Re: [PATCH v3 09/25] midx: avoid opening multiple MIDXs when writing

I think we'd need something like this:

diff --git a/midx.c b/midx.c
index 6dfafe7a8c..bfb6afea2e 100644
--- a/midx.c
+++ b/midx.c

@@ -1123,8 +1123,7 @@ static int write_midx_internal(const char *object_dir,
 	hold_lock_file_for_update(&lk, midx_name, LOCK_DIE_ON_ERROR);
 	f = hashfd(get_lock_file_fd(&lk), get_lock_file_path(&lk));
 
-	if (ctx.m)
-		close_midx(ctx.m);
+	close_object_store(the_repository->objects);
 
 	if (ctx.nr - dropped_packs == 0) {
 		error(_("no pack files to index."));

though I'm not sure:

 - if this should be unconditional or dependent on ctx.m (I think the
   latter, because if we are renaming over any open midx, we would have
   filled in ctx.m earlier).

 - if this should go below the "no pack files to index" check (i.e., is
   there any point in closing if we know we will not write?). In fact,
   its purpose might be more obvious right before finalize_hashfile(),
   but I am OK either way on that.