Clear stale sign data when signs break

tests/test_signs.py

@@ -3,6 +3,7 @@ from unittest.mock import patch
 from munch import Munch
 
 from mosfet.commands import Commands
+from mosfet.info import blocks
 from mosfet.protocol.managers import EntityManager, WorldState
 from mosfet.world import World
 
@@ -45,6 +46,47 @@ class SignReadingTests(unittest.TestCase):
         signs = World(Munch(chunks=chunks)).find_signs((4, 65, -2), 1)
         self.assertEqual(signs[0]['lines'], ('Farm', '', '', ''))
 
+    def test_broken_sign_block_update_removes_cached_sign_entity(self):
+        position = (4, 65, -2)
+        sign_state = blocks.MCD_BLOCKS['oak_wall_sign']['minStateId']
+
+        class FakeColumn:
+            def __init__(self):
+                self.states = {}
+
+            def get_block_at(self, x, y, z):
+                return self.states.get((x, y, z), blocks.AIR)
+
+            def set_block_at(self, x, y, z, state_id):
+                previous = self.get_block_at(x, y, z)
+                self.states[(x, y, z)] = state_id
+                return previous
+
+        chunks = WorldState()
+        chunks.chunks[(0, -1)] = FakeColumn()
+        chunks.set_block_at(*position, sign_state)
+        chunks.set_block_entity(position, 7, {
+            'front_text': {'messages': ['mosfet1 stay', 'within 8 square', '', '']},
+        })
+        world = World(Munch(chunks=chunks))
+
+        self.assertEqual(len(world.find_signs(position, 1)), 1)
+        chunks.set_block_at(*position, blocks.AIR)
+
+        self.assertNotIn(position, chunks.block_entities)
+        self.assertEqual(world.find_signs(position, 1), [])
+
+    def test_sign_search_ignores_stale_block_entity_for_non_sign_block(self):
+        position = (4, 65, -2)
+        chunks = Munch(
+            block_entities={position: {'data': {'front_text': {
+                'messages': ['mosfet1 stay', 'within 8 square', '', ''],
+            }}}},
+            get_block_at=lambda *_position: blocks.AIR,
+        )
+
+        self.assertEqual(World(Munch(chunks=chunks)).find_signs(position, 1), [])
+
     def test_group_and_individual_sign_addresses(self):
         group_world = World(Munch(chunks=Munch(block_entities={
             (0, 64, 0): {'data': {'front_text': {'messages': [