nodemap: add a (python) index class for persistent nodemap testing

Using the persistent nodemap require a compeling performance boost and an
existing implementation. The benefit of the persistent nodemap for pure python
code is unclear and we don't have a C implementation for it. Yet we would like
to actually start testing it in more details and define an API for using that
persistent nodemap.

We introduce a new `devel` config option to use an index class dedicated to
Nodemap Testing. This feature is "pure" only because having using a pure-python
index with the `cext` policy proved more difficult than I would like.

There is nothing going on in that class for now, but the coming changeset will
change that.

Differential Revision: https://phab.mercurial-scm.org/D7840

mercurial/configitems.py

@@ -406,6 +406,9 b' coreconfigitem('
     b'devel', b'legacy.exchange', default=list,
 )
 coreconfigitem(
+    b'devel', b'persistent-nodemap', default=False,
+)
+coreconfigitem(
     b'devel', b'servercafile', default=b'',
 )
 coreconfigitem(

mercurial/localrepo.py

@@ -934,6 +934,8 b' def resolverevlogstorevfsoptions(ui, req'
         options[b'rust.index'] = True
     if ui.configbool(b'experimental', b'exp-persistent-nodemap'):
         options[b'exp-persistent-nodemap'] = True
+    if ui.configbool(b'devel', b'persistent-nodemap'):
+        options[b'devel-force-nodemap'] = True
 
     return options
 

mercurial/pure/parsers.py

@@ -141,6 +141,15 b' class IndexObject(BaseIndexObject):'
             self._extra = self._extra[: i - self._lgt]
 
 
+class PersistentNodeMapIndexObject(IndexObject):
+    """a Debug oriented class to test persistent nodemap
+
+    We need a simple python object to test API and higher level behavior. See
+    the Rust implementation for  more serious usage. This should be used only
+    through the dedicated `devel.persistent-nodemap` config.
+    """
+
+
 class InlinedIndexObject(BaseIndexObject):
     def __init__(self, data, inline=0):
         self._data = data
@@ -188,6 +197,12 b' def parse_index2(data, inline):'
     return InlinedIndexObject(data, inline), (0, data)
 
 
+def parse_index_devel_nodemap(data, inline):
+    """like parse_index2, but alway return a PersistentNodeMapIndexObject
+    """
+    return PersistentNodeMapIndexObject(data), None
+
+
 def parse_dirstate(dmap, copymap, st):
     parents = [st[:20], st[20:40]]
     # dereference fields so they will be local in loop

mercurial/revlog.py

@@ -352,6 +352,21 b' class revlogio(object):'
         return p
 
 
+NodemapRevlogIO = None
+
+if util.safehasattr(parsers, 'parse_index_devel_nodemap'):
+
+    class NodemapRevlogIO(revlogio):
+        """A debug oriented IO class that return a PersistentNodeMapIndexObject
+
+        The PersistentNodeMapIndexObject object is meant to test the persistent nodemap feature.
+        """
+
+        def parseindex(self, data, inline):
+            index, cache = parsers.parse_index_devel_nodemap(data, inline)
+            return index, cache
+
+
 class rustrevlogio(revlogio):
     def parseindex(self, data, inline):
         index, cache = super(rustrevlogio, self).parseindex(data, inline)
@@ -596,9 +611,17 b' class revlog(object):'
 
         self._storedeltachains = True
 
+        devel_nodemap = (
+            self.nodemap_file
+            and opts.get(b'devel-force-nodemap', False)
+            and NodemapRevlogIO is not None
+        )
+
         self._io = revlogio()
         if self.version == REVLOGV0:
             self._io = revlogoldio()
+        elif devel_nodemap:
+            self._io = NodemapRevlogIO()
         elif rustrevlog is not None and self.opener.options.get(b'rust.index'):
             self._io = rustrevlogio()
         try:

tests/test-persistent-nodemap.t

@@ -8,6 +8,8 b' Test the persistent on-disk nodemap'
   $ cat << EOF >> .hg/hgrc
   > [experimental]
   > exp-persistent-nodemap=yes
+  > [devel]
+  > persistent-nodemap=yes
   > EOF
   $ hg debugbuilddag .+5000
   $ f --size .hg/store/00changelog.n