Re: [PATCH] clone: do faster object check for partial clones

[Jonathan Tan <jonathantanmy@google.com>]

> On Wed, Apr 03, 2019 at 10:27:21AM -0700, Josh Steadmon wrote:
> 
> > For partial clones, doing a full connectivity check is wasteful; we skip
> > promisor objects (which, for a partial clone, is all known objects), and
> > excluding them all from the connectivity check can take a significant
> > amount of time on large repos.
> > 
> > At most, we want to make sure that we get the objects referred to by any
> > wanted refs. For partial clones, just check that these objects were
> > transferred.
> 
> This isn't strictly true, since we could get objects from elsewhere via
> --shared or --reference. Those might not be promisor objects.

I don't think local clones (which --shared or --reference implies) can
be partial, but the bigger point is below.

> Shouldn't we be able to stop a traversal as soon as we see that an
> object is in a promisor pack?
> 
> I.e., here:
> 
> > +	if (opt->check_refs_only) {
> > +		/*
> > +		 * For partial clones, we don't want to walk the full commit
> > +		 * graph because we're skipping promisor objects anyway. We
> > +		 * should just check that objects referenced by wanted refs were
> > +		 * transferred.
> > +		 */
> > +		do {
> > +			if (!repo_has_object_file(the_repository, &oid))
> > +				return 1;
> > +		} while (!fn(cb_data, &oid));
> > +		return 0;
> > +	}
> 
> for each object where you ask "do we have this?" we could, for the same
> cost, ask "do we have this in a promisor pack?". And the answer would be
> yes for each in a partial clone.
> 
> But that would also cleanly handle --shared, etc, that I mentioned. And
> more importantly, it would _also_ work on fetches. If I fetch from you
> and get a promisor pack, then there is no point in me enumerating every
> tree you sent. As long as you gave me a tip tree, then you have promised
> that you'd give me all the others if I ask.
> 
> So it seems like this should be a feature of the child rev-list, to stop
> walking the graph at any object that is in a promisor pack.

We currently already do a less optimal version of this - we pass
--exclude-promisor-objects to rev-list, which indeed stops traversal at
any promisor objects (whether in a promisor pack or referenced by one).
As far as I know, the problem is that to do so, we currently enumerate
all the objects in all promisor packs, and all objects that those
objects reference (which means we inflate them too) - so that we have an
oidset that we can check objects against.

A partial solution is for is_promisor_object() to first check if the
given object is in a promisor pack, avoiding generating the set of
promisor objects until necessary. This would work in a blob:none clone
with the refs pointing all to commits or all to blobs, but would not
work in a tree:none clone (or maybe, in this case, the clone would be
small enough that performance is not a concern, hmm).

Maybe the ideal solution is for rev-list to check if an object is in a
promisor pack and if --exclude-promisor-objects is active, we do not
follow any outgoing links.