nodemap: gate the feature behind a new requirement

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 However having out of date cache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/mercurial/localrepo.py	Tue Apr 14 03:05:54 2020 +0200
+++ b/mercurial/localrepo.py	Tue Apr 14 03:16:23 2020 +0200
@@ -445,6 +445,9 @@
 # copies related information in changeset's sidedata.
 COPIESSDC_REQUIREMENT = b'exp-copies-sidedata-changeset'
 
+# The repository use persistent nodemap for the changelog and the manifest.
+NODEMAP_REQUIREMENT = b'persistent-nodemap'
+
 # Functions receiving (ui, features) that extensions can register to impact
 # the ability to load repositories with custom requirements. Only
 # functions defined in loaded extensions are called.
@@ -933,7 +936,7 @@
 
     if ui.configbool(b'experimental', b'rust.index'):
         options[b'rust.index'] = True
-    if ui.configbool(b'experimental', b'exp-persistent-nodemap'):
+    if NODEMAP_REQUIREMENT in requirements:
         options[b'exp-persistent-nodemap'] = True
     if ui.configbool(b'experimental', b'exp-persistent-nodemap.mmap'):
         options[b'exp-persistent-nodemap.mmap'] = True
@@ -1023,6 +1026,7 @@
         REVLOGV2_REQUIREMENT,
         SIDEDATA_REQUIREMENT,
         SPARSEREVLOG_REQUIREMENT,
+        NODEMAP_REQUIREMENT,
         bookmarks.BOOKMARKS_IN_STORE_REQUIREMENT,
     }
     _basesupported = supportedformats | {
@@ -3660,6 +3664,9 @@
     if ui.configbool(b'format', b'bookmarks-in-store'):
         requirements.add(bookmarks.BOOKMARKS_IN_STORE_REQUIREMENT)
 
+    if ui.configbool(b'experimental', b'exp-persistent-nodemap'):
+        requirements.add(NODEMAP_REQUIREMENT)
+
     return requirements
 
 

--- 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 !)