--- a/hgext3rd/evolve/__init__.py	Sun Mar 26 04:59:36 2017 +0200
+++ b/hgext3rd/evolve/__init__.py	Tue Mar 21 23:44:30 2017 +0100
@@ -111,6 +111,7 @@
 from mercurial.node import nullid
 
 from . import (
+    checkheads,
     debugcmd,
     obsdiscovery,
     obsexchange,
@@ -147,6 +148,7 @@
 eh.merge(debugcmd.eh)
 eh.merge(obsdiscovery.eh)
 eh.merge(obsexchange.eh)
+eh.merge(checkheads.eh)
 uisetup = eh.final_uisetup
 extsetup = eh.final_extsetup
 reposetup = eh.final_reposetup

--- /dev/null	Thu Jan 01 00:00:00 1970 +0000
+++ b/hgext3rd/evolve/checkheads.py	Tue Mar 21 23:44:30 2017 +0100
@@ -0,0 +1,220 @@
+# Code dedicated to the postprocessing new heads check with obsolescence
+#
+# Copyright 2017 Pierre-Yves David <pierre-yves.david@ens-lyon.org>
+#
+# 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 (
+    discovery,
+    error,
+    extensions,
+    node as nodemod,
+    obsolete,
+    phases,
+    util,
+)
+
+from mercurial.i18n import _
+
+from . import exthelper
+
+nullid = nodemod.nullid
+short = nodemod.short
+_headssummary = discovery._headssummary
+_oldheadssummary = discovery._oldheadssummary
+_nowarnheads = discovery._nowarnheads
+
+eh = exthelper.exthelper()
+
+@eh.uisetup
+def setupcheckheadswrapper(ui):
+    if util.safehasattr(discovery, '_postprocessobsolete'):
+        extensions.wrapfunction(discovery, '_postprocessobsolete',
+                                checkheadslightoverlay)
+    else:
+        extensions.wrapfunction(discovery, 'checkheads',
+                                checkheadsfulloverlay)
+
+# have dedicated wrapper to keep the rest as close as core as possible
+def checkheadsfulloverlay(orig, pushop):
+    if pushop.repo.obsstore:
+        return corecheckheads(pushop)
+    else:
+        return orig(pushop)
+
+def checkheadslightoverlay(orig, *args, **kwargs):
+    return _postprocessobsolete(*args, **kwargs)
+
+# copied from mercurial.discovery.checkheads as in a5bad127128d (4.1)
+#
+# The only differences are:
+# * the _postprocessobsolete section have been extracted,
+# * minor test adjustment to please flake8
+def corecheckheads(pushop):
+    """Check that a push won't add any outgoing head
+
+    raise Abort error and display ui message as needed.
+    """
+
+    repo = pushop.repo.unfiltered()
+    remote = pushop.remote
+    outgoing = pushop.outgoing
+    remoteheads = pushop.remoteheads
+    newbranch = pushop.newbranch
+    inc = bool(pushop.incoming)
+
+    # Check for each named branch if we're creating new remote heads.
+    # To be a remote head after push, node must be either:
+    # - unknown locally
+    # - a local outgoing head descended from update
+    # - a remote head that's known locally and not
+    #   ancestral to an outgoing head
+    if remoteheads == [nullid]:
+        # remote is empty, nothing to check.
+        return
+
+    if remote.capable('branchmap'):
+        headssum = _headssummary(repo, remote, outgoing)
+    else:
+        headssum = _oldheadssummary(repo, remoteheads, outgoing, inc)
+    newbranches = [branch for branch, heads in headssum.iteritems()
+                   if heads[0] is None]
+    # 1. Check for new branches on the remote.
+    if newbranches and not newbranch:  # new branch requires --new-branch
+        branchnames = ', '.join(sorted(newbranches))
+        raise error.Abort(_("push creates new remote branches: %s!")
+                          % branchnames,
+                          hint=_("use 'hg push --new-branch' to create"
+                                 " new remote branches"))
+
+    # 2. Find heads that we need not warn about
+    nowarnheads = _nowarnheads(pushop)
+
+    # 3. Check for new heads.
+    # If there are more heads after the push than before, a suitable
+    # error message, depending on unsynced status, is displayed.
+    errormsg = None
+    # If there is no obsstore, allfuturecommon won't be used, so no
+    # need to compute it.
