add: Fix error recovery rollback to not move the injested file content out of the annex back to the file, because other files may point to that same content. Instead, copy the injected file content out to recover.

That was not a data loss, but it came close!

Command/Add.hs

@@ -31,6 +31,7 @@ import Utility.InodeCache
 import Annex.FileMatcher
 import Annex.ReplaceFile
 import Utility.Tmp
+import Utility.CopyFile
 
 import Control.Exception (IOException)
 
@@ -244,15 +245,13 @@ undo :: FilePath -> Key -> SomeException -> Annex a
 undo file key e = do
 	whenM (inAnnex key) $ do
 		liftIO $ nukeFile file
-		catchNonAsync (fromAnnex key file) tryharder
-		logStatus key InfoMissing
+		-- The key could be used by other files too, so leave the
+		-- content in the annex, and make a copy back to the file.
+		obj <- calcRepo $ gitAnnexLocation key
+		unlessM (liftIO $ copyFileExternal CopyTimeStamps obj file) $
+			warning $ "Unable to restore content of " ++ file ++ "; it should be located in " ++ obj
+		thawContent file
 	throwM e
-  where
-	-- fromAnnex could fail if the file ownership is weird
-	tryharder :: SomeException -> Annex ()
-	tryharder _ = do
-		src <- calcRepo $ gitAnnexLocation key
-		liftIO $ moveFile src file
 
 {- Creates the symlink to the annexed content, returns the link target. -}
 link :: FilePath -> Key -> Maybe InodeCache -> Annex String