--- a/mercurial/discovery.py	Fri Jan 07 11:53:23 2022 +0300
+++ b/mercurial/discovery.py	Tue Jan 04 23:38:39 2022 +0300
@@ -19,6 +19,7 @@
     bookmarks,
     branchmap,
     error,
+    obsolete,
     phases,
     pycompat,
     scmutil,
@@ -545,12 +546,16 @@
     if len(localcandidate) == 1:
         return unknownheads | set(candidate_newhs), set()
 
+    obsrevs = obsolete.getrevs(unfi, b'obsolete')
+    futurenonobsolete = frozenset(futurecommon) - obsrevs
+
     # actually process branch replacement
     while localcandidate:
         nh = localcandidate.pop()
+        r = torev(nh)
         current_branch = unfi[nh].branch()
         # run this check early to skip the evaluation of the whole branch
-        if torev(nh) in futurecommon or ispublic(torev(nh)):
+        if ispublic(r) or r not in obsrevs:
             newhs.add(nh)
             continue
 
@@ -572,7 +577,7 @@
         # * if we have no markers to push to obsolete it.
         if (
             any(ispublic(r) for r in branchrevs)
-            or any(torev(n) in futurecommon for n in branchnodes)
+            or any(torev(n) in futurenonobsolete for n in branchnodes)
             or any(not hasoutmarker(n) for n in branchnodes)
         ):
             newhs.add(nh)

--- /dev/null	Thu Jan 01 00:00:00 1970 +0000
+++ b/tests/test-single-head-obsolescence-named-branch-A1.t	Tue Jan 04 23:38:39 2022 +0300
@@ -0,0 +1,114 @@
+=========================================
+Testing single head enforcement: Case A-1
+=========================================
+
+A repository is set to only accept a single head per name (typically named
+branch). However, obsolete changesets can make this enforcement more
+complicated, because they can be kept visible by other changeset on other
+branch.
+
+This case is part of a series of tests checking this behavior.
+
+Category A: Involving obsolescence
+TestCase 1: A fully obsolete branch kept visible by another one
+
+.. old-state:
+..
+.. * 2 changesets on branch default
+.. * 2 changesets on branch Z on top of them
+..
+.. new-state:
+..
+.. * 2 changesets on branch Z at the same location
+.. * 2 changesets on branch default superseding the other ones
+..
+.. expected-result:
+..
+.. * only one head detected
+..
+.. graph-summary:
+..
+..   D ●      (branch Z)
+..     |
+..   C ●      (branch Z)
+..     |
+..   B ø⇠◔ B'
+..     | |
+..   A ø⇠◔ A'
+..     |/
+..     ●
+
+  $ . $TESTDIR/testlib/push-checkheads-util.sh
+
+  $ cat >> $HGRCPATH << EOF
+  > [command-templates]
+  > log = "{node|short} [{branch}] ({phase}): {desc}\n"
+  > EOF
+
+Test setup
+----------
+
+  $ mkdir A1
+  $ cd A1
+  $ setuprepos single-head
+  creating basic server and client repo
+  updating to branch default
+  2 files updated, 0 files merged, 0 files removed, 0 files unresolved
+  $ cd client
+  $ mkcommit B0
+  $ hg branch Z
+  marked working directory as branch Z
+  (branches are permanent and global, did you want a bookmark?)
+  $ mkcommit C0
+  $ mkcommit D0
+  $ hg push --new-branch
+  pushing to $TESTTMP/A1/server
+  searching for changes
+  adding changesets
+  adding manifests
+  adding file changes
+  added 3 changesets with 3 changes to 3 files
+  $ hg up 0
+  0 files updated, 0 files merged, 4 files removed, 0 files unresolved
+  $ mkcommit A1
+  created new head
+  $ mkcommit B1
+  $ hg debugobsolete `getid "desc(A0)"` `getid "desc(A1)"`
+  1 new obsolescence markers
+  obsoleted 1 changesets
+  3 new orphan changesets
+  $ hg debugobsolete `getid "desc(B0)"` `getid "desc(B1)"`
+  1 new obsolescence markers
+  obsoleted 1 changesets
+  $ hg heads
+  262c8c798096 [default] (draft): B1
+  cdf1dbb37a67 [Z] (draft): D0
+  $ hg log -G --hidden
+  @  262c8c798096 [default] (draft): B1
+  |
+  o  f6082bc4ffef [default] (draft): A1
+  |
+  | *  cdf1dbb37a67 [Z] (draft): D0
+  | |
+  | *  3213e3e16c67 [Z] (draft): C0
+  | |
+  | x  d73caddc5533 [default] (draft): B0
+  | |
+  | x  8aaa48160adc [default] (draft): A0
+  |/
+  o  1e4be0697311 [default] (public): root
+  
+
+Actual testing
+--------------
+
+  $ hg push -r 'desc("B1")'
+  pushing to $TESTTMP/A1/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
+  2 new orphan changesets

