--- a/CHANGELOG	Tue Feb 04 16:22:40 2020 -0800
+++ b/CHANGELOG	Wed Feb 19 01:35:23 2020 +0100
@@ -14,6 +14,7 @@
   * compat: compatibility with some changes of the upcoming Mercurial 5.3
   * evolve: add content divergence checking to the standard pre-rewrite check,
   * evolve: improve the message associated with content divergence.
+  * topic: add a `experimental.topic.server-gate-topic-changesets` config
 
 9.2.2 -- 2020-01-31
 -------------------

--- a/hgext3rd/topic/__init__.py	Tue Feb 04 16:22:40 2020 -0800
+++ b/hgext3rd/topic/__init__.py	Wed Feb 19 01:35:23 2020 +0100
@@ -110,6 +110,19 @@
     [experimental]
     topic.allow-publish = no
 
+Server side visibility
+======================
+
+Serving changesets with topics to clients without topic extension can get
+confusing. Such clients will have multiple anonymous heads without a clear way
+to distinguish them. They will also "lose" the canonical heads of the branch.
+
+To avoid this confusion, server can be configured to only serve changesets with
+topics to clients with the topic extension (version 9.3+). This might become
+the default in future::
+
+    [experimental]
+    topic.server-gate-topic-changesets = yes
 """
 
 from __future__ import absolute_import
@@ -155,6 +168,7 @@
     flow,
     randomname,
     revset as topicrevset,
+    server,
     stack,
     topicmap,
 )
@@ -222,6 +236,9 @@
     configitem(b'experimental', b'topic-mode.server',
                default=configitems.dynamicdefault,
     )
+    configitem(b'experimental', b'topic.server-gate-topic-changesets',
+               default=False,
+    )
 
     def extsetup(ui):
         # register config that strictly belong to other code (thg, core, etc)
@@ -364,6 +381,8 @@
     # Wrap changelog.add to drop empty topic
     extensions.wrapfunction(changelog.changelog, 'add', wrapadd)
 
+    server.setupserver(ui)
+
 def reposetup(ui, repo):
     if not isinstance(repo, localrepo.localrepository):
         return # this can be a peer in the ssh case (puzzling)

--- /dev/null	Thu Jan 01 00:00:00 1970 +0000
+++ b/hgext3rd/topic/server.py	Wed Feb 19 01:35:23 2020 +0100
@@ -0,0 +1,98 @@
+# topic/server.py - server specific behavior with topic
+#
+# This software may be used and distributed according to the terms of the
+# GNU General Public License version 2 or any later version.
+from mercurial import (
+    extensions,
+    repoview,
+    repoviewutil,
+    wireprototypes,
+    wireprotov1peer,
+    wireprotov1server,
+)
+
+from . import (
+    common,
+    constants,
+)
+
+### Visibility restriction
+#
+# Serving draft changesets with topics to clients without topic extension can
+# confuse them, because they won't see the topic label and will consider them
+# normal anonymous heads. Instead we have the option to not serve changesets
+# with topics to clients without topic support.
+#
+# To achieve this, we alter the behavior of the standard `heads` commands and
+# introduce a new `heads` command that only clients with topic will know about.
+
+# compat version of the wireprotocommand decorator, taken from evolve compat
+
+FILTERNAME = b'served-no-topic'
+
+def computeunservedtopic(repo, visibilityexceptions=None):
+    assert not repo.changelog.filteredrevs
+    filteredrevs = repoview.filtertable[b'served'](repo, visibilityexceptions).copy()
+    mutable = repoview.filtertable[b'immutable'](repo, visibilityexceptions)
+    consider = mutable - filteredrevs
+    cl = repo.changelog
+    extrafiltered = set()
+    for r in consider:
+        if cl.changelogrevision(r).extra.get(constants.extrakey, b''):
+            extrafiltered.add(r)
+    if extrafiltered:
+        filteredrevs = frozenset(filteredrevs | extrafiltered)
+    return filteredrevs
+
+def wireprotocommand(name, args=b'', permission=b'pull'):
+    try:
+        from mercurial.wireprotov1server import wireprotocommand
+    except (ImportError, AttributeError):
+        # hg <= 4.6 (b4d85bc122bd)
+        from mercurial.wireproto import wireprotocommand
+    return wireprotocommand(name, args, permission=permission)
+
+def wrapheads(orig, repo, proto):
+    """wrap head to hide topic^W draft changeset to old client"""
+    hidetopics = repo.ui.configbool(b'experimental', b'topic.server-gate-topic-changesets')
+    if common.hastopicext(repo) and hidetopics:
+        h = repo.filtered(FILTERNAME).heads()
+        return wireprototypes.bytesresponse(wireprototypes.encodelist(h) + b'\n')
+    return orig(repo, proto)
+
+def topicheads(repo, proto):
+    """Same as the normal wireprotocol command, but accessing with a different end point."""
+    h = repo.heads()
+    return wireprototypes.bytesresponse(wireprototypes.encodelist(h) + b'\n')
+
+def wireprotocaps(orig, repo, proto):
+    """advertise the new topic specific `head` command for client with topic"""
+    caps = orig(repo, proto)
+    if common.hastopicext(repo) and repo.peer().capable(b'topics'):
+        caps.append(b'_exttopics_heads')
+    return caps
+
+def setupserver(ui):
+    extensions.wrapfunction(wireprotov1server, 'heads', wrapheads)
+    wireprotov1server.commands.pop(b'heads')
+    wireprotocommand(b'heads', permission=b'pull')(wireprotov1server.heads)
+    wireprotocommand(b'_exttopics_heads', permission=b'pull')(topicheads)
+    extensions.wrapfunction(wireprotov1server, '_capabilities', wireprotocaps)
+
+    class topicpeerexecutor(wireprotov1peer.peerexecutor):
+
+        def callcommand(self, command, args):
+            if command == b'heads':
+                if self._peer.capable(b'_exttopics_heads'):
+                    command = b'_exttopics_heads'
+                    if getattr(self._peer, '_exttopics_heads', None) is None:
+                        self._peer._exttopics_heads = self._peer.heads
+            s = super(topicpeerexecutor, self)
+            return s.callcommand(command, args)
+
+    wireprotov1peer.peerexecutor = topicpeerexecutor
+
+    if FILTERNAME not in repoview.filtertable:
+        repoview.filtertable[FILTERNAME] = computeunservedtopic
+        repoviewutil.subsettable[FILTERNAME] = b'immutable'
+        repoviewutil.subsettable[b'served'] = FILTERNAME

