Freeze rather than just clone

synapse/module_api/__init__.py

@@ -12,7 +12,6 @@
 # WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
 # See the License for the specific language governing permissions and
 # limitations under the License.
-import copy
 import email.utils
 import logging
 from typing import (
@@ -120,6 +119,7 @@
 from synapse.util import Clock
 from synapse.util.async_helpers import maybe_awaitable
 from synapse.util.caches.descriptors import cached
+from synapse.util.frozenutils import freeze
 
 if TYPE_CHECKING:
     from synapse.app.generic_worker import GenericWorkerSlavedStore
@@ -1432,9 +1432,9 @@ async def get_global(self, user_id: str, data_type: str) -> Optional[JsonDict]:
         data = await self._store.get_global_account_data_by_type_for_user(
             user_id, data_type
         )
-        # We clone to prevent the module accidentally mutating the dict that
-        # lives in the cache, as that could introduce nasty bugs.
-        return copy.deepcopy(data)
+        # We clone and freeze to prevent the module accidentally mutating the
+        # dict that lives in the cache, as that could introduce nasty bugs.
+        return freeze(data)
 
     async def put_global(
         self, user_id: str, data_type: str, new_data: JsonDict

tests/module_api/test_account_data_manager.py

@@ -85,31 +85,13 @@ def test_get_global_no_mutability(self) -> None:
             )
         )
 
-        # Request that account data from the normal store; check it's as we expect.
-        self.assertEqual(
-            self.get_success(
-                self._store.get_global_account_data_by_type_for_user(
-                    self.user_id, "test.data"
-                )
-            ),
-            {"wombat": True},
-        )
-
-        # Now, as a module, request that data and then mutate it (out of negligence or otherwise).
+        # Now request that data and then mutate it (out of negligence or otherwise).
         the_data = self.get_success(
             self._account_data_mgr.get_global(self.user_id, "test.data")
         )
-        the_data["wombat"] = False
-
-        # As Synapse, request that account data once more from the normal store; check it's as we expect.
-        self.assertEqual(
-            self.get_success(
-                self._store.get_global_account_data_by_type_for_user(
-                    self.user_id, "test.data"
-                )
-            ),
-            {"wombat": True},
-        )
+        with self.assertRaises(TypeError):
+            # This throws an exception because it's a frozen dict.
+            the_data["wombat"] = False
 
     def test_put_global(self) -> None:
         """