Subject: Re: [PATCH v2 2/2] shallow.c: use '{commit,rollback}_shallow_file'

From: Taylor Blau <[email-masked]>
Date: Wed, 3 Jun 2020 16:14:53 -0600

On Wed, Jun 03, 2020 at 01:51:51PM -0700, Jonathan Nieder wrote:
> Taylor Blau wrote:
>
> > Ah, this only sort of has to do with the object cache. In
> > 'parse_commit_buffer()' we stop parsing parents in the case that the
> > repository is shallow (this goes back to 7f3140cd23 (git repack: keep
> > commits hidden by a graft, 2009-07-23)).
>
> Ah, good analysis.  (In fact, the behavior is older: it's from
> 5da5c8f4cf4 (Teach parse_commit_buffer about grafting., 2005-07-30).)
> So this is additional "cached" data that needs to be invalidated by
> reset_repository_shallow.
>
> So the question is, what other information falls into that category?
>
> [...]
> > --- a/shallow.c
> > +++ b/shallow.c
> > @@ -90,6 +90,9 @@ static void reset_repository_shallow(struct repository *r)
> >  {
> >  	r->parsed_objects->is_shallow = -1;
> >  	stat_validity_clear(r->parsed_objects->shallow_stat);
> > +
>
> (nit: the above two lines wouldn't be needed if r->parsed_objects is
> being thrown away.)

Right, thanks. I don't think that it matters since you point out a
legitimate issue with dangling references, but serves me right for
working on this so late at night ;-).

> > +	parsed_object_pool_clear(r->parsed_objects);
> > +	r->parsed_objects = parsed_object_pool_new();
> >  }
>
> Shallows don't affect the ref store.  They only affect object walks.
> So r->parsed_objects does seem like the only place that could be
> affected.
>
> That said, with this change I'd worry about use-after-free from any
> existing references to objects in the pool.
>
> Stepping back, what I think I would like to see is to *not* have
> grafts and shallow state affect the in-memory persisted parsed
> objects.  Instead, act as an overlay in accessors that traverse over
> them.
>
> Lacking that, I like the idea of a "dirty bit" that gets written as
> soon as we have started lying in the parsed object pool.  Something
> like this.  What do you think?
>
> diff --git i/commit-graph.c w/commit-graph.c
> index 2ff042fbf4f..84b49ce903b 100644
> --- i/commit-graph.c
> +++ w/commit-graph.c
> @@ -149,7 +149,8 @@ static int commit_graph_compatible(struct repository *r)
>  	}
>
>  	prepare_commit_graft(r);
> -	if (r->parsed_objects && r->parsed_objects->grafts_nr)
> +	if (r->parsed_objects &&
> +	    (r->parsed_objects->grafts_nr || r->parsed_objects->substituted_parent))

This is a little tricky. Why would we set substituted_parent without
also incrementing grafts_nr? That seems like the real bug here: if we
incremented grafts_nr, then we would return a non-zero value from
'commit_graph_compatible' and rightly stop even without this sticky-bit.

I don't quite understand this myself. If it's an oversight, it's a
remarkably long-lived one. Do you have a better sense of this?

>  		return 0;
>  	if (is_repository_shallow(r))
>  		return 0;
> diff --git i/commit.c w/commit.c
> index 87686a7055b..762f09e53ae 100644
> --- i/commit.c
> +++ w/commit.c
> @@ -423,6 +423,8 @@ int parse_commit_buffer(struct repository *r, struct commit *item, const void *b
>  	pptr = &item->parents;
>
>  	graft = lookup_commit_graft(r, &item->object.oid);
> +	if (graft)
> +		r->parsed_objects->substituted_parent = 1;
>  	while (bufptr + parent_entry_len < tail && !memcmp(bufptr, "parent ", 7)) {
>  		struct commit *new_parent;
>
> @@ -447,6 +449,7 @@ int parse_commit_buffer(struct repository *r, struct commit *item, const void *b
>  	if (graft) {
>  		int i;
>  		struct commit *new_parent;
> +

Nit: unnecessary whitespace change, but I doubt it really matters much.

>  		for (i = 0; i < graft->nr_parent; i++) {
>  			new_parent = lookup_commit(r,
>  						   &graft->parent[i]);
> diff --git i/object.h w/object.h
> index b22328b8383..db02fdcd6b2 100644
> --- i/object.h
> +++ w/object.h
> @@ -26,6 +26,7 @@ struct parsed_object_pool {
>  	char *alternate_shallow_file;
>
>  	int commit_graft_prepared;
> +	int substituted_parent;
>
>  	struct buffer_slab *buffer_slab;
>  };
>
> Thanks,
> Jonathan

Thanks,
Taylor