--- a/mercurial/changegroup.py	Fri Jan 15 01:30:08 2021 +0100
+++ b/mercurial/changegroup.py	Tue Jan 19 00:20:53 2021 +0100
@@ -323,7 +323,10 @@
                     cgnodes.append(node)
 
             def onchangelog(cl, node):
-                efilesset.update(cl.readfiles(node))
+                rev = cl.rev(node)
+                ctx = cl.changelogrevision(rev)
+                efilesset.update(ctx.files)
+                repo.register_changeset(rev, ctx)
 
             self.changelogheader()
             deltas = self.deltaiter()

--- a/mercurial/commit.py	Fri Jan 15 01:30:08 2021 +0100
+++ b/mercurial/commit.py	Tue Jan 19 00:20:53 2021 +0100
@@ -97,6 +97,9 @@
             extra,
         )
         rev = repo[n].rev()
+        if oldtip != repo.changelog.tiprev():
+            repo.register_changeset(rev, repo.changelog.changelogrevision(rev))
+
         xp1, xp2 = p1.hex(), p2 and p2.hex() or b''
         repo.hook(
             b'pretxncommit',

--- a/mercurial/exchangev2.py	Fri Jan 15 01:30:08 2021 +0100
+++ b/mercurial/exchangev2.py	Tue Jan 19 00:20:53 2021 +0100
@@ -372,6 +372,8 @@
         # so we can set the linkrev accordingly when manifests are added.
         manifestnodes[rev] = revision.manifest
 
+        repo.register_changeset(rev, revision)
+
     nodesbyphase = {phase: set() for phase in phases.phasenames.values()}
     remotebookmarks = {}
 

--- a/mercurial/interfaces/repository.py	Fri Jan 15 01:30:08 2021 +0100
+++ b/mercurial/interfaces/repository.py	Tue Jan 19 00:20:53 2021 +0100
@@ -1641,6 +1641,14 @@
     def revbranchcache():
         pass
 
+    def register_changeset(rev, changelogrevision):
+        """Extension point for caches for new nodes.
+
+        Multiple consumers are expected to need parts of the changelogrevision,
+        so it is provided as optimization to avoid duplicate lookups. A simple
+        cache would be fragile when other revisions are accessed, too."""
+        pass
+
     def branchtip(branchtip, ignoremissing=False):
         """Return the tip node for a given branch."""
 

--- a/mercurial/localrepo.py	Fri Jan 15 01:30:08 2021 +0100
+++ b/mercurial/localrepo.py	Tue Jan 19 00:20:53 2021 +0100
@@ -2062,6 +2062,9 @@
             self._revbranchcache = branchmap.revbranchcache(self.unfiltered())
         return self._revbranchcache
 
+    def register_changeset(self, rev, changelogrevision):
+        pass
+
     def branchtip(self, branch, ignoremissing=False):
         """return the tip node for a given branch