nodemap: gate the feature behind a new requirement Now that the feature is working smoothly, a question was still open, should we gate the feature behind a new requirement or just treat it as a cache to be warmed by those who can and ignored by other. The advantage of using the cache approach is a transparent upgrade/downgrade story, making the feature easier to move to. However having out of date cache can come with a significant performance hit for process who expect an up to date cache but found none. In this case the file needs to be stored under `.hg/cache`. The "requirement" approach guarantee that the persistent nodemap is up to date. However, it comes with a less flexible activation story since an explicite upgrade is required. In this case the file can be stored in `.hg/store`. This wiki page is relevant to this questions: https://www.mercurial-scm.org/wiki/ComputedIndexPlan So which one should we take? Another element came into plan, the persistent nodemap use the `add` method of the transaction, it is used to keep track of a file content before a transaction in case we need to rollback it back. It turns out that the `transaction.add` API does not support file stored anywhere than `.hg/store`. Making it support file stored elsewhere is possible, require a change in on disk transaction format. Updating on disk file requires? introducing a new requirements. As a result, we pick the second option "gating the persistent nodemap behind a new requirements". Differential Revision: https://phab.mercurial-scm.org/D8417

--- a/tests/test-persistent-nodemap.t	Tue Apr 14 03:05:54 2020 +0200
+++ b/tests/test-persistent-nodemap.t	Tue Apr 14 03:16:23 2020 +0200
@@ -2,14 +2,14 @@
 Test the persistent on-disk nodemap
 ===================================
 
-  $ hg init test-repo
-  $ cd test-repo
-  $ cat << EOF >> .hg/hgrc
+  $ cat << EOF >> $HGRCPATH
   > [experimental]
   > exp-persistent-nodemap=yes
   > [devel]
   > persistent-nodemap=yes
   > EOF
+  $ hg init test-repo
+  $ cd test-repo
   $ 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 !)