Support rolling back to revision 0

This patch set allows revision rollback functionality to work
with revision 0 in order to allow Deckhand to create a blank
revision (without any documents), without having to delete every
revision in the database (which is currently done here: [1] for
example). This is less than ideal as revision history is lost.
Instead, Deckhand should just support rolling back to revision 0
which simulates a blank slate (though in reality the previous
revision history is retained).

[1] 18ed6674d2/src/bin/shipyard_airflow/shipyard_airflow/control/helpers/configdocs_helper.py (L159)

Change-Id: Ic94b25ca15f88ba492e08c907d4a330aeecca810

deckhand/db/sqlalchemy/api.py

@@ -510,20 +510,22 @@ def revision_get_latest(session=None):
 
     :param session: Database session object.
     :returns: Dictionary representation of latest revision.
-    :raises RevisionNotFound: if the latest revision was not found.
     """
     session = session or get_session()
 
     latest_revision = session.query(models.Revision)\
         .order_by(models.Revision.created_at.desc())\
         .first()
-    if not latest_revision:
-        raise errors.RevisionNotFound(revision_id='latest')
 
-    latest_revision = latest_revision.to_dict()
-
-    latest_revision['documents'] = _update_revision_history(
-        latest_revision['documents'])
+    if latest_revision:
+        latest_revision = latest_revision.to_dict()
+        latest_revision['documents'] = _update_revision_history(
+            latest_revision['documents'])
+    else:
+        # If the latest revision doesn't exist, assume an empty revision
+        # history and return a dummy revision instead for the purposes of
+        # revision rollback.
+        latest_revision = {'documents': [], 'id': 0}
 
     return latest_revision
 
@@ -976,7 +978,11 @@ def revision_rollback(revision_id, latest_revision, session=None):
         LOG.debug('The revision being rolled back to is the current revision.'
                   'Expect no meaningful changes.')
 
-    orig_revision = revision_get(revision_id, session=session)
+    if revision_id == 0:
+        # Placeholder revision as revision_id=0 doesn't exist.
+        orig_revision = {'documents': []}
+    else:
+        orig_revision = revision_get(revision_id, session=session)
 
     # A mechanism for determining whether a particular document has changed
     # between revisions. Keyed with the document_id, the value is True if

deckhand/tests/unit/db/test_revision_rollback.py

@@ -12,6 +12,7 @@
 # See the License for the specific language governing permissions and
 # limitations under the License.
 
+from deckhand import errors
 from deckhand.tests import test_utils
 from deckhand.tests.unit.db import base
 
@@ -98,3 +99,33 @@ class TestRevisionRollback(base.TestDbBase):
                    key=lambda d: d['created_at']),
             ignore=['created_at', 'updated_at', 'revision_id',
                     'orig_revision_id', 'id'])
+
+    def test_rollback_to_revision_0_creates_blank_slate(self):
+        """Rolling back to revision 0 should create an empty revision."""
+        payload = base.DocumentFixture.get_minimal_multi_fixture(count=4)
+        bucket_name = test_utils.rand_name('bucket')
+        created_documents = self.create_documents(bucket_name, payload)
+        orig_revision_id = created_documents[0]['revision_id']
+
+        rollback_revision = self.rollback_revision(0)
+        rollback_documents = self.list_revision_documents(
+            rollback_revision['id'], include_history=False)
+        self.assertEqual(orig_revision_id + 1, rollback_revision['id'])
+        self.assertEmpty(rollback_documents)
+
+    def test_rollback_to_revision_0_with_empty_revision_history(self):
+        """Validate that rolling back to revision_id 0 should work with
+        an empty revision history (zero existing revisions in the DB).
+        """
+        rollback_revision = self.rollback_revision(0)
+        rollback_documents = self.list_revision_documents(
+            rollback_revision['id'], include_history=False)
+        self.assertEqual(1, rollback_revision['id'])
+        self.assertEmpty(rollback_documents)
+
+
+class TestRevisionRollbackNegative(base.TestDbBase):
+
+    def test_rollback_to_missing_revision_raises_exc(self):
+        # revision_id=1 doesn't exist yet since we start from an empty DB.
+        self.assertRaises(errors.RevisionNotFound, self.rollback_revision, 1)