avoid creating content directory when locking content

If the content directory does not exist, then it does not make sense to
lock the content file, as it also does not exist, and so it's ok for the
lock operation to fail.

This avoids potential races where the content file exists but is then
deleted/renamed, while another process sees that it exists and goes to
lock it, resulting in a dangling lock file in an otherwise empty object
directory.

Also renamed modifyContent to modifyContentDir since it is not only
necessarily used for modifying content files, but also other files in
the content directory.

Sponsored-by: Dartmouth College's Datalad project

Command/Lock.hs

@@ -77,13 +77,13 @@ perform file key = do
 	breakhardlink obj = whenM (catchBoolIO $ (> 1) . linkCount <$> liftIO (R.getFileStatus obj)) $ do
 		mfc <- withTSDelta (liftIO . genInodeCache file)
 		unlessM (sameInodeCache obj (maybeToList mfc)) $ do
-			modifyContent obj $ replaceGitAnnexDirFile (fromRawFilePath obj) $ \tmp -> do
+			modifyContentDir obj $ replaceGitAnnexDirFile (fromRawFilePath obj) $ \tmp -> do
 				unlessM (checkedCopyFile key obj (toRawFilePath tmp) Nothing) $
 					giveup "unable to lock file"
 			Database.Keys.storeInodeCaches key [obj]
 
 	-- Try to repopulate obj from an unmodified associated file.
-	repopulate obj = modifyContent obj $ do
+	repopulate obj = modifyContentDir obj $ do
 		g <- Annex.gitRepo
 		fs <- map (`fromTopFilePath` g)
 			<$> Database.Keys.getAssociatedFiles key