$ 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 __future__ import absolute_import

  > from mercurial import bundle2, exchange, pushkey, util

  > def _newhandlechangegroup(op, inpart):

  >     """This function wraps the changegroup part handler for getbundle.

  >     It issues an additional pushkey part to send a new

  >     bookmark back to the client"""

  >     result = bundle2.handlechangegroup(op, inpart)

  >     if b'pushback' in op.reply.capabilities:

  >         params = {b'namespace': b'bookmarks',

  >                   b'key': b'new-server-mark',

  >                   b'old': b'',

  >                   b'new': b'tip'}

  >         encodedparams = [(k, pushkey.encode(v))

  >                           for (k, v) in params.items()]

  >         op.reply.newpart(b'pushkey', mandatoryparams=encodedparams)

  >     else:

  >         op.reply.newpart(b'output', data=b'pushback not enabled')

  >     return result

  > _newhandlechangegroup.params = bundle2.handlechangegroup.params

  > bundle2.parthandlermapping[b'changegroup'] = _newhandlechangegroup

  > EOF

  $ cat >> $HGRCPATH <<EOF

  > [ui]

  > 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

  > 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: adding changesets

  remote: adding manifests

  remote: adding file changes

  remote: added 1 changesets with 1 changes to 1 files

  remote: pushback not enabled

  $ 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]