migrate: Bugfix for case when migrating a file results in a key that is already present in .git/annex/objects.

For example, this could happen if using SHA1 and a file with content
"foo" were added to that backend. Then a file with "content" foo were
migrated from the WORM backend.

Assume that, if a backend assigned the same key, the already annexed
content must be the same. So, the "old" content can be reused.

Command/Migrate.hs

@@ -8,6 +8,7 @@
 module Command.Migrate where
 
 import Control.Monad.State (liftIO)
+import Control.Monad (unless)
 import System.Posix.Files
 import System.Directory
 
@@ -58,7 +59,8 @@ perform file oldkey newbackend = do
 			ok <- getViaTmpUnchecked newkey $ \t -> do
 				-- Make a hard link to the old backend's
 				-- cached key, to avoid wasting disk space.
-				liftIO $ createLink src t
+				exists <- liftIO $ doesFileExist t
+				unless exists $ liftIO $ createLink src t
 				return True
 			if ok
 				then do

debian/changelog

@@ -13,6 +13,8 @@ git-annex (0.20110321) UNRELEASED; urgency=low
   * unused: In fast mode, just show all existing temp files as unused,
     and avoid expensive scan for other unused content.
   * Fix space leak in fsck and drop commands.
+  * migrate: Bugfix for case when migrating a file results in a key that
+    is already present in .git/annex/objects.
 
  -- Joey Hess <joeyh@debian.org>  Tue, 22 Mar 2011 16:52:00 -0400