261 lines
8.1 KiB
Haskell
261 lines
8.1 KiB
Haskell
{- pid-based lock files
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-
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- Copyright 2015 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.PidLock (
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LockHandle,
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tryLock,
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waitLock,
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dropLock,
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LockStatus(..),
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getLockStatus,
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checkLocked,
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checkSaneLock,
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) where
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import Utility.PartialPrelude
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import Utility.Exception
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import Utility.Applicative
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import Utility.Directory
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import Utility.Monad
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import Utility.Path
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import Utility.FileMode
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import Utility.LockFile.LockStatus
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import Utility.ThreadScheduler
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import Utility.Hash
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import Utility.FileSystemEncoding
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import qualified Utility.LockFile.Posix as Posix
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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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import Data.List
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import Network.BSD
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import System.FilePath
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import Control.Applicative
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import Prelude
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type LockFile = FilePath
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data LockHandle = LockHandle LockFile FileStatus SideLockHandle
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type SideLockHandle = Maybe (LockFile, Posix.LockHandle)
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data PidLock = PidLock
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{ lockingPid :: ProcessID
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, lockingHost :: HostName
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}
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deriving (Eq, Read, Show)
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mkPidLock :: IO PidLock
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mkPidLock = PidLock
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<$> getProcessID
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<*> getHostName
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readPidLock :: LockFile -> IO (Maybe PidLock)
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readPidLock lockfile = (readish =<<) <$> catchMaybeIO (readFile lockfile)
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-- To avoid races when taking over a stale pid lock, a side lock is used.
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-- This is a regular posix exclusive lock.
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trySideLock :: LockFile -> (SideLockHandle -> IO a) -> IO a
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trySideLock lockfile a = do
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sidelock <- sideLockFile lockfile
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mlck <- catchDefaultIO Nothing $
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withUmask nullFileMode $
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Posix.tryLockExclusive (Just mode) sidelock
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-- Check the lock we just took, in case we opened a side lock file
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-- belonging to another process that will have since deleted it.
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case mlck of
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Nothing -> a Nothing
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Just lck -> ifM (Posix.checkSaneLock sidelock lck)
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( a (Just (sidelock, lck))
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, a Nothing
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)
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where
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-- Let all users write to the lock file in /dev/shm or /tmp,
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-- so that other users can reuse it to take the lock.
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-- Since /dev/shm and /tmp are sticky dirs, a user cannot
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-- delete another user's lock file there, so could not
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-- delete a stale lock.
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mode = combineModes (readModes ++ writeModes)
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dropSideLock :: SideLockHandle -> IO ()
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dropSideLock Nothing = return ()
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dropSideLock (Just (f, h)) = do
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-- Delete the file first, to ensure that any process that is trying
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-- to take the side lock will only succeed once the file is
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-- deleted, and so will be able to immediately see that it's taken
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-- a stale lock.
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_ <- tryIO $ removeFile f
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Posix.dropLock h
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-- The side lock is put in /dev/shm. This will work on most any
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-- Linux system, even if its whole root filesystem doesn't support posix
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-- locks. /tmp is used as a fallback.
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sideLockFile :: LockFile -> IO LockFile
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sideLockFile lockfile = do
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f <- absPath lockfile
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let base = intercalate "_" (splitDirectories (makeRelative "/" f))
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let shortbase = reverse $ take 32 $ reverse base
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let md5sum = if base == shortbase
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then ""
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else show (md5 (encodeBS base))
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dir <- ifM (doesDirectoryExist "/dev/shm")
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( return "/dev/shm"
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, return "/tmp"
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)
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return $ dir </> md5sum ++ shortbase ++ ".lck"
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-- | Tries to take a lock; does not block when the lock is already held.
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--
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-- Note that stale locks are automatically detected and broken.
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-- However, if the lock file is on a networked file system, and was
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-- created on a different host than the current host (determined by hostname),
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-- this can't be done and stale locks may persist.
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tryLock :: LockFile -> IO (Maybe LockHandle)
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tryLock lockfile = trySideLock lockfile $ \sidelock -> do
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lockfile' <- absPath lockfile
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(tmp, h) <- openTempFile (takeDirectory lockfile') "locktmp"
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setFileMode tmp (combineModes readModes)
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hPutStr h . show =<< mkPidLock
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hClose h
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let failedlock st = do
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dropLock $ LockHandle tmp st sidelock
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return Nothing
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let tooklock st = return $ Just $ LockHandle lockfile' st sidelock
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ifM (linkToLock sidelock tmp lockfile')
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( do
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nukeFile tmp
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-- May not have made a hard link, so stat
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-- the lockfile
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lckst <- getFileStatus lockfile'
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tooklock lckst
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, do
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v <- readPidLock lockfile'
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hn <- getHostName
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tmpst <- getFileStatus tmp
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case v of
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Just pl | isJust sidelock && hn == lockingHost pl -> do
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-- Since we have the sidelock,
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-- and are on the same host that
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-- the pidlock was taken on,
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-- we know that the pidlock is
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-- stale, and can take it over.
