content: {-| Utilities related to livelocks and death detection
  
  -}
  
  {-
  
  Copyright (C) 2014 Google Inc.
  All rights reserved.
  
  Redistribution and use in source and binary forms, with or without
  modification, are permitted provided that the following conditions are
  met:
  
  1. Redistributions of source code must retain the above copyright notice,
  this list of conditions and the following disclaimer.
  
  2. Redistributions in binary form must reproduce the above copyright
  notice, this list of conditions and the following disclaimer in the
  documentation and/or other materials provided with the distribution.
  
  THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS "AS
  IS" AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED
  TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR
  PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL THE COPYRIGHT HOLDER OR
  CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL,
  EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT LIMITED TO,
  PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE, DATA, OR
  PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY THEORY OF
  LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT (INCLUDING
  NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE OF THIS
  SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
  
  -}
  
  module Ganeti.Utils.Livelock
    ( Livelock
    , mkLivelockFile
    , listLiveLocks
    , isDead
    ) where
  
  import Control.Exception (bracket)
  import Control.Monad
  import Control.Monad.Except
  import System.Directory (doesFileExist, getDirectoryContents)
  import System.FilePath.Posix ((</>))
  import System.IO
  import qualified System.Posix.IO as Posix
  import System.Posix.Types (Fd)
  import System.Time (ClockTime(..), getClockTime)
  
  import Ganeti.BasicTypes
  import Ganeti.Logging
  import Ganeti.Compat (openFd, closeFd)
  import Ganeti.Path (livelockFile, livelockDir)
  import Ganeti.Utils (lockFile)
  
  type Livelock = FilePath
  
  -- | Appends the current time to the given prefix, creates
  -- the lockfile in the appropriate directory, and locks it.
  -- Returns its full path and the file's file descriptor.
  mkLivelockFile :: (Error e, MonadError e m, MonadIO m)
                 => FilePath -> m (Fd, Livelock)
  mkLivelockFile prefix = do
    (TOD secs _) <- liftIO getClockTime
    lockfile <- liftIO . livelockFile $ prefix ++ "_" ++ show secs
    fd <- liftIO (lockFile lockfile) >>= \r -> case r of
            Bad msg   -> failError $ "Locking the livelock file " ++ lockfile
                                     ++ ": " ++ msg
            Ok fd     -> return fd
    return (fd, lockfile)
  
  -- | List currently existing livelocks. Underapproximate if
  -- some error occurs.
  listLiveLocks :: IO [FilePath]
  listLiveLocks =
    fmap (genericResult (const [] :: IOError -> [FilePath]) id)
    . runResultT . liftIO $ do
      dir <- livelockDir
      entries <- getDirectoryContents dir
      filterM doesFileExist $ map (dir </>) entries
  
  -- | Detect whether a the process identified by the given path
  -- does not exist any more. This function never fails and only
  -- returns True if it has positive knowledge that the process
  -- does not exist any more (i.e., if it managed successfully
  -- obtain a shared lock on the file).
  isDead :: Livelock -> IO Bool
  isDead fpath = fmap (isOk :: Result () -> Bool) . runResultT . liftIO $ do
    filepresent <- doesFileExist fpath
    when filepresent
      . bracket
            (openFd fpath Posix.ReadOnly Nothing Posix.defaultFileFlags)
            closeFd
                  $ \fd -> do
                      logDebug $ "Attempting to get a lock of " ++ fpath
                      Posix.setLock fd (Posix.ReadLock, AbsoluteSeek, 0, 0)
                      logDebug "Got the lock, the process is dead"
  