diff -r a5de21c9e370 -r a86d21e70b2b mercurial/exchangev2.py
--- /dev/null	Thu Jan 01 00:00:00 1970 +0000
+++ b/mercurial/exchangev2.py	Wed Sep 12 09:58:23 2018 -0700
@@ -0,0 +1,55 @@
+# exchangev2.py - repository exchange for wire protocol version 2
+#
+# Copyright 2018 Gregory Szorc <gregory.szorc@gmail.com>
+#
+# 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
+
+from .node import (
+    nullid,
+)
+from . import (
+    setdiscovery,
+)
+
+def pull(pullop):
+    """Pull using wire protocol version 2."""
+    repo = pullop.repo
+    remote = pullop.remote
+
+    # Figure out what needs to be fetched.
+    common, fetch, remoteheads = _pullchangesetdiscovery(
+        repo, remote, pullop.heads, abortwhenunrelated=pullop.force)
+
+def _pullchangesetdiscovery(repo, remote, heads, abortwhenunrelated=True):
+    """Determine which changesets need to be pulled."""
+
+    if heads:
+        knownnode = repo.changelog.hasnode
+        if all(knownnode(head) for head in heads):
+            return heads, False, heads
+
+    # TODO wire protocol version 2 is capable of more efficient discovery
+    # than setdiscovery. Consider implementing something better.
+    common, fetch, remoteheads = setdiscovery.findcommonheads(
+        repo.ui, repo, remote, abortwhenunrelated=abortwhenunrelated)
+
+    common = set(common)
+    remoteheads = set(remoteheads)
+
+    # If a remote head is filtered locally, put it back in the common set.
+    # See the comment in exchange._pulldiscoverychangegroup() for more.
+
+    if fetch and remoteheads:
+        nodemap = repo.unfiltered().changelog.nodemap
+
+        common |= {head for head in remoteheads if head in nodemap}
+
+        if set(remoteheads).issubset(common):
+            fetch = []
+
+    common.discard(nullid)
+
+    return common, fetch, remoteheads