Combine duplicated code for calculating an event ID from a txn ID (#16023)

Refactoring related to stabilization of MSC3970, refactor to combine
code which has the same logic.

1 files changed, 31 insertions, 8 deletions

diff --git a/synapse/handlers/message.py b/synapse/handlers/message.py

index 187dedae7d..c656e07d37 100644
--- a/synapse/handlers/message.py
+++ b/synapse/handlers/message.py
@@ -878,14 +878,13 @@ class EventCreationHandler:
                 return prev_event
         return None
 
-    async def get_event_from_transaction(
+    async def get_event_id_from_transaction(
         self,
         requester: Requester,
         txn_id: str,
         room_id: str,
-    ) -> Optional[EventBase]:
-        """For the given transaction ID and room ID, check if there is a matching event.
-        If so, fetch it and return it.
+    ) -> Optional[str]:
+        """For the given transaction ID and room ID, check if there is a matching event ID.
 
         Args:
             requester: The requester making the request in the context of which we want
@@ -894,8 +893,9 @@ class EventCreationHandler:
             room_id: The room ID.
 
         Returns:
-            An event if one could be found, None otherwise.
+            An event ID if one could be found, None otherwise.
         """
+        existing_event_id = None
 
         if self._msc3970_enabled and requester.device_id:
             # When MSC3970 is enabled, we lookup for events sent by the same device first,
@@ -909,7 +909,7 @@ class EventCreationHandler:
                 )
             )
             if existing_event_id:
-                return await self.store.get_event(existing_event_id)
+                return existing_event_id
 
         # Pre-MSC3970, we looked up for events that were sent by the same session by
         # using the access token ID.
@@ -922,9 +922,32 @@ class EventCreationHandler:
                     txn_id,
                 )
             )
-            if existing_event_id:
-                return await self.store.get_event(existing_event_id)
 
+        return existing_event_id
+
+    async def get_event_from_transaction(
+        self,
+        requester: Requester,
+        txn_id: str,
+        room_id: str,
+    ) -> Optional[EventBase]:
+        """For the given transaction ID and room ID, check if there is a matching event.
+        If so, fetch it and return it.
+
+        Args:
+            requester: The requester making the request in the context of which we want
+                to fetch the event.
+            txn_id: The transaction ID.
+            room_id: The room ID.
+
+        Returns:
+            An event if one could be found, None otherwise.
+        """
+        existing_event_id = await self.get_event_id_from_transaction(
+            requester, txn_id, room_id
+        )
+        if existing_event_id:
+            return await self.store.get_event(existing_event_id)
         return None
 
     async def create_and_send_nonmember_event(