diff --git a/mercurial/changegroup.py b/mercurial/changegroup.py
--- a/mercurial/changegroup.py
+++ b/mercurial/changegroup.py
@@ -626,7 +626,7 @@
             'treemanifest' not in repo.requirements)
 
         for chunk in self.generatemanifests(commonrevs, clrevorder,
-                fastpathlinkrev, mfs, fnodes):
+                fastpathlinkrev, mfs, fnodes, source):
             yield chunk
         mfs.clear()
         clrevs = set(cl.rev(x) for x in clnodes)
@@ -652,7 +652,12 @@
             repo.hook('outgoing', node=hex(clnodes[0]), source=source)
 
     def generatemanifests(self, commonrevs, clrevorder, fastpathlinkrev, mfs,
-                          fnodes):
+                          fnodes, source):
+        """Returns an iterator of changegroup chunks containing manifests.
+
+        `source` is unused here, but is used by extensions like remotefilelog to
+        change what is sent based in pulls vs pushes, etc.
+        """
         repo = self._repo
         mfl = repo.manifestlog
         dirlog = mfl._revlog.dirlog