Raise an error for empty plan

If the execution graph that solar-worker retrieves from db is empty,
ValueError is raised (it's handled by zerorpc executor).

Additionally, db string used for db connection is logged on start of
solar-worker.

Change-Id: I2acafec0e94c0d2792b34d0485415782eae1b9dd
Closes-bug:1545813

solar/orchestration/__init__.py

@@ -55,6 +55,7 @@ def construct_tasks(extensions, clients):
 def main():
     # NOTE(mkwiek): no arguments should be supplied to solar-worker
     argparse.ArgumentParser().parse_args()
+    log.info('Database in use: {}'.format(C.solar_db))
     runner = loader.get_runner(C.runner)
     constructors = loader.get_constructors()
     clients = loader.get_clients()

solar/orchestration/workers/scheduler.py

@@ -51,6 +51,8 @@ class Scheduler(base.Worker):
         ):
             log.debug('Received *next* event for %s', plan_uid)
             plan = graph.get_graph(plan_uid)
+            if len(plan) == 0:
+                raise ValueError('Plan {} is empty'.format(plan_uid))
             rst = self._next(plan)
             for task_name in rst:
                 self._do_scheduling(plan, task_name)

solar/test/orchestration/test_scheduler.py

@@ -0,0 +1,23 @@
+#    Copyright 2016 Mirantis, Inc.
+#
+#    Licensed under the Apache License, Version 2.0 (the "License"); you may
+#    not use this file except in compliance with the License. You may obtain
+#    a copy of the License at
+#
+#         http://www.apache.org/licenses/LICENSE-2.0
+#
+#    Unless required by applicable law or agreed to in writing, software
+#    distributed under the License is distributed on an "AS IS" BASIS, WITHOUT
+#    WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied. See the
+#    License for the specific language governing permissions and limitations
+#    under the License.
+
+import pytest
+
+from solar.orchestration.workers.scheduler import Scheduler
+
+
+def test_scheduler_next_fails_with_empty_plan():
+    scheduler = Scheduler(None)
+    with pytest.raises(ValueError):
+        scheduler.next({}, 'nonexistent_uid')