repository: formalize wire protocol interface

There are a well-defined set of commands constituting the wire
protocol. Interaction with these and methods for calling them in
batches are exposed via methods on peer instances.

Let's formalize support for these features in abstract classes.

The command parts come from the existing wireproto.wirepeer class.
The batch methods come from peer.peerrepository.

Ample documentation has been added as part of defining the interfaces.

# no-check-commit

Differential Revision: https://phab.mercurial-scm.org/D333

mercurial/repository.py

@@ -64,8 +64,169 b' class _basepeer(object):'
         associated with the peer should be cleaned up.
         """
 
-class peer(_basepeer):
+class _basewirecommands(object):
+    """Client-side interface for communicating over the wire protocol.
+
+    This interface is used as a gateway to the Mercurial wire protocol.
+    methods commonly call wire protocol commands of the same name.
+    """
+    __metaclass__ = abc.ABCMeta
+
+    @abc.abstractmethod
+    def branchmap(self):
+        """Obtain heads in named branches.
+
+        Returns a dict mapping branch name to an iterable of nodes that are
+        heads on that branch.
+        """
+
+    @abc.abstractmethod
+    def capabilities(self):
+        """Obtain capabilities of the peer.
+
+        Returns a set of string capabilities.
+        """
+
+    @abc.abstractmethod
+    def debugwireargs(self, one, two, three=None, four=None, five=None):
+        """Used to facilitate debugging of arguments passed over the wire."""
+
+    @abc.abstractmethod
+    def getbundle(self, source, **kwargs):
+        """Obtain remote repository data as a bundle.
+
+        This command is how the bulk of repository data is transferred from
+        the peer to the local repository
+
+        Returns a generator of bundle data.
+        """
+
+    @abc.abstractmethod
+    def heads(self):
+        """Determine all known head revisions in the peer.
+
+        Returns an iterable of binary nodes.
+        """
+
+    @abc.abstractmethod
+    def known(self, nodes):
+        """Determine whether multiple nodes are known.
+
+        Accepts an iterable of nodes whose presence to check for.
+
+        Returns an iterable of booleans indicating of the corresponding node
+        at that index is known to the peer.
+        """
+
+    @abc.abstractmethod
+    def listkeys(self, namespace):
+        """Obtain all keys in a pushkey namespace.
+
+        Returns an iterable of key names.
+        """
+
+    @abc.abstractmethod
+    def lookup(self, key):
+        """Resolve a value to a known revision.
+
+        Returns a binary node of the resolved revision on success.
+        """
+
+    @abc.abstractmethod
+    def pushkey(self, namespace, key, old, new):
+        """Set a value using the ``pushkey`` protocol.
+
+        Arguments correspond to the pushkey namespace and key to operate on and
+        the old and new values for that key.
+
+        Returns a string with the peer result. The value inside varies by the
+        namespace.
+        """
+
+    @abc.abstractmethod
+    def stream_out(self):
+        """Obtain streaming clone data.
+
+        Successful result should be a generator of data chunks.
+        """
+
+    @abc.abstractmethod
+    def unbundle(self, bundle, heads, url):
+        """Transfer repository data to the peer.
+
+        This is how the bulk of data during a push is transferred.
+
+        Returns the integer number of heads added to the peer.
+        """
+
+class _baselegacywirecommands(object):
+    """Interface for implementing support for legacy wire protocol commands.
+
+    Wire protocol commands transition to legacy status when they are no longer
+    used by modern clients. To facilitate identifying which commands are
+    legacy, the interfaces are split.
+    """
+    __metaclass__ = abc.ABCMeta
+
+    @abc.abstractmethod
+    def between(self, pairs):
+        """Obtain nodes between pairs of nodes.
+
+        ``pairs`` is an iterable of node pairs.
+
+        Returns an iterable of iterables of nodes corresponding to each
+        requested pair.
+        """
+
+    @abc.abstractmethod
+    def branches(self, nodes):
+        """Obtain ancestor changesets of specific nodes back to a branch point.
+
+        For each requested node, the peer finds the first ancestor node that is
+        a DAG root or is a merge.
+
+        Returns an iterable of iterables with the resolved values for each node.
+        """
+
+    @abc.abstractmethod
+    def changegroup(self, nodes, kind):
+        """Obtain a changegroup with data for descendants of specified nodes."""
+
+    @abc.abstractmethod
+    def changegroupsubset(self, bases, heads, kind):
+        pass
+
+class peer(_basepeer, _basewirecommands):
     """Unified interface and base class for peer repositories.
 
-    All peer instances must inherit from this class.
+    All peer instances must inherit from this class and conform to its
+    interface.
     """
+
+    @abc.abstractmethod
+    def iterbatch(self):
+        """Obtain an object to be used for multiple method calls.
+
+        Various operations call several methods on peer instances. If each
+        method call were performed immediately and serially, this would
+        require round trips to remote peers and/or would slow down execution.
+
+        Some peers have the ability to "batch" method calls to avoid costly
+        round trips or to facilitate concurrent execution.
+
+        This method returns an object that can be used to indicate intent to
+        perform batched method calls.
+
+        The returned object is a proxy of this peer. It intercepts calls to
+        batchable methods and queues them instead of performing them
+        immediately. This proxy object has a ``submit`` method that will
+        perform all queued batchable method calls. A ``results()`` method
+        exposes the results of queued/batched method calls. It is a generator
+        of results in the order they were called.
+
+        Not all peers or wire protocol implementations may actually batch method
+        calls. However, they must all support this API.
+        """
+
+class legacypeer(peer, _baselegacywirecommands):
+    """peer but with support for legacy wire protocol commands."""