--- a/mercurial/changegroup.py	Tue May 04 05:18:57 2021 +0200
+++ b/mercurial/changegroup.py	Mon Mar 29 22:40:54 2021 +0200
@@ -295,7 +295,10 @@
         # Only useful if we're adding sidedata categories. If both peers have
         # the same categories, then we simply don't do anything.
         adding_sidedata = (
-            requirements.REVLOGV2_REQUIREMENT in repo.requirements
+            (
+                requirements.REVLOGV2_REQUIREMENT in repo.requirements
+                or requirements.CHANGELOGV2_REQUIREMENT in repo.requirements
+            )
             and self.version == b'04'
             and srctype == b'pull'
         )
@@ -1723,6 +1726,7 @@
     want_v4 = (
         repo.ui.configbool(b'experimental', b'changegroup4')
         or requirements.REVLOGV2_REQUIREMENT in repo.requirements
+        or requirements.CHANGELOGV2_REQUIREMENT in repo.requirements
     )
     if not want_v4:
         versions.discard(b'04')

--- a/mercurial/configitems.py	Tue May 04 05:18:57 2021 +0200
+++ b/mercurial/configitems.py	Mon Mar 29 22:40:54 2021 +0200
@@ -1341,6 +1341,21 @@
     default=lambda: [b'zstd', b'zlib'],
     alias=[(b'experimental', b'format.compression')],
 )
+# Experimental TODOs:
+#
+# * Same as for evlogv2 (but for the reduction of the number of files)
+# * drop the storage of the base
+# * Improvement to investigate
+#   - storing .hgtags fnode
+#   - storing `rank` of changesets
+#   - storing branch related identifier
+
+coreconfigitem(
+    b'format',
+    b'exp-use-changelog-v2',
+    default=None,
+    experimental=True,
+)
 coreconfigitem(
     b'format',
     b'usefncache',

--- a/mercurial/localrepo.py	Tue May 04 05:18:57 2021 +0200
+++ b/mercurial/localrepo.py	Mon Mar 29 22:40:54 2021 +0200
@@ -737,7 +737,10 @@
     storevfs = store.vfs
     storevfs.options = resolvestorevfsoptions(ui, requirements, features)
 
-    if requirementsmod.REVLOGV2_REQUIREMENT in requirements:
+    if (
+        requirementsmod.REVLOGV2_REQUIREMENT in requirements
+        or requirementsmod.CHANGELOGV2_REQUIREMENT in requirements
+    ):
         features.add(repository.REPO_FEATURE_SIDE_DATA)
         # the revlogv2 docket introduced race condition that we need to fix
         features.discard(repository.REPO_FEATURE_STREAM_CLONE)
@@ -1021,6 +1024,8 @@
         options[b'revlogv1'] = True
     if requirementsmod.REVLOGV2_REQUIREMENT in requirements:
         options[b'revlogv2'] = True
+    if requirementsmod.CHANGELOGV2_REQUIREMENT in requirements:
+        options[b'changelogv2'] = True
 
     if requirementsmod.GENERALDELTA_REQUIREMENT in requirements:
         options[b'generaldelta'] = True
@@ -1220,6 +1225,7 @@
         requirementsmod.TREEMANIFEST_REQUIREMENT,
         requirementsmod.COPIESSDC_REQUIREMENT,
         requirementsmod.REVLOGV2_REQUIREMENT,
+        requirementsmod.CHANGELOGV2_REQUIREMENT,
         requirementsmod.SPARSEREVLOG_REQUIREMENT,
         requirementsmod.NODEMAP_REQUIREMENT,
         bookmarks.BOOKMARKS_IN_STORE_REQUIREMENT,
@@ -3529,6 +3535,10 @@
     if ui.configbool(b'experimental', b'treemanifest'):
         requirements.add(requirementsmod.TREEMANIFEST_REQUIREMENT)
 
+    changelogv2 = ui.config(b'format', b'exp-use-changelog-v2')
+    if changelogv2 == b'enable-unstable-format-and-corrupt-my-data':
+        requirements.add(requirementsmod.CHANGELOGV2_REQUIREMENT)
+
     revlogv2 = ui.config(b'experimental', b'revlogv2')
     if revlogv2 == b'enable-unstable-format-and-corrupt-my-data':
         requirements.discard(requirementsmod.REVLOGV1_REQUIREMENT)

