--- a/mercurial/bundle2.py	Mon Nov 24 16:04:44 2014 -0800
+++ b/mercurial/bundle2.py	Fri Nov 21 15:50:38 2014 -0800
@@ -877,7 +877,7 @@
                 'b2x:remote-changegroup': ('http', 'https'),
                }
 
-def getrepocaps(repo):
+def getrepocaps(repo, allowpushback=False):
     """return the bundle2 capabilities for a given repo
 
     Exists to allow extensions (like evolution) to mutate the capabilities.
@@ -887,6 +887,8 @@
     if obsolete.isenabled(repo, obsolete.exchangeopt):
         supportedformat = tuple('V%i' % v for v in obsolete.formats)
         caps['b2x:obsmarkers'] = supportedformat
+    if allowpushback:
+        caps['b2x:pushback'] = ()
     return caps
 
 def bundle2caps(remote):

--- a/mercurial/exchange.py	Mon Nov 24 16:04:44 2014 -0800
+++ b/mercurial/exchange.py	Fri Nov 21 15:50:38 2014 -0800
@@ -572,8 +572,12 @@
     The only currently supported type of data is changegroup but this will
     evolve in the future."""
     bundler = bundle2.bundle20(pushop.ui, bundle2.bundle2caps(pushop.remote))
+    pushback = (pushop.trmanager
+                and pushop.ui.configbool('experimental', 'bundle2.pushback'))
+
     # create reply capability
-    capsblob = bundle2.encodecaps(bundle2.getrepocaps(pushop.repo))
+    capsblob = bundle2.encodecaps(bundle2.getrepocaps(pushop.repo,
+                                                      allowpushback=pushback))
     bundler.newpart('b2x:replycaps', data=capsblob)
     replyhandlers = []
     for partgenname in b2partsgenorder:
@@ -590,7 +594,10 @@
     except error.BundleValueError, exc:
         raise util.Abort('missing support for %s' % exc)
     try:
-        op = bundle2.processbundle(pushop.repo, reply)
+        trgetter = None
+        if pushback:
+            trgetter = pushop.trmanager.transaction
+        op = bundle2.processbundle(pushop.repo, reply, trgetter)
     except error.BundleValueError, exc:
         raise util.Abort('missing support for %s' % exc)
     for rephand in replyhandlers:

--- /dev/null	Thu Jan 01 00:00:00 1970 +0000
+++ b/tests/test-bundle2-pushback.t	Fri Nov 21 15:50:38 2014 -0800
@@ -0,0 +1,111 @@
+  $ cat > bundle2.py << EOF
+  > """A small extension to test bundle2 pushback parts.
+  > Current bundle2 implementation doesn't provide a way to generate those
+  > parts, so they must be created by extensions.
+  > """
+  > from mercurial import bundle2, pushkey, exchange, util
+  > def _newhandlechangegroup(op, inpart):
+  >     """This function wraps the changegroup part handler for getbundle.
+  >     It issues an additional b2x:pushkey part to send a new
+  >     bookmark back to the client"""
+  >     result = bundle2.handlechangegroup(op, inpart)
+  >     if 'b2x:pushback' in op.reply.capabilities:
+  >         params = {'namespace': 'bookmarks',
+  >                   'key': 'new-server-mark',
+  >                   'old': '',
+  >                   'new': 'tip'}
+  >         encodedparams = [(k, pushkey.encode(v)) for (k,v) in params.items()]
+  >         op.reply.newpart('b2x:pushkey', mandatoryparams=encodedparams)
+  >     else:
+  >         op.reply.newpart('b2x:output', data='pushback not enabled')
+  >     return result
+  > _newhandlechangegroup.params = bundle2.handlechangegroup.params
+  > bundle2.parthandlermapping['b2x:changegroup'] = _newhandlechangegroup
+  > EOF
+
+  $ cat >> $HGRCPATH <<EOF
+  > [ui]
+  > ssh = python "$TESTDIR/dummyssh"
+  > username = nobody <no.reply@example.com>
+  > 
+  > [alias]
+  > tglog = log -G -T "{desc} [{phase}:{node|short}]"
+  > EOF
+
+Set up server repository
+
+  $ hg init server
+  $ cd server
+  $ echo c0 > f0
+  $ hg commit -Am 0
+  adding f0
+
+Set up client repository
+
+  $ cd ..
+  $ hg clone ssh://user@dummy/server client -q
+  $ cd client
+
+Enable extension
+  $ cat >> $HGRCPATH <<EOF
+  > [extensions]
+  > bundle2=$TESTTMP/bundle2.py
+  > [experimental]
+  > bundle2-exp = True
+  > EOF
+
+Without config
+
+  $ cd ../client
+  $ echo c1 > f1
+  $ hg commit -Am 1
+  adding f1
+  $ hg push
+  pushing to ssh://user@dummy/server
+  searching for changes
+  remote: pushback not enabled
+  remote: adding changesets
+  remote: adding manifests
+  remote: adding file changes
+  remote: added 1 changesets with 1 changes to 1 files
+  $ hg bookmark
+  no bookmarks set
+
+  $ cd ../server
+  $ hg tglog
+  o  1 [public:2b9c7234e035]
+  |
+  @  0 [public:6cee5c8f3e5b]
+  
+
+
+
+With config
+
+  $ cd ../client
+  $ echo '[experimental]' >> .hg/hgrc
+  $ echo 'bundle2.pushback = True' >> .hg/hgrc
+  $ echo c2 > f2
+  $ hg commit -Am 2
+  adding f2
+  $ hg push
+  pushing to ssh://user@dummy/server
+  searching for changes
+  remote: adding changesets
+  remote: adding manifests
+  remote: adding file changes
+  remote: added 1 changesets with 1 changes to 1 files
+  $ hg bookmark
+     new-server-mark           2:0a76dfb2e179
+
+  $ cd ../server
+  $ hg tglog
+  o  2 [public:0a76dfb2e179]
+  |
+  o  1 [public:2b9c7234e035]
+  |
+  @  0 [public:6cee5c8f3e5b]
+  
+
+
+