--- /dev/null	Thu Jan 01 00:00:00 1970 +0000
+++ b/tests/test-single-head-obsolescence-named-branch-A2.t	Tue Jan 04 23:38:39 2022 +0300
@@ -0,0 +1,113 @@
+=========================================
+Testing single head enforcement: Case A-2
+=========================================
+
+A repository is set to only accept a single head per name (typically named
+branch). However, obsolete changesets can make this enforcement more
+complicated, because they can be kept visible by other changeset on other
+branch.
+
+This case is part of a series of tests checking this behavior.
+
+Category A: Involving obsolescence
+TestCase 2: A branch is split in two, effectively creating two heads
+
+.. old-state:
+..
+.. * 2 changesets on branch default
+.. * 2 changesets on branch Z on top of them
+..
+.. new-state:
+..
+.. * 2 changesets on branch Z at the same location
+.. * 1 changeset on branch default unchanged
+.. * 1 changeset on branch default superseding the other ones
+..
+.. expected-result:
+..
+.. * two heads detected
+..
+.. graph-summary:
+..
+..   D ●      (branch Z)
+..     |
+..   C ●      (branch Z)
+..     |
+..   B ø⇠◔ B'
+..     | |
+..   A ● |
+..     |/
+..     ●
+
+  $ . $TESTDIR/testlib/push-checkheads-util.sh
+
+  $ cat >> $HGRCPATH << EOF
+  > [command-templates]
+  > log = "{node|short} [{branch}] ({phase}): {desc}\n"
+  > EOF
+
+Test setup
+----------
+
+  $ mkdir A2
+  $ cd A2
+  $ setuprepos single-head
+  creating basic server and client repo
+  updating to branch default
+  2 files updated, 0 files merged, 0 files removed, 0 files unresolved
+  $ cd client
+  $ mkcommit B0
+  $ hg branch Z
+  marked working directory as branch Z
+  (branches are permanent and global, did you want a bookmark?)
+  $ mkcommit C0
+  $ mkcommit D0
+  $ hg push --new-branch
+  pushing to $TESTTMP/A2/server
+  searching for changes
+  adding changesets
+  adding manifests
+  adding file changes
+  added 3 changesets with 3 changes to 3 files
+  $ hg up 0
+  0 files updated, 0 files merged, 4 files removed, 0 files unresolved
+  $ mkcommit B1
+  created new head
+  $ hg debugobsolete `getid "desc(B0)"` `getid "desc(B1)"`
+  1 new obsolescence markers
+  obsoleted 1 changesets
+  2 new orphan changesets
+  $ hg heads
+  25c56d33e4c4 [default] (draft): B1
+  cdf1dbb37a67 [Z] (draft): D0
+  8aaa48160adc [default] (draft): A0
+  $ hg log -G --hidden
+  @  25c56d33e4c4 [default] (draft): B1
+  |
+  | *  cdf1dbb37a67 [Z] (draft): D0
+  | |
+  | *  3213e3e16c67 [Z] (draft): C0
+  | |
+  | x  d73caddc5533 [default] (draft): B0
+  | |
+  | o  8aaa48160adc [default] (draft): A0
+  |/
+  o  1e4be0697311 [default] (public): root
+  
+
+Actual testing
+--------------
+
+(force push to make sure we get the changeset on the remote)
+
+  $ hg push -r 'desc("B1")' --force
+  pushing to $TESTTMP/A2/server
+  searching for changes
+  adding changesets
+  adding manifests
+  adding file changes
+  transaction abort!
+  rollback completed
+  abort: rejecting multiple heads on branch "default"
+  (2 heads: 8aaa48160adc 25c56d33e4c4)
+  [255]

