Fix internal server error on deleting nodes with allocations

We allow deleting allocated nodes if maintenance is True, but it
fails with HTTP 500 because of a foreign constraint on the nodes
table.

Story: #2007823
Task: #40102
Change-Id: I7a689c8c403b613fca412fa70e31df40cce8ad95

ironic/db/sqlalchemy/api.py

@@ -564,7 +564,7 @@ class Connection(api.Connection):
 
     @oslo_db_api.retry_on_deadlock
     def destroy_node(self, node_id):
-        with _session_for_write():
+        with _session_for_write() as session:
             query = model_query(models.Node)
             query = add_identity_filter(query, node_id)
 
@@ -573,6 +573,11 @@ class Connection(api.Connection):
             except NoResultFound:
                 raise exception.NodeNotFound(node=node_id)
 
+            # Orphan allocation, if any. On the API level this is only allowed
+            # with maintenance on.
+            node_ref.allocation_id = None
+            node_ref.save(session)
+
             # Get node ID, if an UUID was supplied. The ID is
             # required for deleting all ports, attached to the node.
             if uuidutils.is_uuid_like(node_id):

ironic/tests/unit/db/test_nodes.py

@@ -595,6 +595,8 @@ class DbNodeTestCase(base.DbTestCase):
         node = utils.create_test_node()
 
         allocation = utils.create_test_allocation(node_id=node.id)
+        node = self.dbapi.update_node(node.id,
+                                      {'allocation_id': allocation.id})
 
         self.dbapi.destroy_node(node.uuid)
         self.assertRaises(exception.AllocationNotFound,

releasenotes/notes/allocation-delete-26c7c2f1651759f5.yaml

@@ -0,0 +1,6 @@
+---
+fixes:
+  - |
+    Fixes deleting nodes with maintenance mode on and an allocation present.
+    Previously it caused an internal server error. See `story 2007823
+    <https://storyboard.openstack.org/#!/story/2007823>`_ for details.