Re: [PATCH v2 1/3] refs: keep track of unresolved reference value in iterators

From: Junio C Hamano <gitster@pobox.com>
Date: Aug 5, 2024, 15:40 UTC
Message-ID: <xmqqh6bz567d.fsf@gitster.g>
In-Reply-To: <ZrCwqqLKcwdOYclN@tanuki>

Patrick Steinhardt <ps@pks.im> writes:

Show 15 quoted lines

> On Thu, Aug 01, 2024 at 09:41:03AM -0700, Junio C Hamano wrote:
>> "John Cai via GitGitGadget" <gitgitgadget@gmail.com> writes:
>> > @@ -886,6 +889,9 @@ static int files_ref_iterator_advance(struct ref_iterator *ref_iterator)
>> >  		iter->base.refname = iter->iter0->refname;
>> >  		iter->base.oid = iter->iter0->oid;
>> >  		iter->base.flags = iter->iter0->flags;
>> > +		if (iter->iter0->flags & REF_ISSYMREF)
>> > +			iter->base.referent = iter->iter0->referent;
>> 
>> Presumably base.referent is initialized to NULL so this "if"
>> statement does not need an else clause?
>
> This function typically ends up being called in a loop though. So
> without the else clause, wouldn't we potentially leak the value of a
> preceding ref into subsequent iterations like this?

OK, so this does need to clear it when we tell the caller we have non SYMREF, as we do want to show NULL as base.referent to the caller in such a case. Thanks.

It does reinforce my larger point, which was:

Show 10 quoted lines

>> Makes me wonder if we should follow the same "ignore what the flag
>> says when filling the .referent member; if the ref is not a symref,
>> the referent variable is NULL, and if it is, referent is never NULL"
>> pattern?  Then ref->u.value.referent is _always_ defined---the
>> current code says "the u.value.referent member is undefined for ref
>> that is not a symref", but with the suggested change, it will be
>> "the u.value.referent member is NULL for ref that is not a symref,
>> and for a symref, it is the value of the symref".
>
> Yeah, I think that would be preferable indeed.

In other words, with .referent member introduced, checking for (.flags & REF_ISSYMREF) becomes a redundant&duplicated bit of information, as the bit should exactly match the non-NULL ness of the .referent member.

Thanks.