--- /dev/null	Thu Jan 01 00:00:00 1970 +0000
+++ b/tests/test-single-head-obsolescence-named-branch-A3.t	Tue Jan 04 23:38:39 2022 +0300
@@ -0,0 +1,120 @@
+=========================================
+Testing single head enforcement: Case A-3
+=========================================
+
+A repository is set to only accept a single head per name (typically named
+branch). However, obsolete changesets can make this enforcement more
+complicated, because they can be kept visible by other changeset on other
+branch.
+
+This case is part of a series of tests checking this behavior.
+
+Category A: Involving obsolescence
+TestCase 3: Full superseding of a branch interleaved with another
+
+.. old-state:
+..
+.. * 2 changesets on branch default
+.. * 2 changesets on branch Z interleaved with the other
+..
+.. new-state:
+..
+.. * 2 changesets on branch Z at the same location
+.. * 2 changesets on branch default superseding the other ones
+..
+.. expected-result:
+..
+.. * only one head detected
+..
+.. graph-summary:
+..
+..   D ●      (branch Z)
+..     |
+..   C ø⇠◔ C'
+..     | |
+..   B ● |    (branch Z)
+..     | |
+..   A ø⇠◔ A'
+..     |/
+..     ●
+
+  $ . $TESTDIR/testlib/push-checkheads-util.sh
+
+  $ cat >> $HGRCPATH << EOF
+  > [command-templates]
+  > log = "{node|short} [{branch}] ({phase}): {desc}\n"
+  > EOF
+
+Test setup
+----------
+
+  $ mkdir A3
+  $ cd A3
+  $ setuprepos single-head
+  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
+  $ hg branch Z --force
+  marked working directory as branch Z
+  $ mkcommit D0
+  created new head
+  $ hg push --new-branch
+  pushing to $TESTTMP/A3/server
+  searching for changes
+  adding changesets
+  adding manifests
+  adding file changes
+  added 3 changesets with 3 changes to 3 files
+  $ hg up 0
+  0 files updated, 0 files merged, 4 files removed, 0 files unresolved
+  $ mkcommit A1
+  created new head
+  $ mkcommit C1
+  $ hg debugobsolete `getid "desc(A0)"` `getid "desc(A1)"`
+  1 new obsolescence markers
+  obsoleted 1 changesets
+  3 new orphan changesets
+  $ hg debugobsolete `getid "desc(C0)"` `getid "desc(C1)"`
+  1 new obsolescence markers
+  obsoleted 1 changesets
+  $ hg heads
+  0c76bc104656 [default] (draft): C1
+  78578c4306ce [Z] (draft): D0
+  $ hg log -G --hidden
+  @  0c76bc104656 [default] (draft): C1
+  |
+  o  f6082bc4ffef [default] (draft): A1
+  |
+  | *  78578c4306ce [Z] (draft): D0
+  | |
+  | x  afc55ba2ce61 [default] (draft): C0
+  | |
+  | *  93e5c1321ece [Z] (draft): B0
+  | |
+  | x  8aaa48160adc [default] (draft): A0
+  |/
+  o  1e4be0697311 [default] (public): root
+  
+
+Actual testing
+--------------
+
+  $ hg push -r 'desc("C1")'
+  pushing to $TESTTMP/A3/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
+  2 new orphan changesets

--- /dev/null	Thu Jan 01 00:00:00 1970 +0000
+++ b/tests/test-single-head-obsolescence-named-branch-A4.t	Tue Jan 04 23:38:39 2022 +0300
@@ -0,0 +1,117 @@
+=========================================
+Testing single head enforcement: Case A-4
+=========================================
+
+A repository is set to only accept a single head per name (typically named
+branch). However, obsolete changesets can make this enforcement more
+complicated, because they can be kept visible by other changeset on other
+branch.
+
+This case is part of a series of tests checking this behavior.
+
+Category A: Involving obsolescence
+TestCase 4: Partial rewrite of a branch to deinterleave it
+
+.. old-state:
+..
+.. * 2 changesets on branch default
+.. * 2 changesets on branch Z interleaved with the other one
+..
+.. new-state:
+..
+.. * 2 changesets on branch Z at the same location
+.. * 1 changeset on default untouched (the lower one)
+.. * 1 changeset on default moved on the other one
+..
+.. expected-result:
+..
+.. * only one head detected
+..
+.. graph-summary:
+..
+..   D ●      (branch Z)
+..     |
+..   C ø⇠◔ C'
+..     | |
+..   B ● |    (branch Z)
+..     |/
+..   A ●
+..     |
+..     ●
+
+  $ . $TESTDIR/testlib/push-checkheads-util.sh
+
+  $ cat >> $HGRCPATH << EOF
+  > [command-templates]
+  > log = "{node|short} [{branch}] ({phase}): {desc}\n"
+  > EOF
+
+Test setup
+----------
+
+  $ mkdir A4
+  $ cd A4
+  $ setuprepos single-head
+  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
+  $ hg branch Z --force
+  marked working directory as branch Z
+  $ mkcommit D0
+  created new head
+  $ hg push --new-branch
+  pushing to $TESTTMP/A4/server
+  searching for changes
+  adding changesets
+  adding manifests
+  adding file changes
+  added 3 changesets with 3 changes to 3 files
+  $ hg up 'desc("A0")'
+  0 files updated, 0 files merged, 3 files removed, 0 files unresolved
+  $ mkcommit C1
+  created new head
+  $ hg debugobsolete `getid "desc(C0)"` `getid "desc(C1)"`
+  1 new obsolescence markers
+  obsoleted 1 changesets
+  1 new orphan changesets
+  $ hg heads
+  cfe9ed94fa4a [default] (draft): C1
+  78578c4306ce [Z] (draft): D0
+  $ hg log -G --hidden
+  @  cfe9ed94fa4a [default] (draft): C1
+  |
+  | *  78578c4306ce [Z] (draft): D0
+  | |
+  | x  afc55ba2ce61 [default] (draft): C0
+  | |
+  | o  93e5c1321ece [Z] (draft): B0
+  |/
+  o  8aaa48160adc [default] (draft): A0
+  |
+  o  1e4be0697311 [default] (public): root
+  
+
+Actual testing
+--------------
+
+(force push to make sure we get the changeset on the remote)
+
+  $ hg push -r 'desc("C1")' --force
+  pushing to $TESTTMP/A4/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
+  1 new orphan changesets

