Index: contrib/bazaar/mbprotect/__init__.py
===================================================================
--- contrib/bazaar/mbprotect/__init__.py	(revision bccec987843c8e9c688b8a455c308951fd14bae1)
+++ contrib/bazaar/mbprotect/__init__.py	(revision bccec987843c8e9c688b8a455c308951fd14bae1)
@@ -0,0 +1,65 @@
+#
+# Copyright (c) 2009 Jiri Svoboda
+# All rights reserved.
+#
+# Redistribution and use in source and binary forms, with or without
+# modification, are permitted provided that the following conditions
+# are met:
+#
+# - Redistributions of source code must retain the above copyright
+#   notice, this list of conditions and the following disclaimer.
+# - Redistributions in binary form must reproduce the above copyright
+#   notice, this list of conditions and the following disclaimer in the
+#   documentation and/or other materials provided with the distribution.
+# - The name of the author may not be used to endorse or promote products
+#   derived from this software without specific prior written permission.
+#
+# THIS SOFTWARE IS PROVIDED BY THE AUTHOR ``AS IS'' AND ANY EXPRESS OR
+# IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE IMPLIED WARRANTIES
+# OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE ARE DISCLAIMED.
+# IN NO EVENT SHALL THE AUTHOR BE LIABLE FOR ANY DIRECT, INDIRECT,
+# INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT
+# NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE,
+# DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY
+# THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT
+# (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE OF
+# THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
+#
+
+"""Main Branch Protection plugin for Bazaar."""
+
+#
+# This plugin employs the pre_change_branch_tip hook to protect the main
+# branch (a.k.a. linear history) of bzr repositories from unwanted change,
+# effectively making the main branch append-only.
+# 
+# Specifically we verify that the new main branch contains the old tip.
+# This prevents a branch from inadvertently aquiring the tip of another branch.
+#
+# Install this plugin in ~/.bazaar/plugins/mbprotect
+#
+# See also http://trac.helenos.org/trac.fcgi/wiki/BazaarWorkflow
+#
+
+import bzrlib.branch
+from bzrlib.errors import TipChangeRejected
+
+def pre_change_branch_tip(params):
+    repo = params.branch.repository
+
+    # Check if the old repository was empty.
+    if params.old_revid == 'null:':
+	return
+
+    # Look for old tip in new main branch.
+    for revision_id in repo.iter_reverse_revision_history(params.new_revid):
+	if revision_id == params.old_revid:
+	    return	# Found old tip
+
+    # Old tip was not found. Reject the change.
+    raise TipChangeRejected('Bad tip. Read http://trac.helenos.org/trac.fcgi/' +
+	'wiki/BazaarWorkflow')
+
+# Install hook.
+bzrlib.branch.Branch.hooks.install_named_hook('pre_change_branch_tip',
+    pre_change_branch_tip, 'MB-Protect tip check')