Subject: Re: [RFC PATCH 2/6] bloom: prepare to discard incompatible Bloom filters

From: Taylor Blau <[email-masked]>
Date: Thu, 24 Aug 2023 18:47:47 -0400

On Thu, Aug 24, 2023 at 03:20:51PM -0700, Jonathan Tan wrote:
> Taylor Blau <[email-masked]> writes:
> > diff --git a/bloom.c b/bloom.c
> > index 9b6a30f6f6..739fa093ba 100644
> > --- a/bloom.c
> > +++ b/bloom.c
> > @@ -250,6 +250,23 @@ static void init_truncated_large_filter(struct bloom_filter *filter,
> >  	filter->version = version;
> >  }
> >
> > +struct bloom_filter *get_bloom_filter(struct repository *r, struct commit *c)
> > +{
> > +	struct bloom_filter *filter;
> > +	int hash_version;
> > +
> > +	filter = get_or_compute_bloom_filter(r, c, 0, NULL, NULL);
> > +	if (!filter)
> > +		return NULL;
> > +
> > +	prepare_repo_settings(r);
> > +	hash_version = r->settings.commit_graph_changed_paths_version;
> > +
> > +	if (!(hash_version == -1 || hash_version == filter->version))
> > +		return NULL; /* unusable filter */
> > +	return filter;
> > +}
>
> I missed this the last time, but this should match what fill_bloom_key()
> does. Use get_bloom_filter_settings(),

I'm not sure I'm following. Are you saying that we should use
get_bloom_filter_settings() instead of reading the value from
r->settings directly?

> then compare filter->version to version 2 if hash_version is 2, and to
> version 1 otherwise.

Hmm. I think we're already doing the right thing here, no? IIUC, you're
saying to do something like:

    struct bloom_filter_settings *s = get_bloom_filter_settings(r);
    struct bloom_filter *f = get_or_compute_bloom_filter(r, c, ...);
    int hash_version;

    if (!f)
      return NULL;

    prepare_repo_settings(r);
    hash_version = r->settings.commit_graph_changed_paths_version;

    if (!(hash_version == -1 || hash_version == s->hash_version))
      return NULL; /* incompatible */
    return f;

?

If so, I think that we're already OK here, since s->hash_version is
always going to take the same value as f->version, since f->version is
assigned in bloom.c::load_bloom_filter_from_graph(), which does:

    filter->version = g->bloom_filter_settings->hash_version;

Or are we talking about two different things entirely? ;-)

Thanks,
Taylor