diff --git a/mercurial/wireproto.py b/mercurial/wireproto.py
--- a/mercurial/wireproto.py
+++ b/mercurial/wireproto.py
@@ -523,6 +523,10 @@ def pushkey(repo, proto, namespace, key,
 def _allowstream(ui):
     return ui.configbool('server', 'uncompressed', True, untrusted=True)
 
+def _walkstreamfiles(repo):
+    # this is it's own function so extensions can override it
+    return repo.store.walk()
+
 def stream(repo, proto):
     '''If the server supports streaming clone, it advertises the "stream"
     capability with a value representing the version and flags of the repo
@@ -544,7 +548,7 @@ def stream(repo, proto):
         lock = repo.lock()
         try:
             repo.ui.debug('scanning\n')
-            for name, ename, size in repo.store.walk():
+            for name, ename, size in _walkstreamfiles(repo):
                 if size:
                     entries.append((name, size))
                     total_bytes += size