--- a/hgext3rd/evolve/__init__.py	Mon Apr 06 04:01:58 2020 +0200
+++ b/hgext3rd/evolve/__init__.py	Mon Apr 06 02:17:43 2020 +0200
@@ -292,6 +292,7 @@
     state,
     evolvecmd,
     exthelper,
+    headchecking,
     metadata,
     obscache,
     obsexchange,
@@ -346,6 +347,7 @@
 eh.merge(compat.eh)
 eh.merge(cmdrewrite.eh)
 eh.merge(rewind.eh)
+eh.merge(headchecking.eh)
 uisetup = eh.finaluisetup
 extsetup = eh.finalextsetup
 reposetup = eh.finalreposetup

--- /dev/null	Thu Jan 01 00:00:00 1970 +0000
+++ b/hgext3rd/evolve/headchecking.py	Mon Apr 06 02:17:43 2020 +0200
@@ -0,0 +1,113 @@
+from __future__ import absolute_import
+
+import functools
+
+from mercurial import (
+    discovery,
+    extensions,
+    phases,
+)
+
+
+from . import (
+    compat,
+    exthelper,
+)
+
+eh = exthelper.exthelper()
+
+
+@eh.uisetup
+def uisetup(ui):
+    extensions.wrapfunction(discovery, '_postprocessobsolete', _postprocessobsolete)
+
+def branchinfo(pushop, repo, node):
+    return repo[node].branch()
+
+# taken from 7d5455b988ec + branchinfo abstraction.
+def _postprocessobsolete(orig, pushop, futurecommon, candidate_newhs):
+    """post process the list of new heads with obsolescence information
+
+    Exists as a sub-function to contain the complexity and allow extensions to
+    experiment with smarter logic.
+
+    Returns (newheads, discarded_heads) tuple
+    """
+    pushingmarkerfor = discovery.pushingmarkerfor
+    # known issue
+    #
+    # * We "silently" skip processing on all changeset unknown locally
+    #
+    # * if <nh> is public on the remote, it won't be affected by obsolete
+    #     marker and a new is created
+
+    # define various utilities and containers
+    repo = pushop.repo
+    unfi = repo.unfiltered()
+    torev = compat.getgetrev(unfi.changelog)
+    public = phases.public
+    getphase = unfi._phasecache.phase
+    ispublic = lambda r: getphase(unfi, r) == public
+    ispushed = lambda n: torev(n) in futurecommon
+    hasoutmarker = functools.partial(pushingmarkerfor, unfi.obsstore, ispushed)
+    successorsmarkers = unfi.obsstore.successors
+    newhs = set()  # final set of new heads
+    discarded = set()  # new head of fully replaced branch
+
+    localcandidate = set()  # candidate heads known locally
+    unknownheads = set()  # candidate heads unknown locally
+    for h in candidate_newhs:
+        if h in unfi:
+            localcandidate.add(h)
+        else:
+            if successorsmarkers.get(h) is not None:
+                msg = (
+                    b'checkheads: remote head unknown locally has'
+                    b' local marker: %s\n'
+                )
+                repo.ui.debug(msg % hex(h))
+            unknownheads.add(h)
+
+    # fast path the simple case
+    if len(localcandidate) == 1:
+        return unknownheads | set(candidate_newhs), set()
+
+    # actually process branch replacement
+    while localcandidate:
+        nh = localcandidate.pop()
+        current_branch = branchinfo(pushop, unfi, nh)
+        # run this check early to skip the evaluation of the whole branch
+        if torev(nh) in futurecommon or ispublic(torev(nh)):
+            newhs.add(nh)
+            continue
+
+        # Get all revs/nodes on the branch exclusive to this head
+        # (already filtered heads are "ignored"))
+        branchrevs = unfi.revs(
+            b'only(%n, (%ln+%ln))', nh, localcandidate, newhs
+        )
+
+        branchnodes = []
+        for r in branchrevs:
+            ctx = unfi[r]
+            if ctx.branch() == current_branch:
+                branchnodes.append(ctx.node())
+
+        # The branch won't be hidden on the remote if
+        # * any part of it is public,
+        # * any part of it is considered part of the result by previous logic,
+        # * if we have no markers to push to obsolete it.
+        if (
+            any(ispublic(torev(n)) for n in branchnodes)
+            or any(torev(n) in futurecommon for n in branchnodes)
+            or any(not hasoutmarker(n) for n in branchnodes)
+        ):
+            newhs.add(nh)
+        else:
+            # note: there is a corner case if there is a merge in the branch.