--- /dev/null	Thu Jan 01 00:00:00 1970 +0000
+++ b/tests/test-single-head-obsolescence-named-branch-A5.t	Tue Jan 04 23:38:39 2022 +0300
@@ -0,0 +1,108 @@
+=========================================
+Testing single head enforcement: Case A-5
+=========================================
+
+A repository is set to only accept a single head per name (typically named
+branch). However, obsolete changesets can make this enforcement more
+complicated, because they can be kept visible by other changeset on other
+branch.
+
+This case is part of a series of tests checking this behavior.
+
+Category A: Involving obsolescence
+TestCase 5: Obsoleting a merge reveals two heads
+
+.. old-state:
+..
+.. * 3 changesets on branch default (2 on their own branch + 1 merge)
+.. * 1 changeset on branch Z (children of the merge)
+..
+.. new-state:
+..
+.. * 2 changesets on branch default (merge is obsolete) each a head
+.. * 1 changeset on branch Z keeping the merge visible
+..
+.. expected-result:
+..
+.. * 2 heads detected (because we skip the merge)
+..
+.. graph-summary:
+..
+..   C ●      (branch Z)
+..     |
+..   M ⊗
+..     |\
+..   A ● ● B
+..     |/
+..     ●
+
+  $ . $TESTDIR/testlib/push-checkheads-util.sh
+
+  $ cat >> $HGRCPATH << EOF
+  > [command-templates]
+  > log = "{node|short} [{branch}] ({phase}): {desc}\n"
+  > EOF
+
+Test setup
+----------
+
+  $ mkdir A5
+  $ cd A5
+  $ setuprepos single-head
+  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 up 0
+  0 files updated, 0 files merged, 1 files removed, 0 files unresolved
+  $ mkcommit B0
+  created new head
+  $ hg merge
+  1 files updated, 0 files merged, 0 files removed, 0 files unresolved
+  (branch merge, don't forget to commit)
+  $ hg ci -m 'M0'
+  $ hg branch Z
+  marked working directory as branch Z
+  (branches are permanent and global, did you want a bookmark?)
+  $ mkcommit C0
+  $ hg push --new-branch
+  pushing to $TESTTMP/A5/server
+  searching for changes
+  adding changesets
+  adding manifests
+  adding file changes
+  added 3 changesets with 2 changes to 2 files
+  $ hg debugobsolete `getid "desc(M0)"` --record-parents
+  1 new obsolescence markers
+  obsoleted 1 changesets
+  1 new orphan changesets
+  $ hg heads
+  61c95483cc12 [Z] (draft): C0
+  74ff5441d343 [default] (draft): B0
+  8aaa48160adc [default] (draft): A0
+  $ hg log -G --hidden
+  @  61c95483cc12 [Z] (draft): C0
+  |
+  x    14d3d4d41d1a [default] (draft): M0
+  |\
+  | o  74ff5441d343 [default] (draft): B0
+  | |
+  o |  8aaa48160adc [default] (draft): A0
+  |/
+  o  1e4be0697311 [default] (public): root
+  
+
+Actual testing
+--------------
+
+(force push to make sure we get the changeset on the remote)
+
+  $ hg push -r 'desc("C0")' --force
+  pushing to $TESTTMP/A5/server
+  searching for changes
+  no changes found
+  transaction abort!
+  rollback completed
+  abort: rejecting multiple heads on branch "default"
+  (2 heads: 8aaa48160adc 74ff5441d343)
+  [255]

--- a/tests/testlib/push-checkheads-util.sh	Fri Jan 07 11:53:23 2022 +0300
+++ b/tests/testlib/push-checkheads-util.sh	Tue Jan 04 23:38:39 2022 +0300
@@ -41,4 +41,10 @@
     mkcommit A0
     cd ..
     hg clone server client
+
+    if [ "$1" = "single-head" ]; then
+        echo >> "server/.hg/hgrc" "[experimental]"
+        echo >> "server/.hg/hgrc" "# enforce a single name per branch"
+        echo >> "server/.hg/hgrc" "single-head-per-branch = yes"
+    fi
 }