import Backend
+
+backend = Backend {
+	name = "url",
+	keyvalue = keyValue,
+	retrievekey = downloadUrl
+}
+
+-- cannot generate url from filename
+keyValue k = Nothing
+
+downloadUrl k = error "unimplemented"

GitRepo.hs

@@ -8,14 +8,16 @@ import System.Path
 import Data.String.Utils
 import Utility
 
+type GitRepo = FilePath
+
 {- Long-term state is stored in files inside the .git-annex directory
  - in the git repository. -}
 stateLoc = ".git-annex"
-gitStateDir :: String -> String
+gitStateDir :: GitRepo -> FilePath
 gitStateDir repo = repo ++ "/" ++ stateLoc ++ "/"
 
 {- Path to a repository's gitattributes file. -}
-gitAttributes :: FilePath -> IO String
+gitAttributes :: GitRepo -> IO String
 gitAttributes repo = do
 	bare <- isBareRepo repo
 	if (bare)
@@ -25,7 +27,7 @@ gitAttributes repo = do
 {- Path to a repository's .git directory.
  - (For a bare repository, that is the root of the repository.)
  - TODO: support GIT_DIR -}
-gitDir :: FilePath -> IO String
+gitDir :: GitRepo -> IO String
 gitDir repo = do
 	bare <- isBareRepo repo
 	if (bare)
@@ -35,7 +37,7 @@ gitDir repo = do
 {- Given a relative or absolute filename, calculates the name to use
  - relative to a git repository directory (which must be absolute).
  - This is the same form displayed and used by git. -}
-gitRelative :: FilePath -> String -> String
+gitRelative :: GitRepo -> String -> String
 gitRelative repo file = drop (length absrepo) absfile
 	where
 		-- normalize both repo and file, so that repo
@@ -48,7 +50,7 @@ gitRelative repo file = drop (length absrepo) absfile
 			Nothing -> error $ file ++ " is not located inside git repository " ++ absrepo
 
 {- Sets up a git repo for git-annex. May be called repeatedly. -}
-gitPrep :: FilePath -> IO ()
+gitPrep :: GitRepo -> IO ()
 gitPrep repo = do
 	-- configure git to use union merge driver on state files
 	let attrLine = stateLoc ++ "/* merge=union"
@@ -66,7 +68,7 @@ gitPrep repo = do
 
 {- Finds the top of the current git repository, which may be in a parent
  - directory. -}
-repoTop :: IO String
+repoTop :: IO GitRepo
 repoTop = do
 	cwd <- getCurrentDirectory
 	top <- seekUp cwd isRepoTop

git-annex.hs

@@ -3,6 +3,12 @@
 
 import LocationLog
 import GitRepo
+import Backend
+
+-- When adding a new backend, import it here and add it to the backends list.
+import qualified BackendFile
+import qualified BackendUrl
+backends = [BackendFile.backend, BackendUrl.backend]
 
 main = do
 	repo <- repoTop