flows: introduce FlowPlan markers, which indicate when a stage needs … (#79)

* flows: introduce FlowPlan markers, which indicate when a stage needs re-evaluation

Implement re_evaluate_policies
add unittests for several different scenarios
closes #78

* flows: move markers to separate files, cleanup formatting

* flows: fix self.next is not callable

passbook/flows/markers.py

@@ -0,0 +1,49 @@
+"""Stage Markers"""
+from dataclasses import dataclass
+from typing import TYPE_CHECKING, Optional
+
+from structlog import get_logger
+
+from passbook.core.models import User
+from passbook.flows.models import Stage
+from passbook.policies.engine import PolicyEngine
+from passbook.policies.models import PolicyBinding
+
+if TYPE_CHECKING:
+    from passbook.flows.planner import FlowPlan
+
+LOGGER = get_logger()
+
+
+@dataclass
+class StageMarker:
+    """Base stage marker class, no extra attributes, and has no special handler."""
+
+    # pylint: disable=unused-argument
+    def process(self, plan: "FlowPlan", stage: Stage) -> Optional[Stage]:
+        """Process callback for this marker. This should be overridden by sub-classes.
+        If a stage should be removed, return None."""
+        return stage
+
+
+@dataclass
+class ReevaluateMarker(StageMarker):
+    """Reevaluate Marker, forces stage's policies to be evaluated again."""
+
+    binding: PolicyBinding
+    user: User
+
+    def process(self, plan: "FlowPlan", stage: Stage) -> Optional[Stage]:
+        """Re-evaluate policies bound to stage, and if they fail, remove from plan"""
+        engine = PolicyEngine(self.binding, self.user)
+        engine.request.context = plan.context
+        engine.build()
+        result = engine.result
+        if result.passing:
+            return stage
+        LOGGER.warning(
+            "f(plan_inst)[re-eval marker]: stage failed re-evaluation",
+            stage=stage,
+            messages=result.messages,
+        )
+        return None