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.

1 files changed, 3 insertions, 1 deletions

diff --git a/Command/Migrate.hs b/Command/Migrate.hs
index 56147113b..38dfd06b2 100644
--- a/Command/Migrate.hs
+++ b/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