Factor out a `copy_power_levels_contents` method

I'm going to need another copy (hah!) of this.

synapse/events/utils.py

@@ -12,8 +12,9 @@
 # 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 collections
 import re
+from typing import Mapping, Union
 
 from six import string_types
 
@@ -422,3 +423,37 @@ class EventClientSerializer(object):
         return yieldable_gather_results(
             self.serialize_event, events, time_now=time_now, **kwargs
         )
+
+
+def copy_power_levels_contents(
+    old_power_levels: Mapping[str, Union[int, Mapping[str, int]]]
+):
+    """Copy the content of a power_levels event, unfreezing frozendicts along the way
+
+    Raises:
+        TypeError if the input does not look like a valid power levels event content
+    """
+    if not isinstance(old_power_levels, collections.Mapping):
+        raise TypeError("Not a valid power-levels content: %r" % (old_power_levels,))
+
+    power_levels = {}
+    for k, v in old_power_levels.items():
+
+        if isinstance(v, int):
+            power_levels[k] = v
+            continue
+
+        if isinstance(v, collections.Mapping):
+            power_levels[k] = h = {}
+            for k1, v1 in v.items():
+                # we should only have one level of nesting
+                if not isinstance(v1, int):
+                    raise TypeError(
+                        "Invalid power_levels value for %s.%s: %r" % (k, k1, v)
+                    )
+                h[k1] = v1
+            continue
+
+        raise TypeError("Invalid power_levels value for %s: %r" % (k, v))
+
+    return power_levels

synapse/handlers/room.py

@@ -30,6 +30,7 @@ from twisted.internet import defer
 from synapse.api.constants import EventTypes, JoinRules, RoomCreationPreset
 from synapse.api.errors import AuthError, Codes, NotFoundError, StoreError, SynapseError
 from synapse.api.room_versions import KNOWN_ROOM_VERSIONS, RoomVersion
+from synapse.events.utils import copy_power_levels_contents
 from synapse.http.endpoint import parse_and_validate_server_name
 from synapse.storage.state import StateFilter
 from synapse.types import (
@@ -367,6 +368,15 @@ class RoomCreationHandler(BaseHandler):
             if old_event:
                 initial_state[k] = old_event.content
 
+        # deep-copy the power-levels event before we start modifying it
+        # note that if frozen_dicts are enabled, `power_levels` will be a frozen
+        # dict so we can't just copy.deepcopy it.
+        initial_state[
+            (EventTypes.PowerLevels, "")
+        ] = power_levels = copy_power_levels_contents(
+            initial_state[(EventTypes.PowerLevels, "")]
+        )
+
         # Resolve the minimum power level required to send any state event
         # We will give the upgrading user this power level temporarily (if necessary) such that
         # they are able to copy all of the state events over, then revert them back to their
@@ -375,8 +385,6 @@ class RoomCreationHandler(BaseHandler):
         # Copy over user power levels now as this will not be possible with >100PL users once
         # the room has been created
 
-        power_levels = initial_state[(EventTypes.PowerLevels, "")]
-
         # Calculate the minimum power level needed to clone the room
         event_power_levels = power_levels.get("events", {})
         state_default = power_levels.get("state_default", 0)
@@ -386,16 +394,7 @@ class RoomCreationHandler(BaseHandler):
         # Raise the requester's power level in the new room if necessary
         current_power_level = power_levels["users"][user_id]
         if current_power_level < needed_power_level:
-            # make sure we copy the event content rather than overwriting it.
-            # note that if frozen_dicts are enabled, `power_levels` will be a frozen
-            # dict so we can't just copy.deepcopy it.
-
-            new_power_levels = {k: v for k, v in power_levels.items() if k != "users"}
-            new_power_levels["users"] = {
-                k: v for k, v in power_levels.get("users", {}).items() if k != user_id
-            }
-            new_power_levels["users"][user_id] = needed_power_level
-            initial_state[(EventTypes.PowerLevels, "")] = new_power_levels
+            power_levels["users"][user_id] = needed_power_level
 
         yield self._send_events_for_new_room(
             requester,

tests/events/test_utils.py

@@ -15,9 +15,14 @@
 
 
 from synapse.events import FrozenEvent
-from synapse.events.utils import prune_event, serialize_event
+from synapse.events.utils import (
+    copy_power_levels_contents,
+    prune_event,
+    serialize_event,
+)
+from synapse.util.frozenutils import freeze
 
-from .. import unittest
+from tests import unittest
 
 
 def MockEvent(**kwargs):
@@ -241,3 +246,39 @@ class SerializeEventTestCase(unittest.TestCase):
             self.serialize(
                 MockEvent(room_id="!foo:bar", content={"foo": "bar"}), ["room_id", 4]
             )
+
+
+class CopyPowerLevelsContentTestCase(unittest.TestCase):
+    def setUp(self) -> None:
+        self.test_content = {
+            "ban": 50,
+            "events": {"m.room.name": 100, "m.room.power_levels": 100},
+            "events_default": 0,
+            "invite": 50,
+            "kick": 50,
+            "notifications": {"room": 20},
+            "redact": 50,
+            "state_default": 50,
+            "users": {"@example:localhost": 100},
+            "users_default": 0,
+        }
+
+    def _test(self, input):
+        a = copy_power_levels_contents(input)
+
+        self.assertEqual(a["ban"], 50)
+        self.assertEqual(a["events"]["m.room.name"], 100)
+
+        # make sure that changing the copy changes the copy and not the orig
+        a["ban"] = 10
+        a["events"]["m.room.power_levels"] = 20
+
+        self.assertEqual(input["ban"], 50)
+        self.assertEqual(input["events"]["m.room.power_levels"], 100)
+
+    def test_unfrozen(self):
+        self._test(self.test_content)
+
+    def test_frozen(self):
+        input = freeze(self.test_content)
+        self._test(input)