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|
{- Posix lock files
-
- Copyright 2014 Joey Hess <id@joeyh.name>
-
- License: BSD-2-clause
-}
module Utility.LockFile.Posix (
LockHandle,
lockShared,
lockExclusive,
tryLockExclusive,
createLockFile,
openExistingLockFile,
isLocked,
checkLocked,
getLockStatus,
dropLock,
checkSaneLock,
) where
import Utility.Exception
import Utility.Applicative
import System.IO
import System.Posix
import Data.Maybe
type LockFile = FilePath
newtype LockHandle = LockHandle Fd
-- Takes a shared lock, blocking until the lock is available.
lockShared :: Maybe FileMode -> LockFile -> IO LockHandle
lockShared = lock ReadLock
-- Takes an exclusive lock, blocking until the lock is available.
lockExclusive :: Maybe FileMode -> LockFile -> IO LockHandle
lockExclusive = lock WriteLock
-- Tries to take an exclusive lock, but does not block.
tryLockExclusive :: Maybe FileMode -> LockFile -> IO (Maybe LockHandle)
tryLockExclusive mode lockfile = do
l <- openLockFile mode lockfile
v <- tryIO $ setLock l (WriteLock, AbsoluteSeek, 0, 0)
case v of
Left _ -> do
closeFd l
return Nothing
Right _ -> return $ Just $ LockHandle l
-- Setting the FileMode allows creation of a new lock file.
-- If it's Nothing then this only succeeds when the lock file already exists.
lock :: LockRequest -> Maybe FileMode -> LockFile -> IO LockHandle
lock lockreq mode lockfile = do
l <- openLockFile mode lockfile
waitToSetLock l (lockreq, AbsoluteSeek, 0, 0)
return (LockHandle l)
-- Create and opens lock file; does not lock it.
createLockFile :: FileMode -> LockFile -> IO Fd
createLockFile filemode = openLockFile (Just filemode)
-- Opens an existing lock file; does not lock it, and if it does not exist,
-- returns Nothing.
openExistingLockFile :: LockFile -> IO (Maybe Fd)
openExistingLockFile = catchMaybeIO . openLockFile Nothing
-- Close on exec flag is set so child processes do not inherit the lock.
openLockFile :: Maybe FileMode -> LockFile -> IO Fd
openLockFile filemode lockfile = do
l <- openFd lockfile ReadWrite filemode defaultFileFlags
setFdOption l CloseOnExec True
return l
-- Check if a file is locked, either exclusively, or with shared lock.
-- When the file doesn't exist, it's considered not locked.
isLocked :: LockFile -> IO Bool
isLocked = fromMaybe False <$$> checkLocked
-- Returns Nothing when the file doesn't exist, for cases where
-- that is different from it not being locked.
checkLocked :: LockFile -> IO (Maybe Bool)
checkLocked = maybe Nothing (Just . isJust) <$$> getLockStatus'
getLockStatus :: LockFile -> IO (Maybe (ProcessID, FileLock))
getLockStatus = fromMaybe Nothing <$$> getLockStatus'
getLockStatus' :: LockFile -> IO (Maybe (Maybe (ProcessID, FileLock)))
getLockStatus' lockfile = go =<< catchMaybeIO open
where
open = openFd lockfile ReadOnly Nothing defaultFileFlags
go Nothing = return Nothing
go (Just h) = do
ret <- getLock h (ReadLock, AbsoluteSeek, 0, 0)
closeFd h
return (Just ret)
dropLock :: LockHandle -> IO ()
dropLock (LockHandle fd) = closeFd fd
-- Checks that the lock file still exists, and is the same file that was
-- locked to get the LockHandle.
--
-- This check is useful if the lock file might get deleted by something
-- else.
checkSaneLock :: LockFile -> LockHandle -> IO Bool
checkSaneLock lockfile (LockHandle fd) =
go =<< catchMaybeIO (getFileStatus lockfile)
where
go Nothing = return False
go (Just st) = do
fdst <- getFdStatus fd
return $ deviceID fdst == deviceID st && fileID fdst == fileID st
|
