Change engine 'self._check' into a decorator

Instead of having 'self._check' be called at the stat
of every using function it can be made into a decorator
and placed on the function instead. This way it is a little
more explicit than having it called at the start of each
method.

This also adds a check to ensure the engine has been validated
before it is ran (and the decorator takes into this into
account as well).

Change-Id: I67ee8fe8629392003f7777d4d4fab918b1a071dd

taskflow/engines/action_engine/engine.py

@@ -56,6 +56,32 @@ def _start_stop(task_executor, retry_executor):
         task_executor.stop()
 
 
+def _pre_check(check_compiled=True, check_storage_ensured=True,
+               check_validated=True):
+    """Engine state precondition checking decorator."""
+
+    def decorator(meth):
+        do_what = meth.__name__
+
+        @six.wraps(meth)
+        def wrapper(self, *args, **kwargs):
+            if check_compiled and not self._compiled:
+                raise exc.InvalidState("Can not %s an engine which"
+                                       " has not been compiled" % do_what)
+            if check_storage_ensured and not self._storage_ensured:
+                raise exc.InvalidState("Can not %s an engine"
+                                       " which has not had its storage"
+                                       " populated" % do_what)
+            if check_validated and not self._validated:
+                raise exc.InvalidState("Can not %s an engine which"
+                                       " has not been validated" % do_what)
+            return meth(self, *args, **kwargs)
+
+        return wrapper
+
+    return decorator
+
+
 class ActionEngine(base.Engine):
     """Generic action-based engine.
 
@@ -131,24 +157,21 @@ class ActionEngine(base.Engine):
         self._lock = threading.RLock()
         self._state_lock = threading.RLock()
         self._storage_ensured = False
+        self._validated = False
         # Retries are not *currently* executed out of the engines process
         # or thread (this could change in the future if we desire it to).
         self._retry_executor = executor.SerialRetryExecutor()
         self._inject_transient = strutils.bool_from_string(
             self._options.get('inject_transient', True))
 
-    def _check(self, name, check_compiled, check_storage_ensured):
-        """Check (and raise) if the engine has not reached a certain stage."""
-        if check_compiled and not self._compiled:
-            raise exc.InvalidState("Can not %s an engine which"
-                                   " has not been compiled" % name)
-        if check_storage_ensured and not self._storage_ensured:
-            raise exc.InvalidState("Can not %s an engine"
-                                   " which has not has its storage"
-                                   " populated" % name)
-
+    @_pre_check(check_compiled=True,
+                # NOTE(harlowja): We can alter the state of the
+                # flow without ensuring its storage is setup for
+                # its atoms (since this state change does not affect
+                # those units).
+                check_storage_ensured=False,
+                check_validated=False)
     def suspend(self):
-        self._check('suspend', True, False)
         self._change_state(states.SUSPENDING)
 
     @property
@@ -300,8 +323,8 @@ class ActionEngine(base.Engine):
                                               transient=self._inject_transient)
 
     @fasteners.locked
+    @_pre_check(check_validated=False)
     def validate(self):
-        self._check('validate', True, True)
         # At this point we can check to ensure all dependencies are either
         # flow/task provided or storage provided, if there are still missing
         # dependencies then this flow will fail at runtime (which we can avoid
@@ -341,10 +364,11 @@ class ActionEngine(base.Engine):
                 raise exc.MissingDependencies(self._flow,
                                               sorted(missing),
                                               cause=last_cause)
+        self._validated = True
 
     @fasteners.locked
+    @_pre_check(check_storage_ensured=False, check_validated=False)
     def prepare(self):
-        self._check('prepare', True, False)
         if not self._storage_ensured:
             # Set our own state to resuming -> (ensure atoms exist
             # in storage) -> suspended in the storage unit and notify any
@@ -358,8 +382,8 @@ class ActionEngine(base.Engine):
             self.reset()
 
     @fasteners.locked
+    @_pre_check(check_validated=False)
     def reset(self):
-        self._check('reset', True, True)
         # This transitions *all* contained atoms back into the PENDING state
         # with an intention to EXECUTE (or dies trying to do that) and then
         # changes the state of the flow to PENDING so that it can then run...