add HEAD sentinel file that contains migration revision

This change wraps the command to generate a migration with code to
update a file called HEAD. The HEAD file will contain the revision ID
of the head of the migraton timeline. Additionally, check migrations has
been altered to verify the contents of this file against the timeline
head.

This file helps the OpenStack gate detect potential migration branches
without running alembic via git merge conflicts.

Closes-Bug:1288427
Change-Id: If382c57baea061753d3f4fcd6faec1a31fbfb7ed

neutron/db/migration/alembic_migrations/versions/HEAD

@@ -0,0 +1 @@
+2447ad0e9585

neutron/db/migration/cli.py

@@ -18,11 +18,14 @@ import os
 
 from alembic import command as alembic_command
 from alembic import config as alembic_config
+from alembic import script as alembic_script
 from alembic import util as alembic_util
 from oslo.config import cfg
 
 from neutron.common import legacy
 
+HEAD_FILENAME = 'HEAD'
+
 
 _core_opts = [
     cfg.StrOpt('core_plugin',
@@ -61,6 +64,7 @@ def do_alembic_command(config, cmd, *args, **kwargs):
 
 def do_check_migration(config, cmd):
     do_alembic_command(config, 'branches')
+    validate_head_file(config)
 
 
 def do_upgrade_downgrade(config, cmd):
@@ -89,6 +93,30 @@ def do_revision(config, cmd):
                        message=CONF.command.message,
                        autogenerate=CONF.command.autogenerate,
                        sql=CONF.command.sql)
+    update_head_file(config)
+
+
+def validate_head_file(config):
+    script = alembic_script.ScriptDirectory.from_config(config)
+    if len(script.get_heads()) > 1:
+        alembic_util.err(_('Timeline branches unable to generate timeline'))
+
+    head_path = os.path.join(script.versions, HEAD_FILENAME)
+    if (os.path.isfile(head_path) and
+        open(head_path).read().strip() == script.get_current_head()):
+        return
+    else:
+        alembic_util.err(_('HEAD file does not match migration timeline head'))
+
+
+def update_head_file(config):
+    script = alembic_script.ScriptDirectory.from_config(config)
+    if len(script.get_heads()) > 1:
+        alembic_util.err(_('Timeline branches unable to generate timeline'))
+
+    head_path = os.path.join(script.versions, HEAD_FILENAME)
+    with open(head_path, 'w+') as f:
+        f.write(script.get_current_head())
 
 
 def add_command_parsers(subparsers):

neutron/tests/unit/test_db_migration.py

@@ -40,6 +40,8 @@ class TestCli(base.BaseTestCase):
         super(TestCli, self).setUp()
         self.do_alembic_cmd_p = mock.patch.object(cli, 'do_alembic_command')
         self.do_alembic_cmd = self.do_alembic_cmd_p.start()
+        self.mock_alembic_err = mock.patch('alembic.util.err').start()
+        self.mock_alembic_err.side_effect = SystemExit
         self.addCleanup(self.do_alembic_cmd_p.stop)
         self.addCleanup(cli.CONF.reset)
 
@@ -72,22 +74,28 @@ class TestCli(base.BaseTestCase):
         self._main_test_helper(['prog', 'history'], 'history')
 
     def test_check_migration(self):
-        self._main_test_helper(['prog', 'check_migration'], 'branches')
+        with mock.patch.object(cli, 'validate_head_file') as validate:
+            self._main_test_helper(['prog', 'check_migration'], 'branches')
+            validate.assert_called_once_with(mock.ANY)
 
     def test_database_sync_revision(self):
-        self._main_test_helper(
-            ['prog', 'revision', '--autogenerate', '-m', 'message'],
-            'revision',
-            (),
-            {'message': 'message', 'sql': False, 'autogenerate': True}
-        )
+        with mock.patch.object(cli, 'update_head_file') as update:
+            self._main_test_helper(
+                ['prog', 'revision', '--autogenerate', '-m', 'message'],
+                'revision',
+                (),
+                {'message': 'message', 'sql': False, 'autogenerate': True}
+            )
+            update.assert_called_once_with(mock.ANY)
 
-        self._main_test_helper(
-            ['prog', 'revision', '--sql', '-m', 'message'],
-            'revision',
-            (),
-            {'message': 'message', 'sql': True, 'autogenerate': False}
-        )
+            update.reset_mock()
+            self._main_test_helper(
+                ['prog', 'revision', '--sql', '-m', 'message'],
+                'revision',
+                (),
+                {'message': 'message', 'sql': True, 'autogenerate': False}
+            )
+            update.assert_called_once_with(mock.ANY)
 
     def test_upgrade(self):
         self._main_test_helper(
@@ -118,3 +126,61 @@ class TestCli(base.BaseTestCase):
             ('-2',),
             {'sql': False}
         )
+
+    def _test_validate_head_file_helper(self, heads, file_content=None):
+        with mock.patch('alembic.script.ScriptDirectory.from_config') as fc:
+            fc.return_value.get_heads.return_value = heads
+            fc.return_value.get_current_head.return_value = heads[0]
+            with mock.patch('__builtin__.open') as mock_open:
+                mock_open.return_value.__enter__ = lambda s: s
+                mock_open.return_value.__exit__ = mock.Mock()
+                mock_open.return_value.read.return_value = file_content
+
+                with mock.patch('os.path.isfile') as is_file:
+                    is_file.return_value = file_content is not None
+
+                    if file_content in heads:
+                        cli.validate_head_file(mock.sentinel.config)
+                    else:
+                        self.assertRaises(
+                            SystemExit,
+                            cli.validate_head_file,
+                            mock.sentinel.config
+                        )
+                        self.mock_alembic_err.assert_called_once_with(mock.ANY)
+            fc.assert_called_once_with(mock.sentinel.config)
+
+    def test_validate_head_file_multiple_heads(self):
+        self._test_validate_head_file_helper(['a', 'b'])
+
+    def test_validate_head_file_missing_file(self):
+        self._test_validate_head_file_helper(['a'])
+
+    def test_validate_head_file_wrong_contents(self):
+        self._test_validate_head_file_helper(['a'], 'b')
+
+    def test_validate_head_success(self):
+        self._test_validate_head_file_helper(['a'], 'a')
+
+    def test_update_head_file_multiple_heads(self):
+        with mock.patch('alembic.script.ScriptDirectory.from_config') as fc:
+            fc.return_value.get_heads.return_value = ['a', 'b']
+            self.assertRaises(
+                SystemExit,
+                cli.update_head_file,
+                mock.sentinel.config
+            )
+            self.mock_alembic_err.assert_called_once_with(mock.ANY)
+            fc.assert_called_once_with(mock.sentinel.config)
+
+    def test_update_head_file_success(self):
+        with mock.patch('alembic.script.ScriptDirectory.from_config') as fc:
+            fc.return_value.get_heads.return_value = ['a']
+            fc.return_value.get_current_head.return_value = 'a'
+            with mock.patch('__builtin__.open') as mock_open:
+                mock_open.return_value.__enter__ = lambda s: s
+                mock_open.return_value.__exit__ = mock.Mock()
+
+                cli.update_head_file(mock.sentinel.config)
+                mock_open.write.called_once_with('a')
+            fc.assert_called_once_with(mock.sentinel.config)

tox.ini

@@ -25,6 +25,7 @@ downloadcache = ~/cache/pip
 [testenv:pep8]
 commands =
   flake8
+  neutron-db-manage check_migration
 
 [testenv:i18n]
 commands = python ./tools/check_i18n.py ./neutron ./tools/i18n_cfg.py