1 files changed, 25 insertions, 5 deletions
diff --git a/synapse/events/utils.py b/synapse/events/utils.py
index e0613d0dbc..0772472312 100644
--- a/synapse/events/utils.py
+++ b/synapse/events/utils.py
@@ -49,7 +49,7 @@ from synapse.api.errors import Codes, SynapseError
 from synapse.api.room_versions import RoomVersion
 from synapse.types import JsonDict, Requester
 
-from . import EventBase
+from . import EventBase, make_event_from_dict
 
 if TYPE_CHECKING:
     from synapse.handlers.relations import BundledAggregations
@@ -82,17 +82,14 @@ def prune_event(event: EventBase) -> EventBase:
     """
     pruned_event_dict = prune_event_dict(event.room_version, event.get_dict())
 
-    from . import make_event_from_dict
-
     pruned_event = make_event_from_dict(
         pruned_event_dict, event.room_version, event.internal_metadata.get_dict()
     )
 
-    # copy the internal fields
+    # Copy the bits of `internal_metadata` that aren't returned by `get_dict`
     pruned_event.internal_metadata.stream_ordering = (
         event.internal_metadata.stream_ordering
     )
-
     pruned_event.internal_metadata.outlier = event.internal_metadata.outlier
 
     # Mark the event as redacted
@@ -101,6 +98,29 @@ def prune_event(event: EventBase) -> EventBase:
     return pruned_event
 
 
+def clone_event(event: EventBase) -> EventBase:
+    """Take a copy of the event.
+
+    This is mostly useful because it does a *shallow* copy of the `unsigned` data,
+    which means it can then be updated without corrupting the in-memory cache. Note that
+    other properties of the event, such as `content`, are *not* (currently) copied here.
+    """
+    # XXX: We rely on at least one of `event.get_dict()` and `make_event_from_dict()`
+    #   making a copy of `unsigned`. Currently, both do, though I don't really know why.
+    #   Still, as long as they do, there's not much point doing yet another copy here.
+    new_event = make_event_from_dict(
+        event.get_dict(), event.room_version, event.internal_metadata.get_dict()
+    )
+
+    # Copy the bits of `internal_metadata` that aren't returned by `get_dict`.
+    new_event.internal_metadata.stream_ordering = (
+        event.internal_metadata.stream_ordering
+    )
+    new_event.internal_metadata.outlier = event.internal_metadata.outlier
+
+    return new_event
+
+
 def prune_event_dict(room_version: RoomVersion, event_dict: JsonDict) -> JsonDict:
     """Redacts the event_dict in the same way as `prune_event`, except it
     operates on dicts rather than event objects