changeset 33800:f257943e47ab

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

--- /dev/null	Thu Jan 01 00:00:00 1970 +0000
+++ b/mercurial/repository.py	Sun Aug 13 10:58:48 2017 -0700
@@ -0,0 +1,71 @@
+# repository.py - Interfaces and base classes for repositories and peers.
+#
+# Copyright 2017 Gregory Szorc <[e-mail]>
+#
+# This software may be used and distributed according to the terms of the
+# GNU General Public License version 2 or any later version.
+
+from __future__ import absolute_import
+
+import abc
+
+class _basepeer(object):
+    """Represents a "connection" to a repository.
+
+    This is the base interface for representing a connection to a repository.
+    It holds basic properties and methods applicable to all peer types.
+
+    This is not a complete interface definition and should not be used
+    outside of this module.
+    """
+    __metaclass__ = abc.ABCMeta
+
+    @abc.abstractproperty
+    def ui(self):
+        """ui.ui instance."""
+
+    @abc.abstractmethod
+    def url(self):
+        """Returns a URL string representing this peer.
+
+        Currently, implementations expose the raw URL used to construct the
+        instance. It may contain credentials as part of the URL. The
+        expectations of the value aren't well-defined and this could lead to
+        data leakage.
+
+        TODO audit/clean consumers and more clearly define the contents of this
+        value.
+        """
+
+    @abc.abstractmethod
+    def local(self):
+        """Returns a local repository instance.
+
+        If the peer represents a local repository, returns an object that
+        can be used to interface with it. Otherwise returns ``None``.
+        """
+
+    @abc.abstractmethod
+    def peer(self):
+        """Returns an object conforming to this interface.
+
+        Most implementations will ``return self``.
+        """
+
+    @abc.abstractmethod
+    def canpush(self):
+        """Returns a boolean indicating if this peer can be pushed to."""
+
+    @abc.abstractmethod
+    def close(self):
+        """Close the connection to this peer.
+
+        This is called when the peer will no longer be used. Resources
+        associated with the peer should be cleaned up.
+        """
+
+class peer(_basepeer):
+    """Unified interface and base class for peer repositories.
+
+    All peer instances must inherit from this class.
+    """