repository: formalize peer interface with abstract base class There are various interfaces for interacting with repositories and peers. They form a contract for how one should interact with a repo or peer object. The contracts today aren't very well-defined or enforced. There have been several bugs over the years where peers or repo types have forgotten to implement certain methods. In addition, the inheritance of some classes is wonky. For example, localrepository doesn't inherit from an interface and the god-object nature of that class means the repository interface isn't well-defined. Other repository types inherit from localrepository then stub out methods that don't make sense (e.g. statichttprepository re-defining locking methods to fail fast). Not having well-defined interfaces makes implementing alternate storage backends, wire protocol transports, and repository types difficult because it isn't clear what exactly needs to be implemented. This patch starts the process of attempting to establish more order to the type system around repositories and peers. Our first patch starts with a problem space that already has a partial solution: peers. The peer.peerrepository class already somewhat defines a peer interface. But it is missing a few things and the total interface isn't well-defined because it is combined with wireproto.wirepeer. Our newly-established basepeer class uses the abc module to declare an abstract base class with the properties and methods that a generic peer must implement. We create a new class that inherits from it. This class will hold our other future abstract base classes / interfaces so we can expose a unified base class/interface. We don't yet use the new interface because subsequent additions will break existing code without some refactoring first. A new module (repository.py) was created to hold the interfaces. I could have put things in peer.py. However, I have plans to eventually add interfaces to define repository and storage types. These almost certainly require a new module. And I figured having all the interfaces live in one module makes sense. So I created repository.py to be that future home. Differential Revision: https://phab.mercurial-scm.org/D332