+    if repo.obsstore:
+        allmissing = set(outgoing.missing)
+        cctx = repo.set('%ld', outgoing.common)
+        allfuturecommon = set(c.node() for c in cctx)
+        allfuturecommon.update(allmissing)
+    for branch, heads in sorted(headssum.iteritems()):
+        remoteheads, newheads, unsyncedheads = heads
+        candidate_newhs = set(newheads)
+        # add unsynced data
+        if remoteheads is None:
+            oldhs = set()
+        else:
+            oldhs = set(remoteheads)
+        oldhs.update(unsyncedheads)
+        candidate_newhs.update(unsyncedheads)
+        dhs = None # delta heads, the new heads on branch
+        if not repo.obsstore:
+            discardedheads = set()
+            newhs = candidate_newhs
+        else:
+            newhs, discardedheads = _postprocessobsolete(pushop,
+                                                         allfuturecommon,
+                                                         candidate_newhs)
+        unsynced = sorted(h for h in unsyncedheads if h not in discardedheads)
+        if unsynced:
+            if None in unsynced:
+                # old remote, no heads data
+                heads = None
+            elif len(unsynced) <= 4 or repo.ui.verbose:
+                heads = ' '.join(short(h) for h in unsynced)
+            else:
+                heads = (' '.join(short(h) for h in unsynced[:4]) +
+                         ' ' + _("and %s others") % (len(unsynced) - 4))
+            if heads is None:
+                repo.ui.status(_("remote has heads that are "
+                                 "not known locally\n"))
+            elif branch is None:
+                repo.ui.status(_("remote has heads that are "
+                                 "not known locally: %s\n") % heads)
+            else:
+                repo.ui.status(_("remote has heads on branch '%s' that are "
+                                 "not known locally: %s\n") % (branch, heads))
+        if remoteheads is None:
+            if len(newhs) > 1:
+                dhs = list(newhs)
+                if errormsg is None:
+                    errormsg = (_("push creates new branch '%s' "
+                                  "with multiple heads") % (branch))
+                    hint = _("merge or"
+                             " see 'hg help push' for details about"
+                             " pushing new heads")
+        elif len(newhs) > len(oldhs):
+            # remove bookmarked or existing remote heads from the new heads list
+            dhs = sorted(newhs - nowarnheads - oldhs)
+        if dhs:
+            if errormsg is None:
+                if branch not in ('default', None):
+                    errormsg = _("push creates new remote head %s "
+                                 "on branch '%s'!") % (short(dhs[0]), branch)
+                elif repo[dhs[0]].bookmarks():
+                    errormsg = (_("push creates new remote head %s "
+                                  "with bookmark '%s'!")
+                                % (short(dhs[0]), repo[dhs[0]].bookmarks()[0]))
+                else:
+                    errormsg = _("push creates new remote head %s!"
+                                 ) % short(dhs[0])
+                if unsyncedheads:
+                    hint = _("pull and merge or"
+                             " see 'hg help push' for details about"
+                             " pushing new heads")
+                else:
+                    hint = _("merge or"
+                             " see 'hg help push' for details about"
+                             " pushing new heads")
+            if branch is None:
+                repo.ui.note(_("new remote heads:\n"))
+            else:
+                repo.ui.note(_("new remote heads on branch '%s':\n") % branch)
+            for h in dhs:
+                repo.ui.note((" %s\n") % short(h))
+    if errormsg:
+        raise error.Abort(errormsg, hint=hint)
+
+def _postprocessobsolete(pushop, common, candidate):
+    """post process the list of new heads with obsolescence information
+
+    Exist as a subfunction to contains the complexity and allow extensions to
+    experiment with smarter logic.
+    Returns (newheads, discarded_heads) tuple
+    """
+    # remove future heads which are actually obsoleted by another
+    # pushed element:
+    #
+    # XXX as above, There are several cases this code does not handle
+    # XXX properly
+    #
+    # (1) if <nh> is public, it won't be affected by obsolete marker
+    #     and a new is created
+    #
+    # (2) if the new heads have ancestors which are not obsolete and
+    #     not ancestors of any other heads we will have a new head too.
+    #
+    # These two cases will be easy to handle for known changeset but
+    # much more tricky for unsynced changes.
+    #
+    # In addition, this code is confused by prune as it only looks for
+    # successors of the heads (none if pruned) leading to issue4354
+    repo = pushop.repo
+    newhs = set()
+    discarded = set()
+    for nh in candidate:
+        if nh in repo and repo[nh].phase() <= phases.public:
+            newhs.add(nh)
+        else:
+            for suc in obsolete.allsuccessors(repo.obsstore, [nh]):
+                if suc != nh and suc in common:
+                    discarded.add(nh)
+                    break
+            else:
+                newhs.add(nh)
+    return newhs, discarded