diff --git a/mercurial/revlogutils/nodemap.py b/mercurial/revlogutils/nodemap.py
--- a/mercurial/revlogutils/nodemap.py
+++ b/mercurial/revlogutils/nodemap.py
@@ -36,11 +36,11 @@
     if version != ONDISK_VERSION:
         return None
     offset += S_VERSION.size
-    (uuid_size,) = S_HEADER.unpack(pdata[offset : offset + S_HEADER.size])
+    (uid_size,) = S_HEADER.unpack(pdata[offset : offset + S_HEADER.size])
     offset += S_HEADER.size
-    uid = pdata[offset : offset + uuid_size]
+    docket = NodeMapDocket(pdata[offset : offset + uid_size])
 
-    filename = _rawdata_filepath(revlog, uid)
+    filename = _rawdata_filepath(revlog, docket)
     return revlog.opener.tryread(filename)
 
 
@@ -73,9 +73,9 @@
         data = revlog.index.nodemap_data_all()
     else:
         data = persistent_data(revlog.index)
-    uid = _make_uid()
-    datafile = _rawdata_filepath(revlog, uid)
-    olds = _other_rawdata_filepath(revlog, uid)
+    target_docket = NodeMapDocket()
+    datafile = _rawdata_filepath(revlog, target_docket)
+    olds = _other_rawdata_filepath(revlog, target_docket)
     if olds:
         realvfs = getattr(revlog, '_realopener', revlog.opener)
 
@@ -92,7 +92,7 @@
     # EXP-TODO: if this is a cache, this should use a cache vfs, not a
     # store vfs
     with revlog.opener(revlog.nodemap_file, b'w', atomictemp=True) as fp:
-        fp.write(_serialize_docket(uid))
+        fp.write(target_docket.serialize())
     # EXP-TODO: if the transaction abort, we should remove the new data and
     # reinstall the old one.
 
@@ -135,25 +135,39 @@
     return nodemod.hex(os.urandom(ID_SIZE))
 
 
-def _serialize_docket(uid):
-    """return serialized bytes for a docket using the passed uid"""
-    data = []
-    data.append(S_VERSION.pack(ONDISK_VERSION))
-    data.append(S_HEADER.pack(len(uid)))
-    data.append(uid)
-    return b''.join(data)
+class NodeMapDocket(object):
+    """metadata associated with persistent nodemap data
+
+    The persistent data may come from disk or be on their way to disk.
+    """
+
+    def __init__(self, uid=None):
+        if uid is None:
+            uid = _make_uid()
+        self.uid = uid
+
+    def copy(self):
+        return NodeMapDocket(uid=self.uid)
+
+    def serialize(self):
+        """return serialized bytes for a docket using the passed uid"""
+        data = []
+        data.append(S_VERSION.pack(ONDISK_VERSION))
+        data.append(S_HEADER.pack(len(self.uid)))
+        data.append(self.uid)
+        return b''.join(data)
 
 
-def _rawdata_filepath(revlog, uid):
+def _rawdata_filepath(revlog, docket):
     """The (vfs relative) nodemap's rawdata file for a given uid"""
     prefix = revlog.nodemap_file[:-2]
-    return b"%s-%s.nd" % (prefix, uid)
+    return b"%s-%s.nd" % (prefix, docket.uid)
 
 
-def _other_rawdata_filepath(revlog, uid):
+def _other_rawdata_filepath(revlog, docket):
     prefix = revlog.nodemap_file[:-2]
     pattern = re.compile(b"(^|/)%s-[0-9a-f]+\.nd$" % prefix)
-    new_file_path = _rawdata_filepath(revlog, uid)
+    new_file_path = _rawdata_filepath(revlog, docket)
     new_file_name = revlog.opener.basename(new_file_path)
     dirpath = revlog.opener.dirname(new_file_path)
     others = []