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

[Jeff King <peff@peff.net>]

On Sun, Dec 09, 2018 at 09:51:28AM +0900, Junio C Hamano wrote:

> > -static int parse_commit_in_graph_one(struct commit_graph *g, struct commit *item)
> > +static struct commit *parse_commit_in_graph_one(struct repository *r,
> > +						struct commit_graph *g,
> > +						struct commit *shell,
> > +						const struct object_id *oid)
> 
> Now the complexity of the behaviour of this function deserves to be
> documented in a comment in front.  Let me see if I can get it
> correctly without such a comment by explaining the function aloud.
> 
> The caller may or may not have already obtained an in-core commit
> object for a given object name, so shell could be NULL but otherwise
> it could be used for optimization.  When shell==NULL, the function
> looks up the commit object using the oid parameter instead.  The
> returned in-core commit has the parents etc. filled as if we ran
> parse_commit() on it.  If the commit is not yet in the graph, the
> caller may get a NULL even if the commit exists.

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).

-Peff