convert lockContent to use new LockPools

Also cleaned up the code, avoiding creating a lock file if we're going to
open it for create later anyway.

And, if there's an exception while preparing to lock the file, but not at
the point of actually taking the lock, throw an exception, instead of
silently not locking and pretending to succeed.

And, on Windows, always use lock file, even if the repo somehow got into
indirect mode (maybe with cygwin git..)

Annex/Content.hs

@@ -150,10 +150,14 @@ inAnnexSafe key = inAnnex' (fromMaybe False) (Just False) go key
  - file from the content, since locking the actual content file
  - would interfere with the user's use of it. -}
 contentLockFile :: Key -> Annex (Maybe FilePath)
+#ifndef mingw32_HOST_OS
 contentLockFile key = ifM isDirect
 	( Just <$> calcRepo (gitAnnexContentLock key)
 	, return Nothing
 	)
+#else
+contentLockFile key = Just <$> calcRepo (gitAnnexContentLock key)
+#endif
 
 newtype ContentLock = ContentLock Key
 
@@ -164,43 +168,40 @@ lockContent :: Key -> (ContentLock -> Annex a) -> Annex a
 lockContent key a = do
 	contentfile <- calcRepo $ gitAnnexLocation key
 	lockfile <- contentLockFile key
-	maybe noop setuplockfile lockfile
 	bracket
 		(lock contentfile lockfile)
 		(unlock lockfile)
 		(const $ a $ ContentLock key )
   where
 	alreadylocked = error "content is locked"
-	setuplockfile lockfile = modifyContent lockfile $
-		void $ liftIO $ tryIO $
-			writeFile lockfile ""
+	failedtolock e = error $ "failed to lock content: " ++ show e
+	trylock locker = locker `catchIO` failedtolock
 	cleanuplockfile lockfile = modifyContent lockfile $
 		void $ liftIO $ tryIO $
 			nukeFile lockfile
 #ifndef mingw32_HOST_OS
-	lock contentfile Nothing = liftIO $
-		opencontentforlock contentfile >>= dolock
-	lock _ (Just lockfile) = do
-		mode <- annexFileMode
-		liftIO $ createLockFile mode lockfile >>= dolock . Just
 	{- Since content files are stored with the write bit disabled, have
 	 - to fiddle with permissions to open for an exclusive lock. -}
-	opencontentforlock f = catchDefaultIO Nothing $ 
-		withModifiedFileMode f
-			(`unionFileModes` ownerWriteMode)
-			(openExistingLockFile f)
-	dolock Nothing = return Nothing
-	dolock (Just fd) = do
-		v <- tryIO $ setLock fd (WriteLock, AbsoluteSeek, 0, 0)
-		case v of
-			Left _ -> alreadylocked
-			Right _ -> return $ Just fd
-	unlock mlockfile mfd = do
+	lock contentfile Nothing = trylock $ liftIO $
+		withModifiedFileMode contentfile
+			(`unionFileModes` ownerWriteMode) $
+			maybe alreadylocked return
+				=<< tryLockExclusive Nothing contentfile
+	lock _ (Just lockfile) = trylock $ do
+		mode <- annexFileMode
+		maybe alreadylocked return 
+			=<< modifyContent lockfile
+				(liftIO $ tryLockExclusive (Just mode) lockfile)
+	unlock mlockfile lck = do
 		maybe noop cleanuplockfile mlockfile
-		liftIO $ maybe noop closeFd mfd
+		liftIO $ dropLock lck
 #else
-	lock _ (Just lockfile) = liftIO $
+	lock _ (Just lockfile) = do
+		modifyContent lockfile $
+			void $ liftIO $ tryIO $
+				writeFile lockfile ""
 		maybe alreadylocked (return . Just) =<< lockExclusive lockfile
+	-- never reached; windows always uses a separate lock file
 	lock _ Nothing = return Nothing
 	unlock mlockfile mlockhandle = do
 		liftIO $ maybe noop dropLock mlockhandle

Utility/LockFile/Posix.hs

@@ -10,8 +10,6 @@ module Utility.LockFile.Posix (
 	lockShared,
 	lockExclusive,
 	tryLockExclusive,
-	createLockFile,
-	openExistingLockFile,
 	checkLocked,
 	getLockStatus,
 	dropLock,
@@ -56,15 +54,6 @@ lock lockreq mode lockfile = do
 	waitToSetLock l (lockreq, AbsoluteSeek, 0, 0)
 	return (LockHandle l)
 
--- Create and opens lock file; does not lock it.
-createLockFile :: FileMode -> LockFile -> IO Fd
-createLockFile filemode = openLockFile (Just filemode)
-
--- 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 Nothing
-
 -- Close on exec flag is set so child processes do not inherit the lock.
 openLockFile :: Maybe FileMode -> LockFile -> IO Fd
 openLockFile filemode lockfile = do