diff -r 8b8a845c85fc -r 6b08cf6b900f mercurial/httppeer.py
--- a/mercurial/httppeer.py	Tue Apr 10 13:11:40 2018 -0700
+++ b/mercurial/httppeer.py	Tue Apr 10 13:07:13 2018 -0700
@@ -620,9 +620,12 @@
 
     return respurl, set(rawcaps.split())
 
-def makepeer(ui, path, requestbuilder=urlreq.request):
+def makepeer(ui, path, opener=None, requestbuilder=urlreq.request):
     """Construct an appropriate HTTP peer instance.
 
+    ``opener`` is an ``url.opener`` that should be used to establish
+    connections, perform HTTP requests.
+
     ``requestbuilder`` is the type used for constructing HTTP requests.
     It exists as an argument so extensions can override the default.
     """
@@ -635,7 +638,7 @@
     url, authinfo = u.authinfo()
     ui.debug('using %s\n' % url)
 
-    opener = urlmod.opener(ui, authinfo)
+    opener = opener or urlmod.opener(ui, authinfo)
 
     respurl, caps = performhandshake(ui, url, opener, requestbuilder)