# HG changeset patch
# User Pierre-Yves David <pierre-yves.david@octobus.net>
# Date 2020-04-14 00:45:05
# Node ID 6493f0a567c20f1d3ddde2b59e40e850a6be4c7d
# Parent  97ebdb192b0028e44f4b6d7236aaa4c9a837645a

nodemap: add a new mode option, with an optional "warn" value

When "warn" is set, user will get notified when the slow code, used for
compatibility is used. This can help people to detect situation were using that
feature will give them a slowdown instead of a speedup.

Differential Revision: https://phab.mercurial-scm.org/D8414

diff --git a/mercurial/configitems.py b/mercurial/configitems.py
--- a/mercurial/configitems.py
+++ b/mercurial/configitems.py
@@ -681,6 +681,9 @@ coreconfigitem(
     b'experimental', b'exp-persistent-nodemap.mmap', default=True,
 )
 coreconfigitem(
+    b'experimental', b'exp-persistent-nodemap.mode', default=b'compat',
+)
+coreconfigitem(
     b'experimental', b'server.filesdata.recommended-batch-size', default=50000,
 )
 coreconfigitem(
diff --git a/mercurial/localrepo.py b/mercurial/localrepo.py
--- a/mercurial/localrepo.py
+++ b/mercurial/localrepo.py
@@ -937,6 +937,8 @@ def resolverevlogstorevfsoptions(ui, req
         options[b'exp-persistent-nodemap'] = True
     if ui.configbool(b'experimental', b'exp-persistent-nodemap.mmap'):
         options[b'exp-persistent-nodemap.mmap'] = True
+    epnm = ui.config(b'experimental', b'exp-persistent-nodemap.mode')
+    options[b'exp-persistent-nodemap.mode'] = epnm
     if ui.configbool(b'devel', b'persistent-nodemap'):
         options[b'devel-force-nodemap'] = True
 
diff --git a/mercurial/revlogutils/nodemap.py b/mercurial/revlogutils/nodemap.py
--- a/mercurial/revlogutils/nodemap.py
+++ b/mercurial/revlogutils/nodemap.py
@@ -13,6 +13,8 @@ import os
 import re
 import struct
 
+from ..i18n import _
+
 from .. import (
     error,
     node as nodemod,
@@ -105,6 +107,9 @@ class _NoTransaction(object):
     def addabort(self, *args, **kwargs):
         pass
 
+    def _report(self, *args):
+        pass
+
 
 def update_persistent_nodemap(revlog):
     """update the persistent nodemap right now
@@ -138,6 +143,11 @@ def _persist_nodemap(tr, revlog, pending
     ondisk_docket = revlog._nodemap_docket
     feed_data = util.safehasattr(revlog.index, "update_nodemap_data")
     use_mmap = revlog.opener.options.get(b"exp-persistent-nodemap.mmap")
+    mode = revlog.opener.options.get(b"exp-persistent-nodemap.mode")
+    if not can_incremental:
+        msg = _(b"persistent nodemap in strict mode without efficient method")
+        if mode == b'warn':
+            tr._report(b"%s\n" % msg)
 
     data = None
     # first attemp an incremental update of the data
diff --git a/tests/test-persistent-nodemap.t b/tests/test-persistent-nodemap.t
--- a/tests/test-persistent-nodemap.t
+++ b/tests/test-persistent-nodemap.t
@@ -10,7 +10,9 @@ Test the persistent on-disk nodemap
   > [devel]
   > persistent-nodemap=yes
   > EOF
-  $ hg debugbuilddag .+5000 --new-file
+  $ hg debugbuilddag .+5000 --new-file --config "experimental.exp-persistent-nodemap.mode=warn"
+  persistent nodemap in strict mode without efficient method (no-rust no-pure !)
+  persistent nodemap in strict mode without efficient method (no-rust no-pure !)
   $ hg debugnodemap --metadata
   uid: ???????????????? (glob)
   tip-rev: 5000