69 lines
2.2 KiB
Haskell
69 lines
2.2 KiB
Haskell
{- Windows lock files
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-
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- Copyright 2014 Joey Hess <joey@kitenet.net>
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-
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- License: BSD-2-clause
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-}
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module Utility.WinLock (
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lockShared,
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lockExclusive,
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dropLock,
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waitToLock,
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LockHandle
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) where
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import System.Win32.Types
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import System.Win32.File
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import Control.Concurrent
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{- Locking is exclusive, and prevents the file from being opened for read
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- or write by any other process. So for advisory locking of a file, a
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- different LockFile should be used. -}
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type LockFile = FilePath
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type LockHandle = HANDLE
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{- Tries to lock a file with a shared lock, which allows other processes to
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- also lock it shared. Fails is the file is exclusively locked. -}
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lockShared :: LockFile -> IO (Maybe LockHandle)
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lockShared = openLock fILE_SHARE_READ
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{- Tries to take an exclusive lock on a file. Fails if another process has
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- a shared or exclusive lock. -}
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lockExclusive :: LockFile -> IO (Maybe LockHandle)
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lockExclusive = openLock fILE_SHARE_NONE
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{- Windows considers just opening a file enough to lock it. This will
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- create the LockFile if it does not already exist.
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-
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- Will fail if the file is already open with an incompatable ShareMode.
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- Note that this may happen if an unrelated process, such as a virus
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- scanner, even looks at the file. See http://support.microsoft.com/kb/316609
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-
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- Note that createFile busy-waits to try to avoid failing when some other
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- process briefly has a file open. But that would make checking locks
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- much more expensive, so is not done here. Thus, the use of c_CreateFile.
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-}
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openLock :: ShareMode -> LockFile -> IO (Maybe LockHandle)
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openLock sharemode f = do
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h <- withTString f $ \c_f ->
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c_CreateFile c_f gENERIC_READ sharemode (maybePtr Nothing)
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oPEN_ALWAYS fILE_ATTRIBUTE_NORMAL (maybePtr Nothing)
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return $ if h == iNVALID_HANDLE_VALUE
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then Nothing
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else Just h
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dropLock :: LockHandle -> IO ()
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dropLock = closeHandle
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{- If the initial lock fails, this is a BUSY wait, and does not
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- guarentee FIFO order of waiters. In other news, Windows is a POS. -}
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waitToLock :: IO (Maybe LockHandle) -> IO LockHandle
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waitToLock locker = takelock
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where
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takelock = go =<< locker
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go (Just lck) = return lck
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go Nothing = do
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threadDelay (500000) -- half a second
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takelock
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