fix #3854 MAU transaction errors

changelog.d/3961.bugfix

@@ -0,0 +1 @@
+Fix errors due to concurrent monthly_active_user upserts 

synapse/storage/monthly_active_users.py

@@ -172,6 +172,10 @@ def upsert_monthly_active_user(self, user_id):
             Deferred[bool]: True if a new entry was created, False if an
                 existing one was updated.
         """
+        # Am consciously deciding to lock the table on the basis that is ought
+        # never be a big table and alternative approaches (batching multiple
+        # upserts into a single txn) introduced a lot of extra complexity.
+        # See https://github.com/matrix-org/synapse/issues/3854 for more
         is_insert = yield self._simple_upsert(
             desc="upsert_monthly_active_user",
             table="monthly_active_users",
@@ -181,7 +185,6 @@ def upsert_monthly_active_user(self, user_id):
             values={
                 "timestamp": int(self._clock.time_msec()),
             },
-            lock=False,
         )
         if is_insert:
             self.user_last_seen_monthly_active.invalidate((user_id,))