feat: model error on testing module if invalid status set by charm

ops/model.py

@@ -1790,6 +1790,14 @@ class UnknownStatus(StatusBase):
     A unit-agent has finished calling install, config-changed and start, but the
     charm has not called status-set yet.
 
+    This status is for READ ONLY and should not be used to set a unit status. The following
+    charm code would be an invalid charm code.
+
+        ```
+        # Raises ops.model.ModelError : ERROR invalid status "unknown", expected one of
+        # [maintenance blocked waiting active]
+        self.unit.status = UnknownStatus()
+        ```
     """
     name = 'unknown'
 
@@ -1807,6 +1815,15 @@ class ErrorStatus(StatusBase):
 
     The unit-agent has encountered an error (the application or unit requires
     human intervention in order to operate correctly).
+
+    This status is for READ ONLY and should not be used to set a unit status. The following
+    charm code would be an invalid charm code.
+
+        ```
+        # Raises ops.model.ModelError : ERROR invalid status "error", expected one of
+        # [maintenance blocked waiting active]
+        self.unit.status = ErrorStatus()
+        ```
     """
     name = 'error'
 

ops/testing.py

@@ -446,7 +446,7 @@ def begin_with_initial_hooks(self) -> None:
         # the unit status from "Maintenance" to "Unknown". See gh#726
         post_setup_sts = self._backend.status_get()
         if post_setup_sts.get("status") == "maintenance" and not post_setup_sts.get("message"):
-            self._backend.status_set("unknown", "", is_app=False)
+            self._backend.status_set("unknown", "", is_app=False, by_testing_backend=True)
         all_ids = list(self._backend._relation_names.items())
         random.shuffle(all_ids)
         for rel_id, rel_name in all_ids:
@@ -2222,7 +2222,17 @@ def status_get(self, *, is_app: bool = False):
         else:
             return self._unit_status
 
-    def status_set(self, status: '_StatusName', message: str = '', *, is_app: bool = False):
+    def status_set(
+        self,
+        status: "_StatusName",
+        message: str = "",
+        *,
+        is_app: bool = False,
+        by_testing_backend: bool = False,
+    ):
+        if not by_testing_backend and status in {model.ErrorStatus.name, model.UnknownStatus.name}:
+            raise model.ModelError(f'ERROR invalid status "{status}", expected one of'
+                                   ' [maintenance blocked waiting active]')
         if is_app:
             self._app_status = {'status': status, 'message': message}
         else:

test/test_testing.py

@@ -2851,6 +2851,29 @@ def _on_collect_unit_status(self, event: ops.CollectStatusEvent):
         self.assertEqual(harness.model.app.status, ops.ActiveStatus('active app'))
         self.assertEqual(harness.model.unit.status, ops.ActiveStatus('active unit'))
 
+    def test_invalid_status_set(self):
+
+        class TestCharm(ops.CharmBase):
+            def __init__(self, framework: ops.Framework):
+                super().__init__(framework)
+                self.framework.observe(self.on.collect_app_status, self._on_collect_app_status)
+                self.framework.observe(self.on.collect_unit_status, self._on_collect_unit_status)
+                self.app_status_to_add = ops.ErrorStatus('errored app')
+                self.unit_status_to_add = ops.ErrorStatus('errored unit')
+
+            def _on_collect_app_status(self, event: ops.CollectStatusEvent):
+                event.add_status(self.app_status_to_add)
+
+            def _on_collect_unit_status(self, event: ops.CollectStatusEvent):
+                event.add_status(self.unit_status_to_add)
+
+        harness = ops.testing.Harness(TestCharm)
+        harness.set_leader(True)
+        harness.begin()
+
+        with self.assertRaises(ops.model.ModelError):
+            harness.evaluate_status()
+
 
 class TestNetwork(unittest.TestCase):
     def setUp(self):