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