getbundle: add a ``cg`` boolean argument to control changegroup inclusion

The ``getbundle`` function was initially design to return a changegroup bundle.
However, bundle2 allows transmitting a wide range of data. Some bundle2
requests may not include a changegroup at all.

Before this changeset, the client would request a changegroup for
``heads=[nullid]`` and receive an empty changegroup.

We introduce an official boolean parameter, ``cg``, that can be set
to false to disable changegroup generation on getbundle. A new bundle2
capability is introduced to let the client know.

mercurial/exchange.py

@@ -726,9 +726,13 b' def getbundle(repo, source, heads=None, '
     The implementation is at a very early stage and will get massive rework
     when the API of bundle is refined.
     """
-    # build changegroup bundle here.
-    cg = changegroup.getbundle(repo, source, heads=heads,
-                               common=common, bundlecaps=bundlecaps)
+    cg = None
+    if kwargs.get('cg', True):
+        # build changegroup bundle here.
+        cg = changegroup.getbundle(repo, source, heads=heads,
+                                   common=common, bundlecaps=bundlecaps)
+    elif 'HG2X' not in bundlecaps:
+        raise ValueError(_('request for bundle10 must include changegroup'))
     if bundlecaps is None or 'HG2X' not in bundlecaps:
         if kwargs:
             raise ValueError(_('unsupported getbundle arguments: %s')

mercurial/localrepo.py

@@ -182,7 +182,9 b' class localrepository(object):'
 
     bundle2caps = {'HG2X': (),
                    'b2x:listkeys': (),
-                   'b2x:pushkey': ()}
+                   'b2x:pushkey': (),
+                   'b2x:changegroup': (),
+                  }
 
     # a list of (ui, featureset) functions.
     # only functions defined in module of enabled extensions are invoked

mercurial/wireproto.py

@@ -203,7 +203,8 b' def unescapearg(escaped):'
 gboptsmap = {'heads':  'nodes',
              'common': 'nodes',
              'bundlecaps': 'csv',
-             'listkeys': 'csv'}
+             'listkeys': 'csv',
+             'cg': 'boolean'}
 
 # client side