tests: verify that peer instances only expose interface members Our abstract interfaces are more useful if we guarantee that implementations conform to certain rules. Namely, we want to ensure that objects implementing interfaces don't expose new public attributes that aren't part of the interface. That way, as long as consumers don't access "internal" attributes (those beginning with "_") then (in theory) objects implementing interfaces can be swapped out and everything will "just work." We add a test that enforces our "no public attributes not part of the abstract interface" rule. We /could/ implement "interface compliance detection" at run-time. However, that is littered with problems. The obvious solutions are custom __new__ and __init__ methods. These rely on derived types actually calling the parent's implementation, which is no sure bet. Furthermore, __new__ and __init__ will likely be called before instance-specific attributes are assigned. In other words, they won't detect public attributes set on self.__dict__. This means public attribute detection won't be robust. We could work around lack of robust self.__dict__ public attribute detection by having our interfaces implement a custom __getattribute__, __getattr__, and/or __setattr__. However, this incurs an undesirable run-time penalty. And, subclasses could override our custom method, bypassing the check. The most robust solution is a non-runtime test. So that's what this commit implements. We have a generic function for validating that an object only has public attributes defined by abstract classes. Then, we instantiate some peers and verify a newly constructed object plays by the rules. Differential Revision: https://phab.mercurial-scm.org/D339