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.

2 files changed, 10 insertions, 10 deletions

diff --git a/synapse/storage/databases/main/events.py b/synapse/storage/databases/main/events.py
index bd3f14fb71..c1353b18c1 100644
--- a/synapse/storage/databases/main/events.py
+++ b/synapse/storage/databases/main/events.py
@@ -127,8 +127,6 @@ class PersistEventsStore:
         self._backfill_id_gen: AbstractStreamIdGenerator = self.store._backfill_id_gen
         self._stream_id_gen: AbstractStreamIdGenerator = self.store._stream_id_gen
 
-        self._msc3970_enabled = hs.config.experimental.msc3970_enabled
-
     @trace
     async def _persist_events_and_state_updates(
         self,
@@ -1012,9 +1010,11 @@ class PersistEventsStore:
                         )
                     )
 
-        # Pre-MSC3970, we rely on the access_token_id to scope the txn_id for events.
-        # Since this is an experimental flag, we still store the mapping even if the
-        # flag is disabled.
+        # Synapse usually relies on the device_id to scope transactions for events,
+        # except for users without device IDs (appservice, guests, and access
+        # tokens minted with the admin API) which use the access token ID instead.
+        #
+        # TODO https://github.com/matrix-org/synapse/issues/16042
         if to_insert_token_id:
             self.db_pool.simple_insert_many_txn(
                 txn,
@@ -1030,10 +1030,7 @@ class PersistEventsStore:
                 values=to_insert_token_id,
             )
 
-        # With MSC3970, we rely on the device_id instead to scope the txn_id for events.
-        # We're only inserting if MSC3970 is *enabled*, because else the pre-MSC3970
-        # behaviour would allow for a UNIQUE constraint violation on this table
-        if to_insert_device_id and self._msc3970_enabled:
+        if to_insert_device_id:
             self.db_pool.simple_insert_many_txn(
                 txn,
                 table="event_txn_id_device_id",
diff --git a/synapse/storage/schema/__init__.py b/synapse/storage/schema/__init__.py
index d3ec648f6d..7de9949a5b 100644
--- a/synapse/storage/schema/__init__.py
+++ b/synapse/storage/schema/__init__.py
@@ -12,7 +12,7 @@
 # See the License for the specific language governing permissions and
 # limitations under the License.
 
-SCHEMA_VERSION = 79  # remember to update the list below when updating
+SCHEMA_VERSION = 80  # remember to update the list below when updating
 """Represents the expectations made by the codebase about the database schema
 
 This should be incremented whenever the codebase changes its requirements on the
@@ -110,6 +110,9 @@ Changes in SCHEMA_VERSION = 78
 Changes in SCHEMA_VERSION = 79
     - Add tables to handle in DB read-write locks.
     - Add some mitigations for a painful race between foreground and background updates, cf #15677.
+
+Changes in SCHEMA_VERSION = 80
+    - The event_txn_id_device_id is always written to for new events.
 """