final scary locking refactoring (for now)

Note that while before checkTransfer this called getLock with WriteLock,
getLockStatus's use of ReadLock will also notice any exclusive locks.
Since transfer info files are only locked exclusively, never shared,
there is no behavior change.

Also, fixes checkLocked to actually return Just False when the file
exists, but is not locked.

Logs/Transfer.hs

@@ -129,17 +129,12 @@ checkTransfer :: Transfer -> Annex (Maybe TransferInfo)
 checkTransfer t = do
 	tfile <- fromRepo $ transferFile t
 #ifndef mingw32_HOST_OS
-	mfd <- liftIO $ openExistingLockFile (transferLockFile tfile)
-	case mfd of
-		Nothing -> return Nothing -- failed to open file; not running
-		Just fd -> do
-			locked <- liftIO $
-				getLock fd (WriteLock, AbsoluteSeek, 0, 0)
-			liftIO $ closeFd fd
-			case locked of
-				Nothing -> return Nothing
-				Just (pid, _) -> liftIO $ catchDefaultIO Nothing $
-					readTransferInfoFile (Just pid) tfile
+	liftIO $ do
+		v <- getLockStatus (transferLockFile tfile)
+		case v of
+			Just (pid, _) -> catchDefaultIO Nothing $
+				readTransferInfoFile (Just pid) tfile
+			Nothing -> return Nothing
 #else
 	v <- liftIO $ lockShared $ transferLockFile tfile
 	liftIO $ case v of