more lock file refactoring

Utility/LockFile/Posix.hs

@@ -6,13 +6,16 @@
  -}
 
 module Utility.LockFile.Posix (
+	LockHandle,
 	lockShared,
 	lockExclusive,
 	dropLock,
 	createLockFile,
-	LockHandle
+	openExistingLockFile,
 ) where
 
+import Utility.Exception
+
 import System.IO
 import System.Posix
 
@@ -35,11 +38,18 @@ lock lockreq mode lockfile = do
 	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.
+-- Create and opens lock file; does not lock it.
 createLockFile :: Maybe FileMode -> LockFile -> IO Fd
-createLockFile mode lockfile = do
-	l <- openFd lockfile ReadWrite mode defaultFileFlags
+createLockFile = openLockFile ReadWrite
+
+-- Opens an existing lock file; does not lock it or create it.
+openExistingLockFile :: LockFile -> IO (Maybe Fd)
+openExistingLockFile = catchMaybeIO . openLockFile ReadOnly Nothing
+
+-- Close on exec flag is set so child processes do not inherit the lock.
+openLockFile :: OpenMode -> Maybe FileMode -> LockFile -> IO Fd
+openLockFile openmode filemode lockfile = do
+	l <- openFd lockfile openmode filemode defaultFileFlags
 	setFdOption l CloseOnExec True
 	return l