--- a/mercurial/requirements.py	Tue May 04 05:18:57 2021 +0200
+++ b/mercurial/requirements.py	Mon Mar 29 22:40:54 2021 +0200
@@ -30,6 +30,10 @@
 
 # Increment the sub-version when the revlog v2 format changes to lock out old
 # clients.
+CHANGELOGV2_REQUIREMENT = b'exp-changelog-v2'
+
+# Increment the sub-version when the revlog v2 format changes to lock out old
+# clients.
 REVLOGV2_REQUIREMENT = b'exp-revlogv2.2'
 
 # A repository with the sparserevlog feature will have delta chains that

--- a/mercurial/revlog.py	Tue May 04 05:18:57 2021 +0200
+++ b/mercurial/revlog.py	Mon Mar 29 22:40:54 2021 +0200
@@ -42,6 +42,7 @@
     FLAG_GENERALDELTA,
     FLAG_INLINE_DATA,
     INDEX_HEADER,
+    KIND_CHANGELOG,
     REVLOGV0,
     REVLOGV1,
     REVLOGV1_FLAGS,
@@ -458,7 +459,9 @@
         mmapindexthreshold = None
         opts = self.opener.options
 
-        if b'revlogv2' in opts:
+        if b'changelogv2' in opts and self.revlog_kind == KIND_CHANGELOG:
+            new_header = REVLOGV2
+        elif b'revlogv2' in opts:
             new_header = REVLOGV2
         elif b'revlogv1' in opts:
             new_header = REVLOGV1 | FLAG_INLINE_DATA

--- a/mercurial/upgrade_utils/actions.py	Tue May 04 05:18:57 2021 +0200
+++ b/mercurial/upgrade_utils/actions.py	Mon Mar 29 22:40:54 2021 +0200
@@ -373,6 +373,15 @@
 
 
 @registerformatvariant
+class changelogv2(requirementformatvariant):
+    name = b'changelog-v2'
+    _requirement = requirements.CHANGELOGV2_REQUIREMENT
+    default = False
+    description = _(b'An iteration of the revlog focussed on changelog needs.')
+    upgrademessage = _(b'quite experimental')
+
+
+@registerformatvariant
 class removecldeltachain(formatvariant):
     name = b'plain-cl-delta'
 

--- a/tests/test-copies-chain-merge.t	Tue May 04 05:18:57 2021 +0200
+++ b/tests/test-copies-chain-merge.t	Mon Mar 29 22:40:54 2021 +0200
@@ -1660,6 +1660,7 @@
   persistent-nodemap: yes    yes      no (rust !)
   copies-sdc:          no    yes      no
   revlog-v2:           no    yes      no
+  changelog-v2:        no     no      no
   plain-cl-delta:     yes    yes     yes
   compression:        * (glob)
   compression-level:  default default default
@@ -1699,6 +1700,7 @@
   persistent-nodemap: yes    yes      no (rust !)
   copies-sdc:          no    yes      no
   revlog-v2:           no    yes      no
+  changelog-v2:        no     no      no
   plain-cl-delta:     yes    yes     yes
   compression:        * (glob)
   compression-level:  default default default

