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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 System.Directory
import Locations
import GitRepo
import Utility
type Key = String
data Backend = Backend {
-- name of this backend
name :: String,
-- converts a filename to a key
getKey :: GitRepo -> FilePath -> IO (Maybe Key),
-- stores a file's contents to a key
storeFileKey :: GitRepo -> FilePath -> Key -> IO (Bool),
-- retrieves a key's contents to a file
retrieveKeyFile :: IO Key -> FilePath -> IO (Bool)
}
instance Show Backend where
show backend = "Backend { name =\"" ++ (name backend) ++ "\" }"
{- 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)
{- Attempts to store a file in one of the backends, and returns
- its key. -}
storeFile :: [Backend] -> GitRepo -> FilePath -> IO (Maybe Key)
storeFile [] _ _ = return Nothing
storeFile (b:bs) repo file = do
try <- (getKey b) repo (gitRelative repo file)
case (try) of
Nothing -> nextbackend
Just key -> do
stored <- (storeFileKey b) repo file key
if (not stored)
then nextbackend
else do
bookkeeping key
return $ Just key
where
nextbackend = storeFile bs repo file
backendfile = backendFile b repo file
bookkeeping key = do
createDirectoryIfMissing True (parentDir backendfile)
writeFile backendfile key
{- Attempts to retrieve an file from one of the backends, saving it to
- a specified location. -}
retrieveFile :: [Backend] -> GitRepo -> FilePath -> FilePath -> IO (Bool)
retrieveFile backends repo file dest = do
result <- lookupBackend backends repo file
case (result) of
Nothing -> return False
Just b -> (retrieveKeyFile b) key dest
where
key = readFile (backendFile b repo file)
{- Looks up the backend used for an already annexed file. -}
lookupBackend :: [Backend] -> GitRepo -> FilePath -> IO (Maybe Backend)
lookupBackend [] _ _ = 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
|