--- a/tests/test-extension-isolation.t	Tue Feb 04 16:22:40 2020 -0800
+++ b/tests/test-extension-isolation.t	Wed Feb 19 01:35:23 2020 +0100
@@ -131,6 +131,7 @@
   $ hg debugcapabilities http://$LOCALIP:$HGPORT/repo-no-ext | egrep 'topics|evoext'
   [1]
   $ hg debugcapabilities http://$LOCALIP:$HGPORT/repo-topic | egrep 'topics|evoext'
+    _exttopics_heads
     topics
   $ hg debugcapabilities http://$LOCALIP:$HGPORT/repo-no-ext | egrep 'topics|evoext'
   [1]
@@ -143,11 +144,13 @@
   $ hg debugcapabilities http://$LOCALIP:$HGPORT/repo-both | egrep 'topics|evoext'
     _evoext_getbundle_obscommon
     _evoext_obshashrange_v1
+    _exttopics_heads
     topics
   $ hg debugcapabilities http://$LOCALIP:$HGPORT/repo-evo | egrep 'topics|evoext'
     _evoext_getbundle_obscommon
     _evoext_obshashrange_v1
   $ hg debugcapabilities http://$LOCALIP:$HGPORT/repo-topic | egrep 'topics|evoext'
+    _exttopics_heads
     topics
   $ hg debugcapabilities http://$LOCALIP:$HGPORT/repo-evo | egrep 'topics|evoext'
     _evoext_getbundle_obscommon