--- a/tests/test-copies-in-changeset.t	Tue May 04 05:18:57 2021 +0200
+++ b/tests/test-copies-in-changeset.t	Mon Mar 29 22:40:54 2021 +0200
@@ -43,6 +43,7 @@
   persistent-nodemap: yes    yes      no (rust !)
   copies-sdc:         yes    yes      no
   revlog-v2:          yes    yes      no
+  changelog-v2:        no     no      no
   plain-cl-delta:     yes    yes     yes
   compression:        zlib   zlib    zlib (no-zstd !)
   compression:        zstd   zstd    zstd (zstd !)
@@ -59,6 +60,7 @@
   persistent-nodemap: yes    yes      no (rust !)
   copies-sdc:          no     no      no
   revlog-v2:           no     no      no
+  changelog-v2:        no     no      no
   plain-cl-delta:     yes    yes     yes
   compression:        zlib   zlib    zlib (no-zstd !)
   compression:        zstd   zstd    zstd (zstd !)
@@ -432,6 +434,7 @@
   persistent-nodemap: yes    yes      no (rust !)
   copies-sdc:         yes    yes      no
   revlog-v2:          yes    yes      no
+  changelog-v2:        no     no      no
   plain-cl-delta:     yes    yes     yes
   compression:        zlib   zlib    zlib (no-zstd !)
   compression:        zstd   zstd    zstd (zstd !)
@@ -461,6 +464,7 @@
   persistent-nodemap: yes    yes      no (rust !)
   copies-sdc:          no     no      no
   revlog-v2:          yes    yes      no
+  changelog-v2:        no     no      no
   plain-cl-delta:     yes    yes     yes
   compression:        zlib   zlib    zlib (no-zstd !)
   compression:        zstd   zstd    zstd (zstd !)
@@ -487,6 +491,7 @@
   persistent-nodemap: yes    yes      no (rust !)
   copies-sdc:         yes    yes      no
   revlog-v2:          yes    yes      no
+  changelog-v2:        no     no      no
   plain-cl-delta:     yes    yes     yes
   compression:        zlib   zlib    zlib (no-zstd !)
   compression:        zstd   zstd    zstd (zstd !)

--- a/tests/test-persistent-nodemap.t	Tue May 04 05:18:57 2021 +0200
+++ b/tests/test-persistent-nodemap.t	Mon Mar 29 22:40:54 2021 +0200
@@ -64,6 +64,7 @@
   persistent-nodemap: yes
   copies-sdc:          no
   revlog-v2:           no
+  changelog-v2:        no
   plain-cl-delta:     yes
   compression:        zlib (no-zstd !)
   compression:        zstd (zstd !)
@@ -583,6 +584,7 @@
   persistent-nodemap: yes     no      no
   copies-sdc:          no     no      no
   revlog-v2:           no     no      no
+  changelog-v2:        no     no      no
   plain-cl-delta:     yes    yes     yes
   compression:        zlib   zlib    zlib (no-zstd !)
   compression:        zstd   zstd    zstd (zstd !)
@@ -630,6 +632,7 @@
   persistent-nodemap:  no    yes      no
   copies-sdc:          no     no      no
   revlog-v2:           no     no      no
+  changelog-v2:        no     no      no
   plain-cl-delta:     yes    yes     yes
   compression:        zlib   zlib    zlib (no-zstd !)
   compression:        zstd   zstd    zstd (zstd !)

--- a/tests/test-sidedata.t	Tue May 04 05:18:57 2021 +0200
+++ b/tests/test-sidedata.t	Mon Mar 29 22:40:54 2021 +0200
@@ -60,6 +60,7 @@
   persistent-nodemap: yes    yes      no (rust !)
   copies-sdc:          no     no      no
   revlog-v2:           no     no      no
+  changelog-v2:        no     no      no
   plain-cl-delta:     yes    yes     yes
   compression:        zlib   zlib    zlib (no-zstd !)
   compression:        zstd   zstd    zstd (zstd !)
@@ -75,6 +76,7 @@
   persistent-nodemap: yes    yes      no (rust !)
   copies-sdc:          no     no      no
   revlog-v2:           no    yes      no
+  changelog-v2:        no     no      no
   plain-cl-delta:     yes    yes     yes
   compression:        zlib   zlib    zlib (no-zstd !)
   compression:        zstd   zstd    zstd (zstd !)
@@ -96,6 +98,7 @@
   persistent-nodemap: yes    yes      no (rust !)
   copies-sdc:          no     no      no
   revlog-v2:          yes     no      no
+  changelog-v2:        no     no      no
   plain-cl-delta:     yes    yes     yes
   compression:        zlib   zlib    zlib (no-zstd !)
   compression:        zstd   zstd    zstd (zstd !)
@@ -111,6 +114,7 @@
   persistent-nodemap: yes    yes      no (rust !)
   copies-sdc:          no     no      no
   revlog-v2:          yes     no      no
+  changelog-v2:        no     no      no
   plain-cl-delta:     yes    yes     yes
   compression:        zlib   zlib    zlib (no-zstd !)
   compression:        zstd   zstd    zstd (zstd !)

