Remove option to skip locking of tables during emulated upserts (#14469)

To perform an emulated upsert into a table safely, we must either:
 * lock the table,
 * be the only writer upserting into the table
 * or rely on another unique index being present.

When the 2nd or 3rd cases were applicable, we previously avoided locking
the table as an optimization. However, as seen in #14406, it is easy to
slip up when adding new schema deltas and corrupt the database.

The only time we lock when performing emulated upserts is while waiting
for background updates on postgres. On sqlite, we do no locking at all.

Let's remove the option to skip locking tables, so that we don't shoot
ourselves in the foot again.

Signed-off-by: Sean Quah <seanq@matrix.org>

synapse/storage/databases/main/pusher.py

@@ -325,14 +325,11 @@ class PusherWorkerStore(SQLBaseStore):
     async def set_throttle_params(
         self, pusher_id: str, room_id: str, params: ThrottleParams
     ) -> None:
-        # no need to lock because `pusher_throttle` has a primary key on
-        # (pusher, room_id) so simple_upsert will retry
         await self.db_pool.simple_upsert(
             "pusher_throttle",
             {"pusher": pusher_id, "room_id": room_id},
             {"last_sent_ts": params.last_sent_ts, "throttle_ms": params.throttle_ms},
             desc="set_throttle_params",
-            lock=False,
         )
 
     async def _remove_deactivated_pushers(self, progress: dict, batch_size: int) -> int:
@@ -589,8 +586,6 @@ class PusherStore(PusherWorkerStore, PusherBackgroundUpdatesStore):
         device_id: Optional[str] = None,
     ) -> None:
         async with self._pushers_id_gen.get_next() as stream_id:
-            # no need to lock because `pushers` has a unique key on
-            # (app_id, pushkey, user_name) so simple_upsert will retry
             await self.db_pool.simple_upsert(
                 table="pushers",
                 keyvalues={"app_id": app_id, "pushkey": pushkey, "user_name": user_id},
@@ -609,7 +604,6 @@ class PusherStore(PusherWorkerStore, PusherBackgroundUpdatesStore):
                     "device_id": device_id,
                 },
                 desc="add_pusher",
-                lock=False,
             )
 
             user_has_pusher = self.get_if_user_has_pusher.cache.get_immediate(