Provide exclusive access to a resource using lock file. https://github.com/trskop/lock-file#readme

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BSD3 licensed by Peter Trsko
Maintained by peter.trsko@gmail.com

Lock File

Haskell Programming Language BSD3 License

Hackage Build


Provide exclusive access to a resource using lock file, which are files whose purpose is to signal by their presence that some resource is locked.

Usage Example

Following example acquires lock file and then waits 1000000 micro seconds before releasing it. Note also that it is possible to specify retry strategy. Here we set it to No and therefore this code won’t retry to acquire lock file after first failure.

{-# LANGUAGE TypeApplications #-}
-- |
-- Module:       Main
-- Description:  Simple example that acquires lock for a short period of time.
-- Copyright:    (c) 2013, 2014 Peter Trsko
-- License:      BSD3
-- Maintainer:   peter.trsko@gmail.com
-- Stability:    experimental
-- Portability:  portable
module Main (main)

import Control.Concurrent (threadDelay)
    -- From base package, but GHC specific.
import qualified Control.Exception as Exception (handle)

import Data.Default.Class (Default(def))
    -- From data-default-class package, alternatively it's possible to use
    -- data-default package version 0.5.2 and above.
    -- http://hackage.haskell.org/package/data-default-class
    -- http://hackage.haskell.org/package/data-default
import System.IO.LockFile
    ( LockingException
    , LockingParameters(retryToAcquireLock)
    , RetryStrategy(No)
    , withLockFile

main :: IO ()
main = handleException
    . withLockFile lockParams lockFile $ threadDelay 1000000
    lockParams = def
        { retryToAcquireLock = No

    lockFile = "/var/run/lock/my-example-lock"

    handleException = Exception.handle
        $ putStrLn . ("Locking failed with: " ++) . show @LockingException

This command line example shows that trying to execute two instances of example at the same time will result in failure of the second one.

stack ghc example/example.hs
[1 of 1] Compiling Main             ( example/example.hs, example/example.o )
Linking example/example ...
$ ./example/example & ./example/example
[1] 7893
Locking failed with: Unable to acquire lock file: "/var/run/lock/my-example-lock"
$ [1]+  Done                    ./example/example

PID File Example

Lock file, as implemented by this library, is created containing PID of the process that created it. PID file, on UNIX-like system, is a special case of lock file. It prevents the same daemon to be started up multiple times.

Thanks to the above fact we can now create function with following type signature:

withPidFile :: IO () -> IO ()

Its purpose is to wrap application main and acquire PID file prior to passing control to the application code, or fail if PID file is already acquired.

{-# LANGUAGE LambdaCase #-}
{-# LANGUAGE NoImplicitPrelude #-}
module Main

import Control.Applicative (pure)
import Control.Concurrent (threadDelay)
import Control.Exception (catch)
import Control.Monad ((>=>), (>>=))
import Data.Function ((.), ($), const)
import Data.Functor ((<$>))
import Data.Monoid ((<>))
import System.Environment (getEnv, getProgName)
import System.Exit (exitFailure)
import System.IO (IO, FilePath, hPutStrLn, putStrLn, stderr)
import Text.Show (show, showString)

import System.Posix.User (getEffectiveUserID)

import Data.Default.Class (def)
import System.IO.LockFile
    ( LockingException(CaughtIOException, UnableToAcquireLockFile)
    , LockingParameters(retryToAcquireLock)
    , RetryStrategy(No)
    , withLockFile

withPidFile :: IO () -> IO ()
withPidFile m = do
    pidFilePath <- mkPidFilePath
    withLockFile def{retryToAcquireLock = No} pidFilePath m
        `catch` (printLockingException pidFilePath >=> const exitFailure)
    mkPidFilePath :: IO FilePath
    mkPidFilePath = do
        fileName <- (<> ".pid") <$> getProgName
        getEffectiveUserID >>= \case
            0 -> pure $ "/var/run/" <> fileName
            _ -> (<> ('/' : '.' : fileName)) <$> getEnv "HOME"
                -- This may throw exception if $HOME environment varialbe is
                -- not set.

    printLockingException :: FilePath -> LockingException -> IO ()
    printLockingException filePath = hPutStrLn stderr . mkMsg . \case
        UnableToAcquireLockFile _ -> "File already exists."
        CaughtIOException       e -> show e
        mkMsg =
            showString filePath . showString ": Unable to create PID file: "

main :: IO ()
main = withPidFile $ do
    putStrLn "Hello World!"
    threadDelay 1000000

Building options

  • -fpedantic (disabled by default)

    Pass additional warning flags to GHC.


ChangeLog / ReleaseNotes


Version (unreleased)

  • Explicitly unpacking strict fields in data types. (change)
  • Correcting name of sleepBetweenRetries field of LockingParameters. (breaking change)
  • Relaxed bounds on base to include versions up to version 5. (change)
  • Bumping upper transformers bound to include 0.5.* versions. (change)
  • Bumping upper bound of directory to include 1.3.* branch. (change)
  • Bumping upper bound of data-default-class to include 0.1.* branch. (change)




Depends on 4 packages:
Used by 1 package:
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