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{- git-annex "WORM" backend -- Write Once, Read Many
- -}
module Backend.WORM (backend) where
import Control.Monad.State
import System.FilePath
import System.Posix.Files
import Data.Digest.Pure.SHA -- slow, but we only checksum filenames
import qualified Data.ByteString.Lazy.Char8 as B
import qualified Backend.File
import BackendTypes
import Utility
backend = Backend.File.backend {
name = "WORM",
getKey = keyValue
}
-- A SHA1 of the basename of the filename, plus the file size and
-- modification time, is used as the unique part of the key. That
-- allows multiple files with the same names to have different keys,
-- while also allowing a file to be moved around while retaining the
-- same key.
--
-- The file size and modification time are also included in the key,
-- unhashed. This could be used as a sanity check.
--
-- The basename of the filename is also included in the key, so it's clear
-- what the original filename was when a user sees the value.
keyValue :: FilePath -> Annex (Maybe Key)
keyValue file = do
stat <- liftIO $ getFileStatus file
return $ Just $ Key ((name backend), key stat)
where
key stat = (checksum $ uniqueid stat) ++ sep ++
uniqueid stat ++ sep ++ base
checksum s = show $ sha1 $ B.pack s
uniqueid stat = (show $ modificationTime stat) ++ sep ++
(show $ fileSize stat)
base = takeFileName file
sep = ":"
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