diff -r 2f357d053df2 -r d90f439ff19f mercurial/setdiscovery.py
--- a/mercurial/setdiscovery.py	Fri Dec 11 13:39:56 2020 -0800
+++ b/mercurial/setdiscovery.py	Fri Dec 11 12:51:09 2020 +0100
@@ -291,9 +291,14 @@
     abortwhenunrelated=True,
     ancestorsof=None,
     samplegrowth=1.05,
+    audit=None,
 ):
     """Return a tuple (common, anyincoming, remoteheads) used to identify
     missing nodes from or in remote.
+
+    The audit argument is an optional dictionnary that a caller can pass. it
+    will be updated with extra data about the discovery, this is useful for
+    debug.
     """
     start = util.timer()
 
@@ -379,6 +384,9 @@
 
     srvheadhashes, yesno = fheads.result(), fknown.result()
 
+    if audit is not None:
+        audit[b'total-roundtrips'] = 1
+
     if cl.tip() == nullid:
         if srvheadhashes != [nullid]:
             return [nullid], True, srvheadhashes
@@ -473,6 +481,9 @@
     missing = set(result) - set(knownsrvheads)
     ui.log(b'discovery', msg, len(result), len(missing), roundtrips, elapsed)
 
+    if audit is not None:
+        audit[b'total-roundtrips'] = roundtrips
+
     if not result and srvheadhashes != [nullid]:
         if abortwhenunrelated:
             raise error.Abort(_(b"repository is unrelated"))