Re: [PATCH on master v2] revision: use commit graph in get_reference()

[Jeff King <peff@peff.net>]

On Wed, Dec 12, 2018 at 11:58:12AM -0800, Jonathan Tan wrote:

> > Yeah, this was the part that took me a bit to figure out, as well. The
> > optimization here is really just avoiding a call to lookup_commit(),
> > which will do a single hash-table lookup. I wonder if that's actually
> > worth this more complex interface (as opposed to just always taking an
> > oid and then always returning a "struct commit", which could be old or
> > new).
> 
> Avoidance of lookup_commit() is more important than an optimization, I
> think. Here, we call lookup_commit() only when we know that that object
> is a commit (by its presence in a commit graph). If we just called it
> blindly, we might mistakenly create a commit for that hash when it is
> actually an object of another type. (We could inline lookup_commit() in
> parse_commit_in_graph_one(), removing the object creation part, but that
> adds complexity as well.)

I was thinking we would only do so in the happy path when we find a
commit. I.e., something like:

  obj = lookup_object(oid); /* does not auto-vivify */
  if (obj && obj->parsed)
	return obj;

  if (we_have_it_in_commit_graph) {
	commit = obj || lookup_commit(oid);
	fill_in_details_from_commit_graph(commit);
	return &commit->obj;
  } else {
	return parse_object(oid);
  }

which is more along the lines of that parse_probably_commit() that
Stolee mentioned.

-Peff