--- a/mercurial/bundle2.py	Fri Apr 04 14:24:11 2014 -0700
+++ b/mercurial/bundle2.py	Tue Mar 25 15:05:11 2014 -0700
@@ -558,7 +558,7 @@
         yield _pack(_fpayloadsize, 0)
 
 @parthandler('changegroup')
-def handlechangegroup(op, part):
+def handlechangegroup(op, inpart):
     """apply a changegroup part on the repo
 
     This is a very early implementation that will massive rework before being
@@ -570,10 +570,21 @@
     # we need to make sure we trigger the creation of a transaction object used
     # for the whole processing scope.
     op.gettransaction()
-    data = StringIO.StringIO(part.data)
+    data = StringIO.StringIO(inpart.data)
     data.seek(0)
     cg = changegroup.readbundle(data, 'bundle2part')
     ret = changegroup.addchangegroup(op.repo, cg, 'bundle2', 'bundle2')
     op.records.add('changegroup', {'return': ret})
+    if op.reply is not None:
+        # This is definitly not the final form of this
+        # return. But one need to start somewhere.
+        op.reply.addpart(part('reply:changegroup', (),
+                         [('in-reply-to', str(inpart.id)),
+                          ('return', '%i' % ret)]))
 
+@parthandler('reply:changegroup')
+def handlechangegroup(op, inpart):
+    p = dict(inpart.advisoryparams)
+    ret = int(p['return'])
+    op.records.add('changegroup', {'return': ret}, int(p['in-reply-to']))
 

--- a/tests/test-bundle2.t	Fri Apr 04 14:24:11 2014 -0700
+++ b/tests/test-bundle2.t	Tue Mar 25 15:05:11 2014 -0700
@@ -626,7 +626,7 @@
   \x87\xcd\xc9n\x8e\xaa\xb6\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x02H (esc)
   \x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00 (no-eol) (esc)
 
-  $ hg unbundle2 < ../rev.hg2
+  $ hg unbundle2 ../rev-replay.hg2 < ../rev.hg2
   adding changesets
   adding manifests
   adding file changes
@@ -634,6 +634,9 @@
   0 unread bytes
   addchangegroup return: 1
 
+  $ cat ../rev-replay.hg2
+  HG20\x00\x00\x00/\x11reply:changegroup\x00\x00\x00\x00\x00\x02\x0b\x01\x06\x01in-reply-to0return1\x00\x00\x00\x00\x00\x00 (no-eol) (esc)
+
 Real world exchange
 =====================