Refresh object state after lock acquisition in WithItemsTask

* We need to refresh task state once we acquired the 'with-items'
  lock to prevent modifying stale values cached by SQLAlchemy
  session. For this purpose, new DB API method refresh() was
  added. Without this change 'with-items' task doesn't work
  properly in case of using multiple engines.

Change-Id: Id5a1d2cf03090279e6df3805562e00d300b89953
Closes-Bug: #1640378
Partial-Bug: #1640379

mistral/db/v2/api.py

@@ -58,6 +58,10 @@ def transaction():
         yield
 
 
+def refresh(model):
+    IMPL.refresh(model)
+
+
 # Locking.
 
 

mistral/db/v2/sqlalchemy/api.py

@@ -115,6 +115,11 @@ def transaction():
         end_tx()
 
 
+@b.session_aware()
+def refresh(model, session=None):
+    session.refresh(model)
+
+
 @b.session_aware()
 def acquire_lock(model, id, session=None):
     # Expire all so all objects queried after lock is acquired

mistral/engine/tasks.py

@@ -438,6 +438,14 @@ class WithItemsTask(RegularTask):
             # from modifying runtime context simultaneously by multiple
             # transactions.
             with db_api.named_lock('with-items-%s' % self.task_ex.id):
+                # NOTE: We need to refresh task execution object right
+                # after the lock is acquired to make sure that we're
+                # working with a fresh state of its runtime context.
+                # Otherwise, SQLAlchemy session can contain a stale
+                # cached version of it so that we don't modify actual
+                # values (i.e. capacity).
+                db_api.refresh(self.task_ex)
+
                 self._on_action_complete(action_ex)
 
     def _on_action_complete(self, action_ex):