first module

LocationLog.hs

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+{- git-annex location log
+ -
+ - git-annex keeps track of on which repository it last saw a file's content.
+ - This can be useful when using it for archiving with offline storage.
+ - When you indicate you --want a file, git-annex will tell you which
+ - repositories have the file's content.
+ -
+ - Location tracking information is stored in `.git-annex/$filename.log`.
+ - Repositories record their name and the date when they --get or --drop
+ - a file's content. (Git is configured to use a union merge for this file,
+ - so the lines may be in arbitrary order, but it will never conflict.)
+ -
+ - A line of the log will look like: "date reponame filename"
+ -
+ -}
+
+module LocationLog where
+
+import Data.DateTime
+import System.IO
+import System.Posix.IO
+
+data LogLine = LogLine {
+	date :: DateTime,
+	repo :: String,
+	file :: String
+} deriving (Eq)
+
+-- a special value representing a log file line that could not be parsed
+unparsable = (LogLine (fromSeconds 0) "" "")
+
+instance Show LogLine where
+	show (LogLine date repo file) = unwords
+		[(show (toSeconds date)), repo, file]
+
+instance Read LogLine where
+	-- this parser is robust in that even unparsable log lines are
+	-- read without an exception being thrown
+	readsPrec _ string = if (length w >= 3)
+				then [((LogLine time repo file), "")]
+				else [(unparsable, "")]
+		where
+			time = fromSeconds $ read $ w !! 0
+			repo = w !! 1
+			file = unwords $ rest w
+			w = words string
+			rest (_:_:l) = l
+
+{- Reads a log file -}
+readLog :: String -> IO [LogLine]
+readLog file = do
+	h <- openLocked file ReadMode
+	s <- hGetContents h
+	-- hClose handle' -- TODO disabled due to lazy IO issue
+	-- filter out any unparsable lines
+	return $ filter ( /= unparsable ) $ map read $ lines s
+
+{- Adds a LogLine to a log file -}
+writeLog :: String -> LogLine -> IO ()
+writeLog file line = do
+	h <- openLocked file AppendMode
+	hPutStrLn h $ show line
+	hClose h
+
+{- Let's just say that Haskell makes reading/writing a file with
+ - file locking excessively difficult. -}
+openLocked file mode = do
+	handle <- openFile file mode
+	lockfd <- handleToFd handle -- closes handle
+	waitToSetLock lockfd (lockType mode, AbsoluteSeek, 0, 0)
+	handle' <- fdToHandle lockfd
+	return handle'
+		where
+			lockType ReadMode = ReadLock
+			lockType _ = WriteLock
+
+{- Generates a new log line with the current date. -}
+logNow :: String -> String -> IO LogLine
+logNow repo file = do
+	now <- getCurrentTime
+	return $ LogLine now repo file