git-annex/Backend.hs

{- 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.
 - 
 - Multiple pluggable backends are supported, and more than one can be used
 - to store different files' contents in a given repository.
 -
 - Copyright 2010 Joey Hess <joey@kitenet.net>
 -
 - Licensed under the GNU GPL version 3 or higher.
 -}

module Backend (
	list,
	storeFileKey,
	retrieveKeyFile,
	removeKey,
	hasKey,
	fsckKey,
	upgradableKey,
	lookupFile,
	chooseBackends,
	keyBackend,
	lookupBackendName,
	maybeLookupBackendName
) where

import Control.Monad.State (liftIO, when)
import System.IO.Error (try)
import System.FilePath
import System.Posix.Files
import System.Directory

import Locations
import qualified GitRepo as Git
import qualified Annex
import Types
import Key
import qualified BackendClass as B
import Messages
import Content
import DataUnits

{- List of backends in the order to try them when storing a new key. -}
list :: Annex [Backend Annex]
list = do
	l <- Annex.getState Annex.backends -- list is cached here
	if not $ null l
		then return l
		else do
			s <- getstandard
			d <- Annex.getState Annex.defaultbackend
			handle d s
	where
		parseBackendList l [] = l
		parseBackendList bs s = map (lookupBackendName bs) $ words s
		handle Nothing s = return s
		handle (Just "") s = return s
		handle (Just name) s = do
			bs <- Annex.getState Annex.supportedBackends
			let l' = (lookupBackendName bs name):s
			Annex.changeState $ \state -> state { Annex.backends = l' }
			return l'
		getstandard = do
			bs <- Annex.getState Annex.supportedBackends
			g <- Annex.gitRepo
			return $ parseBackendList bs $
				Git.configGet g "annex.backends" ""

{- Looks up a backend in a list. May fail if unknown. -}
lookupBackendName :: [Backend Annex] -> String -> Backend Annex
lookupBackendName bs s = maybe unknown id $ maybeLookupBackendName bs s
	where
		unknown = error $ "unknown backend " ++ s
maybeLookupBackendName :: [Backend Annex] -> String -> Maybe (Backend Annex)
maybeLookupBackendName bs s =
	if 1 /= length matches
		then Nothing
		else Just $ head matches
	where matches = filter (\b -> s == B.name b) bs

{- Attempts to store a file in one of the backends. -}
storeFileKey :: FilePath -> Maybe (Backend Annex) -> Annex (Maybe (Key, Backend Annex))
storeFileKey file trybackend = do
	bs <- list
	let bs' = maybe bs (:bs) trybackend
	storeFileKey' bs' file
storeFileKey' :: [Backend Annex] -> FilePath -> Annex (Maybe (Key, Backend Annex))
storeFileKey' [] _ = return Nothing
storeFileKey' (b:bs) file = maybe nextbackend store =<< (B.getKey b) file
	where
		nextbackend = storeFileKey' bs file
		store key = do
			stored <- (B.storeFileKey b) file key
			if (not stored)
				then nextbackend
				else return $ Just (key, b)

{- Attempts to retrieve an key from one of the backends, saving it to
 - a specified location. -}
retrieveKeyFile :: Backend Annex -> Key -> FilePath -> Annex Bool
retrieveKeyFile backend key dest = (B.retrieveKeyFile backend) key dest

{- Removes a key from a backend. -}
removeKey :: Backend Annex -> Key -> Maybe Int -> Annex Bool
removeKey backend key numcopies = (B.removeKey backend) key numcopies

{- Checks if a key is present in its backend. -}
hasKey :: Key -> Annex Bool
hasKey key = do
	backend <- keyBackend key
	(B.hasKey backend) key

{- Checks a key for problems. -}
fsckKey :: Backend Annex -> Key -> Maybe FilePath -> Maybe Int -> Annex Bool
fsckKey backend key file numcopies = do
	size_ok <- checkKeySize key
	backend_ok <-(B.fsckKey backend) key file numcopies
	return $ size_ok && backend_ok

{- Checks if a key is upgradable to a newer representation. -}
upgradableKey :: Backend Annex -> Key -> Annex Bool
upgradableKey backend key = (B.upgradableKey backend) key

{- Looks up the key and backend corresponding to an annexed file,
 - by examining what the file symlinks to. -}
lookupFile :: FilePath -> Annex (Maybe (Key, Backend Annex))
lookupFile file = do
	bs <- Annex.getState Annex.supportedBackends
	tl <- liftIO $ try getsymlink
	case tl of
		Left _ -> return Nothing
		Right l -> makekey bs l
	where
		getsymlink = do
			l <- readSymbolicLink file
			return $ takeFileName l
		makekey bs l = maybe (return Nothing) (makeret bs l) (fileKey l)
		makeret bs l k =
			case maybeLookupBackendName bs bname of
					Just backend -> return $ Just (k, backend)
					Nothing -> do
						when (isLinkToAnnex l) $
							warning skip
						return Nothing
			where
				bname = keyBackendName k
				skip = "skipping " ++ file ++ 
					" (unknown backend " ++ bname ++ ")"

{- Looks up the backends that should be used for each file in a list.
 - That can be configured on a per-file basis in the gitattributes file.
 -}
chooseBackends :: [FilePath] -> Annex [(FilePath, Maybe (Backend Annex))]
chooseBackends fs = do
	g <- Annex.gitRepo
	bs <- Annex.getState Annex.supportedBackends
	pairs <- liftIO $ Git.checkAttr g "annex.backend" fs
	return $ map (\(f,b) -> (f, maybeLookupBackendName bs b)) pairs

{- Returns the backend to use for a key. -}
keyBackend :: Key -> Annex (Backend Annex)
keyBackend key = do
	bs <- Annex.getState Annex.supportedBackends
	return $ lookupBackendName bs $ keyBackendName key

{- The size of the data for a key is checked against the size encoded in
 - the key's metadata, if available. -}
checkKeySize :: Key -> Annex Bool
checkKeySize key = do
	g <- Annex.gitRepo
	let file = gitAnnexLocation g key
	present <- liftIO $ doesFileExist file
	case (present, keySize key) of
		(_, Nothing) -> return True
		(False, _) -> return True
		(True, Just size) -> do
			stat <- liftIO $ getFileStatus file
			let size' = fromIntegral (fileSize stat)
			if size == size'
				then return True
				else do
					dest <- moveBad key
					warning $ "Bad file size (" ++
						compareSizes storageUnits True size size' ++ 
						"); moved to " ++ dest
					return False