fix overwrite race with git-annex add of annex symlink

In the unlikely case where git-annex add is run on an annex symlink that
is not already added, and while it's processing it, the annex symlink is
overwritten with something else, avoid git-annex overwriting that with
the symlink again.

Sponsored-by: Jack Hill on Patreon

Annex/Ingest.hs

@@ -17,6 +17,7 @@ module Annex.Ingest (
 	finishIngestUnlocked,
 	cleanOldKeys,
 	addLink,
+	addSymlink,
 	makeLink,
 	addUnlocked,
 	CheckGitIgnore(..),

Command/Add.hs

@@ -181,10 +181,19 @@ start o si file addunlockedmatcher = do
 			_ -> add
 	fixuplink key = 
 		starting "add" (ActionItemTreeFile file) si $
-			addingExistingLink file key $ do
-				liftIO $ removeFile (fromRawFilePath file)
-				addLink (checkGitIgnoreOption o) file key Nothing
+			addingExistingLink file key $
+				withOtherTmp $ \tmp -> do
+					let tmpf = fromRawFilePath tmp </> fromRawFilePath file
+					liftIO $ moveFile (fromRawFilePath file) tmpf
+					ifM (isSymbolicLink <$> liftIO (getSymbolicLinkStatus tmpf))
+						( do
+							liftIO $ removeFile tmpf
+							addSymlink file key Nothing
 							next $ cleanup key =<< inAnnex key
+						, do
+							liftIO $ moveFile tmpf (fromRawFilePath file)
+							next $ return True
+						)
 	fixuppointer key =
 		starting "add" (ActionItemTreeFile file) si $
 			addingExistingLink file key $ do