+            # we might end up with -more- heads.  However, these heads are not
+            # "added" by the push, but more by the "removal" on the remote so I
+            # think is a okay to ignore them,
+            discarded.add(nh)
+    newhs |= unknownheads
+    return newhs, discarded

--- /dev/null	Thu Jan 01 00:00:00 1970 +0000
+++ b/tests/test-push-checkheads-multibranches-E1.t	Mon Apr 06 02:17:43 2020 +0200
@@ -0,0 +1,93 @@
+====================================
+Testing head checking code: Case E-1
+====================================
+
+Mercurial checks for the introduction of new heads on push. Evolution comes
+into play to detect if existing branches on the server are being replaced by
+some of the new one we push.
+
+This case is part of a series of tests checking this behavior.
+
+Category E: case involving changeset on multiple branch
+TestCase 8: moving a branch to another location
+
+.. old-state:
+..
+.. * 1-changeset on branch default
+.. * 1-changeset on branch Z (above default)
+..
+.. new-state:
+..
+.. * 1-changeset on branch default
+.. * 1-changeset on branch Z (rebased away from A0)
+..
+.. expected-result:
+..
+.. * push allowed
+..
+.. graph-summary:
+..
+..   B ø⇠◔ B'
+..     | |
+..   A ◔ |
+..     |/
+..     ●
+
+  $ . $TESTDIR/testlib/topic_setup.sh
+  $ . $TESTDIR/testlib/push-checkheads-util.sh
+
+Test setup
+----------
+
+  $ mkdir E1
+  $ cd E1
+  $ setuprepos
+  creating basic server and client repo
+  updating to branch default
+  2 files updated, 0 files merged, 0 files removed, 0 files unresolved
+  $ cd client
+  $ hg branch Z
+  marked working directory as branch Z
+  (branches are permanent and global, did you want a bookmark?)
+  $ mkcommit B0
+  $ hg push --new-branch
+  pushing to $TESTTMP/E1/server
+  searching for changes
+  adding changesets
+  adding manifests
+  adding file changes
+  added 1 changesets with 1 changes to 1 files
+  $ hg up 0
+  0 files updated, 0 files merged, 2 files removed, 0 files unresolved
+  $ hg branch --force Z
+  marked working directory as branch Z
+  $ mkcommit B1
+  created new head
+  (consider using topic for lightweight branches. See 'hg help topic')
+  $ hg debugobsolete `getid "desc(B0)" ` `getid "desc(B1)"`
+  1 new obsolescence markers
+  obsoleted 1 changesets
+  $ hg log -G --hidden
+  @  c98b855401e7 (draft): B1
+  |
+  | x  93e5c1321ece (draft): B0
+  | |
+  | o  8aaa48160adc (draft): A0
+  |/
+  o  1e4be0697311 (public): root
+  
+
+Actual testing
+--------------
+
+  $ hg push
+  pushing to $TESTTMP/E1/server
+  searching for changes
+  adding changesets
+  adding manifests
+  adding file changes
+  added 1 changesets with 1 changes to 1 files (+1 heads)
+  1 new obsolescence markers
+  obsoleted 1 changesets
+
+  $ cd ../..

--- /dev/null	Thu Jan 01 00:00:00 1970 +0000
+++ b/tests/test-push-checkheads-multibranches-E2.t	Mon Apr 06 02:17:43 2020 +0200
@@ -0,0 +1,108 @@
+====================================
+Testing head checking code: Case E-2
+====================================
+
+Mercurial checks for the introduction of new heads on push. Evolution comes
+into play to detect if existing branches on the server are being replaced by
+some of the new one we push.
+
+This case is part of a series of tests checking this behavior.
+
+Category E: case involving changeset on multiple branch
+TestCase 8: moving interleaved branch away from each other
+
+.. old-state:
+..
+.. * 2-changeset on branch default
+.. * 1-changeset on branch Z (between the two other)
+..
+.. new-state:
+..
+.. * 2-changeset on branch default, aligned
+.. * 1-changeset on branch Z (at the same location)
+..
+.. expected-result:
+..
+.. * push allowed
+..
+.. graph-summary:
+..
+..   C ø⇠◔ C'
+..     | |
+..   B ◔ |
+..     | |
+..   A ø⇠◔ A'
+..     |/
+..     ●
+
+  $ . $TESTDIR/testlib/topic_setup.sh
+  $ . $TESTDIR/testlib/push-checkheads-util.sh
+
+Test setup
+----------
+
+  $ mkdir E1
+  $ cd E1
+  $ setuprepos
+  creating basic server and client repo
+  updating to branch default
+  2 files updated, 0 files merged, 0 files removed, 0 files unresolved
+  $ cd client
+  $ hg branch Z
+  marked working directory as branch Z
+  (branches are permanent and global, did you want a bookmark?)