--- a/tests/test-transaction-safety.t	Tue May 04 05:18:57 2021 +0200
+++ b/tests/test-transaction-safety.t	Mon Mar 29 22:40:54 2021 +0200
@@ -1,7 +1,7 @@
 Test transaction safety
 =======================
 
-#testcases revlogv1 revlogv2
+#testcases revlogv1 revlogv2 changelogv2
 
 #if revlogv1
 
@@ -21,6 +21,15 @@
 
 #endif
 
+#if changelogv2
+
+  $ cat << EOF >> $HGRCPATH
+  > [format]
+  > exp-use-changelog-v2=enable-unstable-format-and-corrupt-my-data
+  > EOF
+
+#endif
+
 This test basic case to make sure external process do not see transaction
 content until it is committed.
 

--- a/tests/test-upgrade-repo.t	Tue May 04 05:18:57 2021 +0200
+++ b/tests/test-upgrade-repo.t	Mon Mar 29 22:40:54 2021 +0200
@@ -65,6 +65,7 @@
   persistent-nodemap: yes (rust !)
   copies-sdc:          no
   revlog-v2:           no
+  changelog-v2:        no
   plain-cl-delta:     yes
   compression:        zlib
   compression-level:  default
@@ -79,6 +80,7 @@
   persistent-nodemap: yes    yes      no (rust !)
   copies-sdc:          no     no      no
   revlog-v2:           no     no      no
+  changelog-v2:        no     no      no
   plain-cl-delta:     yes    yes     yes
   compression:        zlib   zlib    zlib (no-zstd !)
   compression:        zlib   zlib    zstd (zstd !)
@@ -94,6 +96,7 @@
   persistent-nodemap: yes    yes      no (rust !)
   copies-sdc:          no     no      no
   revlog-v2:           no     no      no
+  changelog-v2:        no     no      no
   plain-cl-delta:     yes    yes     yes
   compression:        zlib   zlib    zlib (no-zstd !)
   compression:        zlib   zlib    zstd (zstd !)
@@ -109,6 +112,7 @@
   [formatvariant.name.mismatchdefault|persistent-nodemap:][formatvariant.repo.mismatchdefault| yes][formatvariant.config.special|    yes][formatvariant.default|      no] (rust !)
   [formatvariant.name.uptodate|copies-sdc:        ][formatvariant.repo.uptodate|  no][formatvariant.config.default|     no][formatvariant.default|      no]
   [formatvariant.name.uptodate|revlog-v2:         ][formatvariant.repo.uptodate|  no][formatvariant.config.default|     no][formatvariant.default|      no]
+  [formatvariant.name.uptodate|changelog-v2:      ][formatvariant.repo.uptodate|  no][formatvariant.config.default|     no][formatvariant.default|      no]
   [formatvariant.name.uptodate|plain-cl-delta:    ][formatvariant.repo.uptodate| yes][formatvariant.config.default|    yes][formatvariant.default|     yes]
   [formatvariant.name.uptodate|compression:       ][formatvariant.repo.uptodate| zlib][formatvariant.config.default|   zlib][formatvariant.default|    zlib] (no-zstd !)
   [formatvariant.name.mismatchdefault|compression:       ][formatvariant.repo.mismatchdefault| zlib][formatvariant.config.special|   zlib][formatvariant.default|    zstd] (zstd !)
@@ -166,6 +170,12 @@
     "repo": false
    },
    {
+    "config": false,
+    "default": false,
+    "name": "changelog-v2",
+    "repo": false
+   },
+   {
     "config": true,
     "default": true,
     "name": "plain-cl-delta",
@@ -324,6 +334,7 @@
   persistent-nodemap:  no
   copies-sdc:          no
   revlog-v2:           no
+  changelog-v2:        no
   plain-cl-delta:     yes
   compression:        zlib
   compression-level:  default
@@ -338,6 +349,7 @@
   persistent-nodemap:  no    yes      no (rust !)
   copies-sdc:          no     no      no
   revlog-v2:           no     no      no
+  changelog-v2:        no     no      no
   plain-cl-delta:     yes    yes     yes
   compression:        zlib   zlib    zlib (no-zstd !)
   compression:        zlib   zlib    zstd (zstd !)
