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

Utility/LockPool/LockHandle.hs

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+{- Handles for lock pools.
+ -
+ - Copyright 2015 Joey Hess <id@joeyh.name>
+ -
+ - License: BSD-2-clause
+ -}
+
+{-# LANGUAGE CPP #-}
+
+module Utility.LockPool.LockHandle where
+
+import qualified Utility.LockPool.STM as P
+#ifndef mingw32_HOST_OS
+import qualified Utility.LockFile.Posix as F
+#else
+import qualified Utility.LockFile.Windows as F
+#endif
+
+import Control.Concurrent.STM
+import Control.Exception
+
+data LockHandle = LockHandle P.LockHandle F.LockHandle
+
+dropLock :: LockHandle -> IO ()
+dropLock (LockHandle ph fh) = P.releaseLock ph (F.dropLock fh)
+
+-- Take a lock, by first updating the lock pool, and then taking the file
+-- lock. If taking the file lock fails for any reason, take care to
+-- release the lock in the lock pool.
+makeLockHandle :: STM P.LockHandle -> IO F.LockHandle -> IO LockHandle
+makeLockHandle pa fa = bracketOnError setup cleanup go
+  where
+	setup = atomically pa
+	cleanup ph = P.releaseLock ph (return ())
+	go ph = do
+		fh <- fa
+		return $ LockHandle ph fh
+
+tryMakeLockHandle :: STM (Maybe P.LockHandle) -> IO (Maybe F.LockHandle) -> IO (Maybe LockHandle)
+tryMakeLockHandle pa fa = bracketOnError setup cleanup go
+  where
+	setup = atomically pa
+	cleanup Nothing = return ()
+	cleanup (Just ph) = P.releaseLock ph (return ())
+	go Nothing = return Nothing
+	go (Just ph) = do
+		mfh <- fa
+		case mfh of
+			Nothing -> do
+				cleanup (Just ph)
+				return Nothing
+			Just fh -> return $ Just $ LockHandle ph fh