+  $ mkcommit B0
+  $ hg branch default --force
+  marked working directory as branch default
+  $ mkcommit C0
+  created new head
+  (consider using topic for lightweight branches. See 'hg help topic')
+  $ hg push --new-branch
+  pushing to $TESTTMP/E1/server
+  searching for changes
+  adding changesets
+  adding manifests
+  adding file changes
+  added 2 changesets with 2 changes to 2 files
+  $ hg up 0
+  0 files updated, 0 files merged, 3 files removed, 0 files unresolved
+  $ mkcommit A1
+  created new head
+  (consider using topic for lightweight branches. See 'hg help topic')
+  $ mkcommit C1
+  $ hg debugobsolete `getid "desc(A0)" ` `getid "desc(A1)"`
+  1 new obsolescence markers
+  obsoleted 1 changesets
+  2 new orphan changesets
+  $ hg debugobsolete `getid "desc(C0)" ` `getid "desc(C1)"`
+  1 new obsolescence markers
+  obsoleted 1 changesets
+  $ hg log -G --hidden
+  @  0c76bc104656 (draft): C1
+  |
+  o  f6082bc4ffef (draft): A1
+  |
+  | x  afc55ba2ce61 (draft): C0
+  | |
+  | *  93e5c1321ece (draft): B0
+  | |
+  | x  8aaa48160adc (draft): A0
+  |/
+  o  1e4be0697311 (public): root
+  
+
+Actual testing
+--------------
+
+  $ hg push -r 'desc("C1")'
+  pushing to $TESTTMP/E1/server
+  searching for changes
+  adding changesets
+  adding manifests
+  adding file changes
+  added 2 changesets with 2 changes to 2 files (+1 heads)
+  2 new obsolescence markers
+  obsoleted 2 changesets
+  1 new orphan changesets
+
+  $ cd ../..

--- /dev/null	Thu Jan 01 00:00:00 1970 +0000
+++ b/tests/test-push-checkheads-multibranches-E3.t	Mon Apr 06 02:17:43 2020 +0200
@@ -0,0 +1,97 @@
+====================================
+Testing head checking code: Case E-3
+====================================
+
+Mercurial checks for the introduction of new heads on push. Evolution comes
+into play to detect if existing branches on the server are being replaced by
+some of the new one we push.
+
+This case is part of a series of tests checking this behavior.
+
+Category E: case involving changeset on multiple branch
+TestCase 8: moving only part of the interleaved branch away, creating 2 heads
+
+.. old-state:
+..
+.. * 2-changeset on branch default
+.. * 1-changeset on branch Z (between the two other)
+..
+.. new-state:
+..
+.. * 2-changeset on branch default, on untouched, the other moved
+.. * 1-changeset on branch Z (at the same location)
+..
+.. expected-result:
+..
+.. * push rejected
+..
+.. graph-summary:
+..
+..   C ø⇠◔ C'
+..     | |
+..   B ◔ |
+..     | |
+..   A ◔ |
+..     |/
+..     ●
+
+  $ . $TESTDIR/testlib/topic_setup.sh
+  $ . $TESTDIR/testlib/push-checkheads-util.sh
+
+Test setup
+----------
+
+  $ mkdir E1
+  $ cd E1
+  $ setuprepos
+  creating basic server and client repo
+  updating to branch default
+  2 files updated, 0 files merged, 0 files removed, 0 files unresolved
+  $ cd client
+  $ hg branch Z
+  marked working directory as branch Z
+  (branches are permanent and global, did you want a bookmark?)
+  $ mkcommit B0
+  $ hg branch default --force
+  marked working directory as branch default
+  $ mkcommit C0
+  created new head
+  (consider using topic for lightweight branches. See 'hg help topic')
+  $ hg push --new-branch
+  pushing to $TESTTMP/E1/server
+  searching for changes
+  adding changesets
+  adding manifests
+  adding file changes
+  added 2 changesets with 2 changes to 2 files
+  $ hg up 0
+  0 files updated, 0 files merged, 3 files removed, 0 files unresolved
+  $ mkcommit C1
+  created new head
+  (consider using topic for lightweight branches. See 'hg help topic')
+  $ hg debugobsolete `getid "desc(C0)" ` `getid "desc(C1)"`
+  1 new obsolescence markers
+  obsoleted 1 changesets
+  $ hg log -G --hidden
+  @  dc44c53142f0 (draft): C1
+  |
+  | x  afc55ba2ce61 (draft): C0
+  | |
+  | o  93e5c1321ece (draft): B0
+  | |
+  | o  8aaa48160adc (draft): A0
+  |/
+  o  1e4be0697311 (public): root
+  
+
+Actual testing
+--------------
+
+  $ hg push -r 'desc("C1")'
+  pushing to $TESTTMP/E1/server
+  searching for changes
+  abort: push creates new remote head dc44c53142f0!
+  (merge or see 'hg help push' for details about pushing new heads)
+  [255]
+
+  $ cd ../..