Jonathan Tan [off-list ref] writes:

quoted

 	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?)