@@ -353,6 +365,7 @@
   persistent-nodemap:  no    yes      no (rust !)
   copies-sdc:          no     no      no
   revlog-v2:           no     no      no
+  changelog-v2:        no     no      no
   plain-cl-delta:     yes    yes     yes
   compression:        zlib   zlib    zlib (no-zstd !)
   compression:        zlib   zlib    zstd (zstd !)
@@ -368,6 +381,7 @@
   [formatvariant.name.mismatchconfig|persistent-nodemap:][formatvariant.repo.mismatchconfig|  no][formatvariant.config.special|    yes][formatvariant.default|      no] (rust !)
   [formatvariant.name.uptodate|copies-sdc:        ][formatvariant.repo.uptodate|  no][formatvariant.config.default|     no][formatvariant.default|      no]
   [formatvariant.name.uptodate|revlog-v2:         ][formatvariant.repo.uptodate|  no][formatvariant.config.default|     no][formatvariant.default|      no]
+  [formatvariant.name.uptodate|changelog-v2:      ][formatvariant.repo.uptodate|  no][formatvariant.config.default|     no][formatvariant.default|      no]
   [formatvariant.name.uptodate|plain-cl-delta:    ][formatvariant.repo.uptodate| yes][formatvariant.config.default|    yes][formatvariant.default|     yes]
   [formatvariant.name.uptodate|compression:       ][formatvariant.repo.uptodate| zlib][formatvariant.config.default|   zlib][formatvariant.default|    zlib] (no-zstd !)
   [formatvariant.name.mismatchdefault|compression:       ][formatvariant.repo.mismatchdefault| zlib][formatvariant.config.special|   zlib][formatvariant.default|    zstd] (zstd !)
@@ -1349,6 +1363,7 @@
   persistent-nodemap: yes    yes      no (rust !)
   copies-sdc:          no     no      no
   revlog-v2:           no     no      no
+  changelog-v2:        no     no      no
   plain-cl-delta:     yes    yes     yes
   compression:        zlib   zlib    zlib (no-zstd !)
   compression:        zstd   zlib    zstd (zstd !)
@@ -1389,6 +1404,7 @@
   persistent-nodemap: yes    yes      no (rust !)
   copies-sdc:          no     no      no
   revlog-v2:           no     no      no
+  changelog-v2:        no     no      no
   plain-cl-delta:     yes    yes     yes
   compression:        zlib   zlib    zlib (no-zstd !)
   compression:        zlib   zlib    zstd (zstd !)
@@ -1432,6 +1448,7 @@
   persistent-nodemap: yes    yes      no (rust !)
   copies-sdc:          no     no      no
   revlog-v2:           no     no      no
+  changelog-v2:        no     no      no
   plain-cl-delta:     yes    yes     yes
   compression:        zlib   zlib    zlib (no-zstd !)
   compression:        zstd   zstd    zstd (zstd !)
@@ -1481,6 +1498,7 @@
   persistent-nodemap: yes    yes      no (rust !)
   copies-sdc:          no     no      no
   revlog-v2:          yes     no      no
+  changelog-v2:        no     no      no
   plain-cl-delta:     yes    yes     yes
   compression:        zlib   zlib    zlib (no-zstd !)
   compression:        zstd   zstd    zstd (zstd !)
@@ -1527,6 +1545,7 @@
   persistent-nodemap: yes    yes      no (rust !)
   copies-sdc:          no     no      no
   revlog-v2:           no     no      no
+  changelog-v2:        no     no      no
   plain-cl-delta:     yes    yes     yes
   compression:        zlib   zlib    zlib (no-zstd !)
   compression:        zstd   zstd    zstd (zstd !)
@@ -1574,6 +1593,7 @@
   persistent-nodemap: yes    yes      no (rust !)
   copies-sdc:          no     no      no
   revlog-v2:          yes    yes      no
+  changelog-v2:        no     no      no
   plain-cl-delta:     yes    yes     yes
   compression:        zlib   zlib    zlib (no-zstd !)
   compression:        zstd   zstd    zstd (zstd !)