lock pools to work around non-concurrency/composition safety of POSIX fcntl

Utility/LockPool/Windows.hs

@@ -0,0 +1,49 @@
+{- Windows lock files, using lock pools.
+ -
+ - Copyright 2015 Joey Hess <id@joeyh.name>
+ -
+ - License: BSD-2-clause
+ -}
+
+module Utility.LockPool.Windows (
+	LockHandle,
+	lockShared,
+	lockExclusive,
+	dropLock,
+	waitToLock,
+) where
+
+import qualified Utility.LockFile.Windows as F
+import qualified Utility.LockPool.STM as P
+import Utility.LockPool.LockHandle
+import Utility.LockPool.STM (LockPool, LockFile, LockMode(..))
+import Utility.Monad
+
+import Control.Concurrent.STM
+import System.IO
+import Data.Maybe
+import Control.Applicative
+import Prelude
+
+{- Tries to lock a file with a shared lock, which allows other processes to
+ - also lock it shared. Fails if the file is exclusively locked. -}
+lockShared :: LockFile -> IO (Maybe LockHandle)
+lockShared file = tryMakeLockHandle
+	(P.tryTakeLock P.lockPool file LockShared)
+	(F.lockShared mode file)
+
+{- Tries to take an exclusive lock on a file. Fails if another process has
+ - a shared or exclusive lock.
+ -
+ - Note that exclusive locking also prevents the file from being opened for
+ - read or write by any other process. So for advisory locking of a file's
+ - content, a separate LockFile should be used. -}
+lockExclusive :: LockFile -> IO (Maybe LockHandle)
+lockExclusive file = tryMakeLockHandle
+	(P.tryTakeLock P.lockPool file LockExclusive)
+	(F.lockExclusive file)
+
+{- If the initial lock fails, this is a BUSY wait, and does not
+ - guarentee FIFO order of waiters. In other news, Windows is a POS. -}
+waitToLock :: IO (Maybe LockHandle) -> IO LockHandle
+waitToLock = F.waitToLock