[PATCH 2 of 2] server: introduce a 'server.single-head' option

[Boris Feld]

# HG changeset patch
# User Boris Feld <boris.feld at octobus.net>
# Date 1510800762 -3600
#      Thu Nov 16 03:52:42 2017 +0100
# Node ID 921ee05859ea9fc17f5a123baa16c2bbe2c49001
# Parent  c46c0f855a64868b90260da603769c8bc1ebdcf7
# EXP-Topic single-heads
# Available At https://bitbucket.org/octobus/mercurial-devel/
#              hg pull https://bitbucket.org/octobus/mercurial-devel/ -r 921ee05859ea
server: introduce a 'server.single-head' option

When the option is set, the repository will reject any transaction adding
multiple heads to the same named branch.

For now we reject all scenario with multiple heads. One could imagine handling
closed branches differently. We prefer to keep things simple for now. The
feature might get extended later. Branch closing is not the best experience
Mercurial has to offer anyway.

diff --git a/mercurial/configitems.py b/mercurial/configitems.py
--- a/mercurial/configitems.py
+++ b/mercurial/configitems.py
@@ -787,6 +787,9 @@ coreconfigitem('server', 'validate',
 coreconfigitem('server', 'zliblevel',
     default=-1,
 )
+coreconfigitem('server', 'single-head',
+    default=False,
+)
 coreconfigitem('share', 'pool',
     default=None,
 )
diff --git a/mercurial/help/config.txt b/mercurial/help/config.txt
--- a/mercurial/help/config.txt
+++ b/mercurial/help/config.txt
@@ -1723,6 +1723,10 @@ Alias definitions for revsets. See :hg:`
 
 Controls generic server settings.
 
+``single-head``
+    Force the repository to always contains a single head for each named
+    branches. Any push/pull commit adding a new heads will be rejected.
+
 ``compressionengines``
     List of compression engines and their relative priority to advertise
     to clients.
diff --git a/mercurial/localrepo.py b/mercurial/localrepo.py
--- a/mercurial/localrepo.py
+++ b/mercurial/localrepo.py
@@ -1244,6 +1244,8 @@ class localrepository(object):
             # gating.
             tracktags(tr2)
             repo = reporef()
+            if repo.ui.configbool('server', 'single-head'):
+                scmutil.enforcesinglehead(repo, tr2)
             if hook.hashook(repo.ui, 'pretxnclose-bookmark'):
                 for name, (old, new) in sorted(tr.changes['bookmarks'].items()):
                     args = tr.hookargs.copy()
diff --git a/mercurial/scmutil.py b/mercurial/scmutil.py
--- a/mercurial/scmutil.py
+++ b/mercurial/scmutil.py
@@ -1277,3 +1277,15 @@ def nodesummaries(repo, nodes, maxnumnod
         return ' '.join(short(h) for h in nodes)
     first = ' '.join(short(h) for h in nodes[:maxnumnodes])
     return _("%s and %s others") % (first, len(nodes) - maxnumnodes)
+
+def enforcesinglehead(repo, tr):
+    """check that no named branch has multiple heads"""
+    visible = repo.filtered('visible')
+    # possible improvement: we could restrict the check to affected branch
+    for name, heads in visible.branchmap().iteritems():
+        if len(heads) > 1:
+            msg = _('rejecting multiple heads on branch "%s"')
+            msg %= name
+            hint = _('%d heads: %s')
+            hint %= (len(heads), nodesummaries(repo, heads))
+            raise error.Abort(msg, hint=hint)
diff --git a/tests/test-single-head.t b/tests/test-single-head.t
new file mode 100644
--- /dev/null
+++ b/tests/test-single-head.t
@@ -0,0 +1,115 @@
+=====================
+Test workflow options
+=====================
+
+  $ . "$TESTDIR/testlib/obsmarker-common.sh"
+
+Test single head enforcing - Setup
+=============================================
+
+  $ cat << EOF >> $HGRCPATH
+  > [experimental]
+  > evolution = all
+  > EOF
+  $ hg init single-head-server
+  $ cd single-head-server
+  $ cat <<EOF >> .hg/hgrc
+  > [phases]
+  > publish = no
+  > [server]
+  > single-head = yes
+  > EOF
+  $ mkcommit ROOT
+  $ mkcommit c_dA0
+  $ cd ..
+
+  $ hg clone single-head-server client
+  updating to branch default
+  2 files updated, 0 files merged, 0 files removed, 0 files unresolved
+
+Test single head enforcing - with branch only
+---------------------------------------------
+
+  $ cd client
+
+continuing the current defaultbranch
+
+  $ mkcommit c_dB0
+  $ hg push
+  pushing to $TESTTMP/single-head-server (glob)
+  searching for changes
+  adding changesets
+  adding manifests
+  adding file changes
+  added 1 changesets with 1 changes to 1 files
+
+creating a new branch
+
+  $ hg up 'desc("ROOT")'
+  0 files updated, 0 files merged, 2 files removed, 0 files unresolved
+  $ hg branch branch_A
+  marked working directory as branch branch_A
+  (branches are permanent and global, did you want a bookmark?)
+  $ mkcommit c_aC0
+  $ hg push --new-branch
+  pushing to $TESTTMP/single-head-server (glob)
+  searching for changes
+  adding changesets
+  adding manifests
+  adding file changes
+  added 1 changesets with 1 changes to 1 files (+1 heads)
+
+Create a new head on the default branch
+
+  $ hg up 'desc("c_dA0")'
+  1 files updated, 0 files merged, 1 files removed, 0 files unresolved
+  $ mkcommit c_dD0
+  created new head
+  $ hg push -f
+  pushing to $TESTTMP/single-head-server (glob)
+  searching for changes
+  adding changesets
+  adding manifests
+  adding file changes
+  added 1 changesets with 1 changes to 1 files (+1 heads)
+  transaction abort!
+  rollback completed
+  abort: rejecting multiple heads on branch "default"
+  (2 heads: 286d02a6e2a2 9bf953aa81f6)
+  [255]
+
+remerge them
+
+  $ hg merge
+  1 files updated, 0 files merged, 0 files removed, 0 files unresolved
+  (branch merge, don't forget to commit)
+  $ mkcommit c_dE0
+  $ hg push
+  pushing to $TESTTMP/single-head-server (glob)
+  searching for changes
+  adding changesets
+  adding manifests
+  adding file changes
+  added 2 changesets with 2 changes to 2 files
+
+Test single head enforcing - after rewrite
+------------------------------------------
+
+  $ mkcommit c_dF0
+  $ hg push
+  pushing to $TESTTMP/single-head-server (glob)
+  searching for changes
+  adding changesets
+  adding manifests
+  adding file changes
+  added 1 changesets with 1 changes to 1 files
+  $ hg commit --amend -m c_dF1
+  $ hg push
+  pushing to $TESTTMP/single-head-server (glob)
+  searching for changes
+  adding changesets
+  adding manifests
+  adding file changes
+  added 1 changesets with 0 changes to 1 files (+1 heads)
+  1 new obsolescence markers
+  obsoleted 1 changesets