--- /dev/null	Thu Jan 01 00:00:00 1970 +0000
+++ b/tests/test-topic-server.t	Wed Feb 19 01:35:23 2020 +0100
@@ -0,0 +1,179 @@
+  $ . "$TESTDIR/testlib/topic_setup.sh"
+
+  $ cat << EOF >> $HGRCPATH
+  > [experimental]
+  > evolution = all
+  > topic.publish-bare-branch = yes
+  > topic.server-gate-topic-changesets = yes
+  > 
+  > [extensions]
+  > evolve =
+  > 
+  > [phases]
+  > publish = no
+  > 
+  > [ui]
+  > ssh = "$PYTHON" "$RUNTESTDIR/dummyssh"
+  > EOF
+
+  $ hg init server
+  $ cd server
+
+  $ echo a > a
+  $ hg commit -qAm root
+  $ hg phase --public -r 'all()'
+
+  $ cd ..
+
+  $ hg clone ssh://user@dummy/server client-topic1
+  requesting all changes
+  adding changesets
+  adding manifests
+  adding file changes
+  added 1 changesets with 1 changes to 1 files
+  new changesets 6569b5a81c7e
+  updating to branch default
+  1 files updated, 0 files merged, 0 files removed, 0 files unresolved
+  $ hg clone ssh://user@dummy/server client-topic2
+  requesting all changes
+  adding changesets
+  adding manifests
+  adding file changes
+  added 1 changesets with 1 changes to 1 files
+  new changesets 6569b5a81c7e
+  updating to branch default
+  1 files updated, 0 files merged, 0 files removed, 0 files unresolved
+  $ hg version -v -R client-topic1
+  warning: --repository ignored
+  Mercurial Distributed SCM (*) (glob)
+  (see https://mercurial-scm.org for more information)
+  
+  Copyright (C) 2005-* Matt Mackall and others (glob)
+  This is free software; see the source for copying conditions. There is NO
+  warranty; not even for MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.
+  
+  Enabled extensions:
+  
+    rebase  internal  
+    topic   external  * (glob)
+    evolve  external  * (glob)
+  $ hg clone ssh://user@dummy/server client-plain
+  requesting all changes
+  adding changesets
+  adding manifests
+  adding file changes
+  added 1 changesets with 1 changes to 1 files
+  new changesets 6569b5a81c7e
+  updating to branch default
+  1 files updated, 0 files merged, 0 files removed, 0 files unresolved
+  $ cat << EOF >> client-plain/.hg/hgrc
+  > [extensions]
+  > topic = !
+  > EOF
+  $ hg version -v -R client-plain
+  warning: --repository ignored
+  Mercurial Distributed SCM (*) (glob)
+  (see https://mercurial-scm.org for more information)
+  
+  Copyright (C) 2005-* Matt Mackall and others (glob)
+  This is free software; see the source for copying conditions. There is NO
+  warranty; not even for MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.
+  
+  Enabled extensions:
+  
+    rebase  internal  
+    evolve  external  * (glob)
+
+Make two commits, with and without a topic, and push them to the server
+
+  $ cd client-topic1
+  $ echo b > b
+  $ hg topic some-work
+  marked working directory as topic: some-work
+  $ hg commit -Am 'adding b (topic)'
+  adding b
+  active topic 'some-work' grew its first changeset
+  (see 'hg help topics' for more information)
+  $ hg up 0
+  0 files updated, 0 files merged, 1 files removed, 0 files unresolved
+  $ echo c > c
+  $ hg commit -Am 'adding c (no topic)'
+  adding c
+  $ hg log -r 'all() - 0'
+  changeset:   1:2a2e8b3520f2
+  topic:       some-work
+  user:        test
+  date:        Thu Jan 01 00:00:00 1970 +0000
+  summary:     adding b (topic)
+  
+  changeset:   2:b46feb4d24f9
+  tag:         tip
+  parent:      0:6569b5a81c7e
+  user:        test
+  date:        Thu Jan 01 00:00:00 1970 +0000
+  summary:     adding c (no topic)
+  
+
+(disable the bare publishing to put a bare draft on the server)
+
+  $ cat << EOF >> ../server/.hg/hgrc
+  > [experimental]
+  > topic.publish-bare-branch = no
+  > EOF
+  $ hg push
+  pushing to ssh://user@dummy/server
+  searching for changes
+  remote: adding changesets
+  remote: adding manifests
+  remote: adding file changes
+  remote: added 2 changesets with 2 changes to 2 files (+1 heads)
+  $ cd ..
+  $ hg --cwd server phase -r 'tip'
+  2: draft
+
+Clients with topic can exchange draft changesets both with and without a topic through the server
+
+  $ hg --cwd client-topic2 pull
+  pulling from ssh://user@dummy/server
+  searching for changes
+  adding changesets
+  adding manifests
+  adding file changes
+  added 2 changesets with 2 changes to 2 files (+1 heads)
+  new changesets 2a2e8b3520f2:b46feb4d24f9 (2 drafts)
+  (run 'hg heads' to see heads)
+  $ hg --cwd client-topic2 log -r 'all() - 0'
+  changeset:   1:2a2e8b3520f2
+  topic:       some-work
+  user:        test
+  date:        Thu Jan 01 00:00:00 1970 +0000
+  summary:     adding b (topic)
+  
+  changeset:   2:b46feb4d24f9
+  tag:         tip
+  parent:      0:6569b5a81c7e
+  user:        test
+  date:        Thu Jan 01 00:00:00 1970 +0000
+  summary:     adding c (no topic)
+  
+
+Client without topic only sees draft changesets if they don't have a topic
+
+  $ hg --cwd client-plain pull
+  pulling from ssh://user@dummy/server
+  searching for changes
+  adding changesets
+  adding manifests
+  adding file changes
+  added 1 changesets with 1 changes to 1 files
+  new changesets b46feb4d24f9 (1 drafts)
+  (run 'hg update' to get a working copy)
+  $ hg --cwd client-plain phase -r 'all() - 0'
+  1: draft
+  $ hg --cwd client-plain log -r 'all() - 0'
+  changeset:   1:b46feb4d24f9
+  tag:         tip
+  user:        test
+  date:        Thu Jan 01 00:00:00 1970 +0000
+  summary:     adding c (no topic)
+  

--- a/tests/test-topic.t	Tue Feb 04 16:22:40 2020 -0800
+++ b/tests/test-topic.t	Wed Feb 19 01:35:23 2020 +0100
@@ -120,6 +120,20 @@
     [experimental]
     topic.allow-publish = no
   
+  Server side visibility
+  ======================
+  
+  Serving changesets with topics to clients without topic extension can get
+  confusing. Such clients will have multiple anonymous heads without a clear way
+  to distinguish them. They will also "lose" the canonical heads of the branch.
+  
+  To avoid this confusion, server can be configured to only serve changesets
+  with topics to clients with the topic extension (version 9.3+). This might
+  become the default in future:
+  
+    [experimental]
+    topic.server-gate-topic-changesets = yes
+  
   list of commands:
   
   Change organization: