Stabilize support for MSC3970: updated transaction semantics (scope to `device_id`) (#15629)

For now this maintains compatible with old Synapses by falling back
to using transaction semantics on a per-access token. A future version
of Synapse will drop support for this.

1 files changed, 5 insertions, 7 deletions

diff --git a/synapse/handlers/message.py b/synapse/handlers/message.py
index c656e07d37..d485f21e49 100644
--- a/synapse/handlers/message.py
+++ b/synapse/handlers/message.py
@@ -561,8 +561,6 @@ class EventCreationHandler:
                 expiry_ms=30 * 60 * 1000,
             )
 
-        self._msc3970_enabled = hs.config.experimental.msc3970_enabled
-
     async def create_event(
         self,
         requester: Requester,
@@ -897,9 +895,8 @@ class EventCreationHandler:
         """
         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,
-            # and fallback to the old behaviour if none were found.
+        # According to the spec, transactions are scoped to a user's device ID.
+        if requester.device_id:
             existing_event_id = (
                 await self.store.get_event_id_from_transaction_id_and_device_id(
                     room_id,
@@ -911,8 +908,9 @@ class EventCreationHandler:
             if 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.
+        # Some requsters don't have device IDs (appservice, guests, and access
+        # tokens minted with the admin API), fallback to checking the access token
+        # ID, which should be close enough.
         if requester.access_token_id:
             existing_event_id = (
                 await self.store.get_event_id_from_transaction_id_and_token_id(