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rename tmp lockfile'
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tooklock tmpst
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_ -> failedlock tmpst
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)
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-- Linux's open(2) man page recommends linking a pid lock into place,
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-- as the most portable atomic operation that will fail if
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-- it already exists.
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--
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-- open(2) suggests that link can sometimes appear to fail
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-- on NFS but have actually succeeded, and the way to find out is to stat
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-- the file and check its link count etc.
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--
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-- However, not all filesystems support hard links. So, first probe
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-- to see if they are supported. If not, use open with O_EXCL.
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linkToLock :: SideLockHandle -> FilePath -> FilePath -> IO Bool
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linkToLock Nothing _ _ = return False
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linkToLock (Just _) src dest = do
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let probe = src ++ ".lnk"
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v <- tryIO $ createLink src probe
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nukeFile probe
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case v of
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Right _ -> do
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_ <- tryIO $ createLink src dest
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ifM (catchBoolIO checklinked)
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( catchBoolIO $ not <$> checkInsaneLustre dest
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, return False
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)
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Left _ -> catchBoolIO $ do
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fd <- openFd dest WriteOnly
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(Just $ combineModes readModes)
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(defaultFileFlags {exclusive = True})
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h <- fdToHandle fd
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readFile src >>= hPutStr h
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hClose h
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return True
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where
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checklinked = do
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x <- getSymbolicLinkStatus src
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y <- getSymbolicLinkStatus dest
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return $ and
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[ deviceID x == deviceID y
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, fileID x == fileID y
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, fileMode x == fileMode y
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, fileOwner x == fileOwner y
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, fileGroup x == fileGroup y
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, fileSize x == fileSize y
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, modificationTime x == modificationTime y
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, isRegularFile x == isRegularFile y
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, linkCount x == linkCount y
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, linkCount x == 2
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]
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-- On a Lustre filesystem, link has been observed to incorrectly *succeed*,
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-- despite the dest already existing. A subsequent stat of the dest
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-- looked like it had been replaced with the src. The process proceeded to
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-- run and then deleted the dest, and after the process was done, the
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-- original file was observed to still be in place.
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--
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-- We can detect this insanity by getting the directory contents after
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-- making the link, and checking to see if 2 copies of the dest file,
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-- with the SAME FILENAME exist.
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checkInsaneLustre :: FilePath -> IO Bool
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checkInsaneLustre dest = do
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fs <- dirContents (takeDirectory dest)
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case length (filter (== dest) fs) of
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1 -> return False -- whew!
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0 -> return True -- wtf?
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_ -> do
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-- Try to clean up the extra copy we made
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-- that has the same name. Egads.
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_ <- tryIO $ removeFile dest
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return True
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-- | Waits as necessary to take a lock.
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--
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-- Uses a 1 second wait-loop.
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--
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-- May wait until timeout if the lock file is stale and is on a network file
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-- system, or on a system where the side lock cannot be taken.
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waitLock :: Seconds -> LockFile -> IO LockHandle
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waitLock (Seconds timeout) lockfile = go timeout
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where
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go n
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| n > 0 = maybe (threadDelaySeconds (Seconds 1) >> go (pred n)) return
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=<< tryLock lockfile
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| otherwise = do
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hPutStrLn stderr $ show timeout ++ " second timeout exceeded while waiting for pid lock file " ++ lockfile
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giveup $ "Gave up waiting for possibly stale pid lock file " ++ lockfile
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dropLock :: LockHandle -> IO ()
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dropLock (LockHandle lockfile _ sidelock) = do
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-- Drop side lock first, at which point the pid lock will be
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-- considered stale.
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dropSideLock sidelock
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nukeFile lockfile
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getLockStatus :: LockFile -> IO LockStatus
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getLockStatus = maybe StatusUnLocked (StatusLockedBy . lockingPid) <$$> readPidLock
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checkLocked :: LockFile -> IO (Maybe Bool)
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checkLocked lockfile = conv <$> getLockStatus lockfile
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where
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conv (StatusLockedBy _) = Just True
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conv _ = Just False
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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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checkSaneLock :: LockFile -> LockHandle -> IO Bool
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checkSaneLock lockfile (LockHandle _ st _) =
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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') = return $
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deviceID st == deviceID st' && fileID st == fileID st'
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