git-annex/Utility/Daemon.hs

{- daemon support
 -
 - Copyright 2012-2014 Joey Hess <joey@kitenet.net>
 -
 - Licensed under the GNU GPL version 3 or higher.
 -}

{-# LANGUAGE CPP #-}

module Utility.Daemon where

import Common
import Utility.PID
#ifndef mingw32_HOST_OS
import Utility.LogFile
#else
import Utility.WinProcess
import Utility.WinLock
#endif

#ifndef mingw32_HOST_OS
import System.Posix
import Control.Concurrent.Async
#endif

#ifndef mingw32_HOST_OS
{- Run an action as a daemon, with all output sent to a file descriptor.
 -
 - Can write its pid to a file, to guard against multiple instances
 - running and allow easy termination.
 -
 - When successful, does not return. -}
daemonize :: Fd -> Maybe FilePath -> Bool -> IO () -> IO ()
daemonize logfd pidfile changedirectory a = do
	maybe noop checkalreadyrunning pidfile
	_ <- forkProcess child1
	out
  where
	checkalreadyrunning f = maybe noop (const $ alreadyRunning) 
		=<< checkDaemon f
	child1 = do
		_ <- createSession
		_ <- forkProcess child2
		out
	child2 = do
		maybe noop lockPidFile pidfile 
		when changedirectory $
			setCurrentDirectory "/"
		nullfd <- openFd "/dev/null" ReadOnly Nothing defaultFileFlags
		redir nullfd stdInput
		redirLog logfd
		{- forkProcess masks async exceptions; unmask them inside
		 - the action. -}
		wait =<< asyncWithUnmask (\unmask -> unmask a)
		out
	out = exitImmediately ExitSuccess
#endif

{- Locks the pid file, with an exclusive, non-blocking lock,
 - and leaves it locked on return.
 -
 - Writes the pid to the file, fully atomically.
 - Fails if the pid file is already locked by another process. -}
lockPidFile :: FilePath -> IO ()
lockPidFile pidfile = do
	createDirectoryIfMissing True (parentDir pidfile)
#ifndef mingw32_HOST_OS
	fd <- openFd pidfile ReadWrite (Just stdFileMode) defaultFileFlags
	locked <- catchMaybeIO $ setLock fd (WriteLock, AbsoluteSeek, 0, 0)
	fd' <- openFd newfile ReadWrite (Just stdFileMode) defaultFileFlags
		{ trunc = True }
	locked' <- catchMaybeIO $ setLock fd' (WriteLock, AbsoluteSeek, 0, 0)
	case (locked, locked') of
		(Nothing, _) -> alreadyRunning
		(_, Nothing) -> alreadyRunning
		_ -> do
			_ <- fdWrite fd' =<< show <$> getPID
			closeFd fd
	rename newfile pidfile
  where
	newfile = pidfile ++ ".new"
#else
	{- Not atomic on Windows, oh well. -}
	unlessM (isNothing <$> checkDaemon pidfile)
		alreadyRunning
	pid <- getPID
	writeFile pidfile (show pid)
	lckfile <- winLockFile pid pidfile
	writeFile lckfile ""
	void $ lockExclusive lckfile
#endif

alreadyRunning :: IO ()
alreadyRunning = error "Daemon is already running."

{- Checks if the daemon is running, by checking that the pid file
 - is locked by the same process that is listed in the pid file.
 -
 - If it's running, returns its pid. -}
checkDaemon :: FilePath -> IO (Maybe PID)
#ifndef mingw32_HOST_OS
checkDaemon pidfile = do
	v <- catchMaybeIO $
		openFd pidfile ReadOnly (Just stdFileMode) defaultFileFlags
	case v of
		Just fd -> do
			locked <- getLock fd (ReadLock, AbsoluteSeek, 0, 0)
			p <- readish <$> readFile pidfile
			closeFd fd `after` return (check locked p)
		Nothing -> return Nothing
  where
	check Nothing _ = Nothing
	check _ Nothing = Nothing
	check (Just (pid, _)) (Just pid')
		| pid == pid' = Just pid
		| otherwise = error $
			"stale pid in " ++ pidfile ++ 
			" (got " ++ show pid' ++ 
			"; expected " ++ show pid ++ " )"
#else
checkDaemon pidfile = maybe (return Nothing) (check . readish)
	=<< catchMaybeIO (readFile pidfile)
  where
	check Nothing = return Nothing
	check (Just pid) = do
		v <- lockShared =<< winLockFile pid pidfile
		case v of
			Just h -> do
				dropLock h
				return Nothing
			Nothing -> return (Just pid)
#endif

{- Stops the daemon, safely. -}
stopDaemon :: FilePath -> IO ()
stopDaemon pidfile = go =<< checkDaemon pidfile
  where
	go Nothing = noop
	go (Just pid) =
#ifndef mingw32_HOST_OS
		signalProcess sigTERM pid
#else
		terminatePID pid
#endif

{- Windows locks a lock file that corresponds with the pid of the process.
 - This allows changing the process in the pid file and taking a new lock
 - when eg, restarting the daemon.
 -}
#ifdef mingw32_HOST_OS
winLockFile :: PID -> FilePath -> IO FilePath
winLockFile pid pidfile = do
	cleanstale
	return $ prefix ++ show pid ++ suffix
  where
  	prefix = pidfile ++ "."
	suffix = ".lck"
	cleanstale = mapM_ (void . tryIO . removeFile) =<<
		(filter iswinlockfile <$> dirContents (parentDir pidfile))
	iswinlockfile f = suffix `isSuffixOf` f && prefix `isPrefixOf` f
#endif