reorganize and refactor lock code

Added a convenience Utility.LockFile that is not a windows/posix
portability shim, but still manages to cut down on the boilerplate around
locking.

This commit was sponsored by Johan Herland.

Utility/LockFile/Posix.hs

@@ -0,0 +1,47 @@
+{- Posix lock files
+ -
+ - Copyright 2014 Joey Hess <joey@kitenet.net>
+ -
+ - License: BSD-2-clause
+ -}
+
+module Utility.LockFile.Posix (
+	lockShared,
+	lockExclusive,
+	dropLock,
+	createLockFile,
+	LockHandle
+) where
+
+import System.IO
+import System.Posix
+
+type LockFile = FilePath
+
+newtype LockHandle = LockHandle Fd
+
+-- Takes a shared lock, blocking until the lock is available.
+lockShared :: Maybe FileMode -> LockFile -> IO LockHandle
+lockShared = lock ReadLock
+
+-- Takes an exclusive lock, blocking until the lock is available.
+lockExclusive :: Maybe FileMode -> LockFile -> IO LockHandle
+lockExclusive = lock WriteLock
+
+-- The FileMode is used when creating a new lock file.
+lock :: LockRequest -> Maybe FileMode -> LockFile -> IO LockHandle
+lock lockreq mode lockfile = do
+	l <- createLockFile mode lockfile
+	waitToSetLock l (lockreq, AbsoluteSeek, 0, 0)
+	return (LockHandle l)
+
+-- Create and opens lock file, does not lock it.
+-- Close on exec flag is set so child processes do not inherit the lock.
+createLockFile :: Maybe FileMode -> LockFile -> IO Fd
+createLockFile mode lockfile = do
+	l <- openFd lockfile ReadWrite mode defaultFileFlags
+	setFdOption l CloseOnExec True
+	return l
+
+dropLock :: LockHandle -> IO ()
+dropLock (LockHandle fd) = closeFd fd