2 files changed, 20 insertions, 5 deletions
diff --git a/synapse/events/__init__.py b/synapse/events/__init__.py
index 3ec4120f85..8f6b955d17 100644
--- a/synapse/events/__init__.py
+++ b/synapse/events/__init__.py
@@ -98,7 +98,7 @@ class DefaultDictProperty(DictProperty):
 
 
 class _EventInternalMetadata:
-    __slots__ = ["_dict", "stream_ordering"]
+    __slots__ = ["_dict", "stream_ordering", "outlier"]
 
     def __init__(self, internal_metadata_dict: JsonDict):
         # we have to copy the dict, because it turns out that the same dict is
@@ -108,7 +108,10 @@ class _EventInternalMetadata:
         # the stream ordering of this event. None, until it has been persisted.
         self.stream_ordering = None  # type: Optional[int]
 
-    outlier = DictProperty("outlier")  # type: bool
+        # whether this event is an outlier (ie, whether we have the state at that point
+        # in the DAG)
+        self.outlier = False
+
     out_of_band_membership = DictProperty("out_of_band_membership")  # type: bool
     send_on_behalf_of = DictProperty("send_on_behalf_of")  # type: str
     recheck_redaction = DictProperty("recheck_redaction")  # type: bool
@@ -129,7 +132,7 @@ class _EventInternalMetadata:
         return dict(self._dict)
 
     def is_outlier(self) -> bool:
-        return self._dict.get("outlier", False)
+        return self.outlier
 
     def is_out_of_band_membership(self) -> bool:
         """Whether this is an out of band membership, like an invite or an invite
diff --git a/synapse/events/utils.py b/synapse/events/utils.py
index 7ca5c9940a..0f8a3b5ad8 100644
--- a/synapse/events/utils.py
+++ b/synapse/events/utils.py
@@ -22,6 +22,7 @@ from synapse.api.constants import EventTypes, RelationTypes
 from synapse.api.errors import Codes, SynapseError
 from synapse.api.room_versions import RoomVersion
 from synapse.util.async_helpers import yieldable_gather_results
+from synapse.util.frozenutils import unfreeze
 
 from . import EventBase
 
@@ -54,6 +55,8 @@ def prune_event(event: EventBase) -> EventBase:
         event.internal_metadata.stream_ordering
     )
 
+    pruned_event.internal_metadata.outlier = event.internal_metadata.outlier
+
     # Mark the event as redacted
     pruned_event.internal_metadata.redacted = True
 
@@ -400,10 +403,19 @@ class EventClientSerializer:
                 # If there is an edit replace the content, preserving existing
                 # relations.
 
+                # Ensure we take copies of the edit content, otherwise we risk modifying
+                # the original event.
+                edit_content = edit.content.copy()
+
+                # Unfreeze the event content if necessary, so that we may modify it below
+                edit_content = unfreeze(edit_content)
+                serialized_event["content"] = edit_content.get("m.new_content", {})
+
+                # Check for existing relations
                 relations = event.content.get("m.relates_to")
-                serialized_event["content"] = edit.content.get("m.new_content", {})
                 if relations:
-                    serialized_event["content"]["m.relates_to"] = relations
+                    # Keep the relations, ensuring we use a dict copy of the original
+                    serialized_event["content"]["m.relates_to"] = relations.copy()
                 else:
                     serialized_event["content"].pop("m.relates_to", None)