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diff --git a/Annex/ReplaceFile.hs b/Annex/ReplaceFile.hs
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+{- git-annex file replacing
+ -
+ - Copyright 2013 Joey Hess <joey@kitenet.net>
+ -
+ - Licensed under the GNU GPL version 3 or higher.
+ -}
+
+module Annex.ReplaceFile where
+
+import Common.Annex
+import Annex.Perms
+import Annex.Exception
+
+{- Replaces a possibly already existing file with a new version, 
+ - atomically, by running an action.
+ - 
+ - The action is passed a temp file, which it can write to, and once
+ - done the temp file is moved into place.
+ -
+ - The action can throw an IO exception, in which case the temp file
+ - will be deleted, and the existing file will be preserved.
+ -
+ - Throws an IO exception when it was unable to replace the file.
+ -}
+replaceFile :: FilePath -> (FilePath -> Annex ()) -> Annex ()
+replaceFile file a = do
+	tmpdir <- fromRepo gitAnnexTmpDir
+	void $ createAnnexDirectory tmpdir
+	bracketIO (setup tmpdir) nukeFile $ \tmpfile -> do
+		a tmpfile
+		liftIO $ catchIO (rename tmpfile file) (fallback tmpfile)
+  where
+  	setup tmpdir = do
+		(tmpfile, h) <- openTempFileWithDefaultPermissions tmpdir "tmp"
+		hClose h
+		return tmpfile
+	fallback tmpfile _ = do
+		createDirectoryIfMissing True $ parentDir file
+		rename tmpfile file