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