more lock file refactoring

Also fixes a test suite failures introduced in recent commits, where
inAnnexSafe failed in indirect mode, since it tried to open the lock file
ReadWrite. This is why the new checkLocked opens it ReadOnly.

This commit was sponsored by Chad Horohoe.

Utility/LockFile/Posix.hs

@@ -9,15 +9,21 @@ module Utility.LockFile.Posix (
 	LockHandle,
 	lockShared,
 	lockExclusive,
-	dropLock,
+	tryLockExclusive,
 	createLockFile,
 	openExistingLockFile,
+	isLocked,
+	checkLocked,
+	dropLock,
 ) where
 
 import Utility.Exception
+import Utility.Applicative
 
 import System.IO
 import System.Posix
+import Data.Maybe
+import Control.Applicative
 
 type LockFile = FilePath
 
@@ -31,27 +37,55 @@ lockShared = lock ReadLock
 lockExclusive :: Maybe FileMode -> LockFile -> IO LockHandle
 lockExclusive = lock WriteLock
 
--- The FileMode is used when creating a new lock file.
+-- Tries to take an exclusive lock, but does not block.
+tryLockExclusive :: Maybe FileMode -> LockFile -> IO (Maybe LockHandle)
+tryLockExclusive mode lockfile = do
+	l <- openLockFile 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.
 lock :: LockRequest -> Maybe FileMode -> LockFile -> IO LockHandle
 lock lockreq mode lockfile = do
-	l <- createLockFile mode lockfile
+	l <- openLockFile mode lockfile
 	waitToSetLock l (lockreq, AbsoluteSeek, 0, 0)
 	return (LockHandle l)
 
 -- Create and opens lock file; does not lock it.
-createLockFile :: Maybe FileMode -> LockFile -> IO Fd
-createLockFile = openLockFile ReadWrite
+createLockFile :: FileMode -> LockFile -> IO Fd
+createLockFile filemode = openLockFile (Just filemode)
 
--- Opens an existing lock file; does not lock it or create it.
+-- Opens an existing lock file; does not lock it, and if it does not exist,
+-- returns Nothing.
 openExistingLockFile :: LockFile -> IO (Maybe Fd)
-openExistingLockFile = catchMaybeIO . openLockFile ReadOnly Nothing
+openExistingLockFile = catchMaybeIO . openLockFile 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
+openLockFile :: Maybe FileMode -> LockFile -> IO Fd
+openLockFile filemode lockfile = do
+	l <- openFd lockfile ReadWrite filemode defaultFileFlags
 	setFdOption l CloseOnExec True
 	return l
 
+-- Check if a file is locked, either exclusively, or with shared lock.
+-- When the file doesn't exist, it's considered not locked.
+isLocked :: LockFile -> IO Bool
+isLocked = fromMaybe False <$$> checkLocked
+
+checkLocked :: LockFile -> IO (Maybe Bool)
+checkLocked lockfile = go =<< catchMaybeIO open
+  where
+	open = openFd lockfile ReadOnly Nothing defaultFileFlags
+	go Nothing = return Nothing
+	go (Just h) = do
+		ret <- isJust <$> getLock h (ReadLock, AbsoluteSeek, 0, 0)
+		closeFd h
+		return $ Just ret
+
 dropLock :: LockHandle -> IO ()
 dropLock (LockHandle fd) = closeFd fd