add tryLockShared

Utility/LockFile/Posix.hs

@@ -9,6 +9,7 @@ module Utility.LockFile.Posix (
 	LockHandle,
 	lockShared,
 	lockExclusive,
+	tryLockShared,
 	tryLockExclusive,
 	checkLocked,
 	getLockStatus,
@@ -36,16 +37,13 @@ lockShared = lock ReadLock
 lockExclusive :: Maybe FileMode -> LockFile -> IO LockHandle
 lockExclusive = lock WriteLock
 
+-- Tries to take a shared lock, but does not block.
+tryLockShared :: Maybe FileMode -> LockFile -> IO (Maybe LockHandle)
+tryLockShared = tryLock ReadLock
+
 -- Tries to take an exclusive lock, but does not block.
 tryLockExclusive :: Maybe FileMode -> LockFile -> IO (Maybe LockHandle)
-tryLockExclusive mode lockfile = do
+tryLockExclusive = tryLock WriteLock
-	l <- openLockFile WriteLock mode lockfile
-	v <- tryIO $ setLock l (WriteLock, AbsoluteSeek, 0, 0)
-	case v of
-		Left _ -> do
-			closeFd l
-			return Nothing
-		Right _ -> return $ Just $ LockHandle l
 
 -- Setting the FileMode allows creation of a new lock file.
 -- If it's Nothing then this only succeeds when the lock file already exists.
@@ -55,6 +53,17 @@ lock lockreq mode lockfile = do
 	waitToSetLock l (lockreq, AbsoluteSeek, 0, 0)
 	return (LockHandle l)
 
+-- Tries to take an lock, but does not block.
+tryLock :: LockRequest -> Maybe FileMode -> LockFile -> IO (Maybe LockHandle)
+tryLock lockreq mode lockfile = do
+	l <- openLockFile lockreq mode lockfile
+	v <- tryIO $ setLock l (lockreq, AbsoluteSeek, 0, 0)
+	case v of
+		Left _ -> do
+			closeFd l
+			return Nothing
+		Right _ -> return $ Just $ LockHandle l
+
 -- Close on exec flag is set so child processes do not inherit the lock.
 openLockFile :: LockRequest -> Maybe FileMode -> LockFile -> IO Fd
 openLockFile lockreq filemode lockfile = do

Utility/LockPool/Posix.hs

@@ -9,6 +9,7 @@ module Utility.LockPool.Posix (
 	LockHandle,
 	lockShared,
 	lockExclusive,
+	tryLockShared,
 	tryLockExclusive,
 	checkLocked,
 	getLockStatus,
@@ -35,11 +36,19 @@ lockShared mode file = makeLockHandle
 	(P.waitTakeLock P.lockPool file LockShared)
 	(F.lockShared mode file)
 
+-- Takes an exclusive lock, blocking until the lock is available.
 lockExclusive :: Maybe FileMode -> LockFile -> IO LockHandle
 lockExclusive mode file = makeLockHandle
 	(P.waitTakeLock P.lockPool file LockExclusive)
 	(F.lockExclusive mode file)
 
+-- Tries to take a shared lock, but does not block.
+tryLockShared :: Maybe FileMode -> LockFile -> IO (Maybe LockHandle)
+tryLockShared mode file = tryMakeLockHandle
+	(P.tryTakeLock P.lockPool file LockShared)
+	(F.tryLockShared mode file)
+
+-- Tries to take an exclusive lock, but does not block.
 tryLockExclusive :: Maybe FileMode -> LockFile -> IO (Maybe LockHandle)
 tryLockExclusive mode file = tryMakeLockHandle
 	(P.tryTakeLock P.lockPool file LockExclusive)