Synnefo » snf-ganeti

  , watchFile

import Control.Concurrent

import Data.IORef

import System.INotify

-- | Until the given point in time (useconds since the epoch), wait

-- for the output of a given method to change and return the new value;

-- make use of the promise that the output only changes if the reference

-- has a value different than the given one.

watchFileEx :: (Eq a, Eq b) => Integer -> b -> IORef b -> a -> IO a -> IO a

watchFileEx endtime base ref old read_fn = do

  current <- getCurrentTimeUSec

  if current > endtime then read_fn else do

    val <- readIORef ref

    if val /= base

      then do

        new <- read_fn

        if new /= old then return new else do

          threadDelay 100000

          watchFileEx endtime val ref old read_fn

      else do 

       threadDelay 100000

       watchFileEx endtime base ref old read_fn

-- | Within the given timeout (in seconds), wait for for the output

-- of the given method to change and return the new value; make use of

-- the promise that the method will only change its value, if

-- the given file changes on disk. If the file does not exist on disk, return

-- immediately.

watchFile :: Eq a => FilePath -> Int -> a -> IO a -> IO a

watchFile fpath timeout old read_fn = do

  current <- getCurrentTimeUSec

  let endtime = current + fromIntegral timeout * 1000000

  fstat <- getFStatSafe fpath

  ref <- newIORef fstat

  inotify <- initINotify

  _ <- addWatch inotify [Modify, Delete] fpath . const $ do

    logDebug $ "Notified of change in " ++ fpath

    fstat' <- getFStatSafe fpath

    writeIORef ref fstat'

  result <- watchFileEx endtime fstat ref old read_fn

  killINotify inotify

  return result