module System.IO.SafeWrite ( withOutputFile , syncFile , allocateTempFile , finalizeTempFile ) where import System.FilePath (takeDirectory, takeBaseName) import System.Posix.IO (openFd, defaultFileFlags, closeFd, OpenMode(..)) import System.Posix.Unistd (fileSynchronise) import Control.Exception (bracket, bracketOnError) import System.IO (Handle, hClose, openTempFile) import System.Directory (renameFile, removeFile) -- | Sync a file to disk -- -- Only supported on Posix (patches with Windows support are welcome) syncFile :: FilePath -- ^ File to sync -> IO () syncFile fname = do bracket (openFd fname ReadWrite Nothing defaultFileFlags) closeFd fileSynchronise -- The code below will not work on Windows bracket (openFd (takeDirectory fname) ReadOnly Nothing defaultFileFlags) closeFd fileSynchronise -- | Variation of 'withFile' for output files. -- -- Output is written to a temporary file. Once the action has completed, this -- file is then sync'ed to disk (see |syncFile|) and renamed to its final -- destination. In Posix, this is an atomic operation. If an exception is -- raised, then the temporary output file will be deleted and not saved to -- disk. Thus, the result file will either contain the complete result or will -- be empty. withOutputFile :: FilePath -- ^ Final desired file path -> (Handle -> IO a) -- ^ action to execute -> IO a withOutputFile finalname act = bracketOnError (allocateTempFile finalname) (finalizeTempFile finalname False) (\tdata@(_, th) -> do r <- act th finalizeTempFile finalname True tdata return r) allocateTempFile :: FilePath -> IO (FilePath, Handle) allocateTempFile finalname = openTempFile (takeDirectory finalname) (takeBaseName finalname) finalizeTempFile :: FilePath -> Bool -> (FilePath, Handle) -> IO () finalizeTempFile finalname ok (tname, th) | ok = do hClose th syncFile tname renameFile tname finalname | otherwise = do hClose th removeFile tname