use locking on Windows

This is all the easy cases, where there was already a separate lock file.

Logs/Transfer.hs

@@ -29,6 +29,7 @@ import System.Posix.Types (ProcessID)
 #else
 import System.Win32.Process (ProcessId)
 import System.Win32.Process.Current (getCurrentProcessId)
+import Utility.WinLock
 #endif
 
 #ifndef mingw32_HOST_OS
@@ -147,7 +148,7 @@ runTransfer t file shouldretry a = do
 			openFd (transferLockFile tfile) ReadWrite (Just mode)
 				defaultFileFlags { trunc = True }
 		case mfd of
-			Nothing -> return (mfd, False)
+			Nothing -> return (Nothing, False)
 			Just fd -> do
 				locked <- catchMaybeIO $
 					setLock fd (WriteLock, AbsoluteSeek, 0, 0)
@@ -158,17 +159,28 @@ runTransfer t file shouldretry a = do
 						return (mfd, False)
 #else
 	prep tfile _mode info = do
-		mfd <- catchMaybeIO $ do
-			writeFile (transferLockFile tfile) ""
-			writeTransferInfoFile info tfile
-		return (mfd, False)
+		v <- catchMaybeIO $ lockExclusive (transferLockFile tfile)
+		case v of
+			Nothing -> return (Nothing, False)
+			Just Nothing -> return (Nothing, True)
+			Just (Just lockhandle) -> do
+				void $ tryIO $ writeTransferInfoFile info tfile
+				return (Just lockhandle, False)
 #endif
 	cleanup _ Nothing = noop
-	cleanup tfile (Just fd) = do
+	cleanup tfile (Just lockhandle) = do
 		void $ tryIO $ removeFile tfile
-		void $ tryIO $ removeFile $ transferLockFile tfile
 #ifndef mingw32_HOST_OS
-		closeFd fd
+		void $ tryIO $ removeFile $ transferLockFile tfile
+		closeFd lockhandle
+#else
+		{- Windows cannot delete the lockfile until the lock
+		 - is closed. So it's possible to race with another
+		 - process that takes the lock before it's removed,
+		 - so ignore failure to remove.
+		 -}
+		dropLock lockhandle
+		void $ tryIO $ removeFile $ transferLockFile tfile
 #endif
 	retry oldinfo metervar run = do
 		v <- tryAnnex run
@@ -246,11 +258,14 @@ checkTransfer t = do
 				Just (pid, _) -> liftIO $ catchDefaultIO Nothing $
 					readTransferInfoFile (Just pid) tfile
 #else
-	ifM (liftIO $ doesFileExist $ transferLockFile tfile)
-		( liftIO $ catchDefaultIO Nothing $
+	v <- liftIO $ lockShared $ transferLockFile tfile
+	liftIO $ case v of
+		Nothing -> catchDefaultIO Nothing $
 			readTransferInfoFile Nothing tfile
-		, return Nothing
-		)
+		Just lockhandle -> do
+			dropLock lockhandle
+			void $ tryIO $ removeFile $ transferLockFile tfile
+			return Nothing
 #endif
 
 {- Gets all currently running transfers. -}