SpiceDB: LookupSubjects may return partial results if a specific kind of relation is used

Background

Use of a relation of the form: relation folder: folder | folder#parent with an arrow such as folder->view can cause LookupSubjects to only return the subjects found under subjects for either folder or folder#parent.

This bug only manifests if the same subject type is used multiple types in a relation, relationships exist for both subject types and an arrow is used over the relation.

Impact

Any user making a negative authorization decision based on the results of a LookupSubjects request with version before v1.30.1 is affected.

Workarounds

Avoid using LookupSubjects for negative authorization decisions and/or avoid using the broken schema.