nodemap: add a optional `nodemap_add_full` method on indexes

This method can be used to obtains persistent data for a full nodemap. The end
goal is for some index implementation to managed the nodemap serialization them
selves (eg: the rust implementation)

Differential Revision: https://phab.mercurial-scm.org/D7841

mercurial/pure/parsers.py

@@ -149,6 +149,13 b' class PersistentNodeMapIndexObject(Index'
     through the dedicated `devel.persistent-nodemap` config.
     """
 
+    def nodemap_data_all(self):
+        """Return bytes containing a full serialization of a nodemap
+
+        The nodemap should be valid for the full set of revisions in the
+        index."""
+        return nodemaputil.persistent_data(self)
+
 
 class InlinedIndexObject(BaseIndexObject):
     def __init__(self, data, inline=0):

mercurial/revlogutils/nodemap.py

@@ -15,7 +15,7 b' import struct'
 from .. import (
     error,
     node as nodemod,
-    pycompat,
+    util,
 )
 
 
@@ -69,6 +69,9 b' def _persist_nodemap(tr, revlog):'
     if revlog.nodemap_file is None:
         msg = "calling persist nodemap on a revlog without the feature enableb"
         raise error.ProgrammingError(msg)
+    if util.safehasattr(revlog.index, "nodemap_data_all"):
+        data = revlog.index.nodemap_data_all()
+    else:
     data = persistent_data(revlog.index)
     uid = _make_uid()
     datafile = _rawdata_filepath(revlog, uid)