Re: [PATCH] revision: allow missing promisor objects on CLI

[Jonathan Tan <jonathantanmy@google.com>]

> Jonathan Tan <jonathantanmy@google.com> writes:
> 
> >  	object = get_reference(revs, arg, &oid, flags ^ local_flags);
> >  	if (!object)
> > -		return revs->ignore_missing ? 0 : -1;
> > +		/*
> > +		 * Either this object is missing and ignore_missing is true, or
> > +		 * this object is a (missing) promisor object and
> > +		 * exclude_promisor_objects is true.
> 
> I had to guess and dig where these assertions are coming from; we
> should not force future readers of the code to.
> 
> At least this comment must say why these assertions hold.  Say
> something like "get_reference() yields NULL on only such and such
> cases" before concluding with "and in any of these cases, we can
> safely ignore it because ...".

OK, will do.

> I think the two cases the comment covers are safe for this caller to
> silently return 0.  Another case get_reference() yields NULL is when
> oid_object_info() says it is a commit but it turns out that the
> object is found by repo_parse_commit() to be a non-commit, isn't it?
> I am not sure if it is safe for this caller to just return 0.  There
> may be some other "unusual-but-not-fatal" cases where get_reference()
> does not hit a die() but returns NULL.

I don't think there is any other case where get_reference() yields NULL,
at least where I based my patch (99c33bed56 ("Git 2.25-rc0",
2019-12-25)). Quoting the entire get_reference():

> static struct object *get_reference(struct rev_info *revs, const char *name,
>                                     const struct object_id *oid,
>                                     unsigned int flags)
> {
>         struct object *object;
> 
>         /*
>          * If the repository has commit graphs, repo_parse_commit() avoids
>          * reading the object buffer, so use it whenever possible.
>          */
>         if (oid_object_info(revs->repo, oid, NULL) == OBJ_COMMIT) {
>                 struct commit *c = lookup_commit(revs->repo, oid);
>                 if (!repo_parse_commit(revs->repo, c))
>                         object = (struct object *) c;
>                 else
>                         object = NULL;
>         } else {
>                 object = parse_object(revs->repo, oid);
>         }

No return statements at all prior to this line.

>         if (!object) {
>                 if (revs->ignore_missing)
>                         return object;

Return NULL (the value of object).

>                 if (revs->exclude_promisor_objects && is_promisor_object(oid))
>                         return NULL;

Return NULL.

>                 die("bad object %s", name);

Die (so this function invocation never returns). In conclusion, if
object is NULL at this point in time, get_reference() either returns
NULL or dies.

>         }

Since get_reference() did not return NULL or die, object is non-NULL
here.

>         object->flags |= flags;
>         return object;

Nothing has overridden object since, so we're returning non-NULL here.

> }

So I think get_reference() only returns NULL in those two safe cases.
(Or did I miss something?)