Re: RFC: New reference iteration paradigm

[Junio C Hamano <gitster@pobox.com>]

Michael Haggerty <mhagger@alum.mit.edu> writes:

> the backend now has to implement
>
>> struct ref_iterator *ref_iterator_begin_fn(const char *submodule,
>>                                            const char *prefix,
>>                                            unsigned int flags);
>
> The ref_iterator itself has to implement two main methods:
>
>> int iterator_advance_fn(struct ref_iterator *ref_iterator);
>> void iterator_free_fn(struct ref_iterator *ref_iterator);
>
> A loop over references now looks something like
>
>> struct ref_iterator *iter = each_ref_in_iterator("refs/tags/");
>> while (ref_iterator_advance(iter)) {
>>         /* code using iter->refname, iter->oid, iter->flags */
>> }

We'd want to take advantage of the tree-like organization of the
refs (i.e. refs/tags/a and refs/tags/b sit next to each other and
they are closer to each other than they are to refs/heads/a) so that
a request "I want to iterate only over tags, even though I may have
millions of other kinds of refs" can be done with cost that is
proportional to how many tags you have.

The current implementation of for_each_tag_ref() that goes down to
do_for_each_entry() in files-backend.c has that propertly, and the
new iteration mechanism with the above design seems to keep it,
which is very nice.