def _do_fetch(self, conn):
        event_list = []
        i = 0
        while True:
            try:
                with self._event_fetch_lock:
                    event_list = self._event_fetch_list
                    self._event_fetch_list = []

                    if not event_list:
                        single_threaded = self.database_engine.single_threaded
                        if single_threaded or i > EVENT_QUEUE_ITERATIONS:
                            self._event_fetch_ongoing -= 1
                            return
                        else:
                            self._event_fetch_lock.wait(EVENT_QUEUE_TIMEOUT_S)
                            i += 1
                            continue
                    i = 0

                event_id_lists = zip(*event_list)[0]
                event_ids = [
                    item for sublist in event_id_lists for item in sublist
                ]

                rows = self._new_transaction(
                    conn, "do_fetch", [], None, self._fetch_event_rows, event_ids
                )

                row_dict = {
                    r["event_id"]: r
                    for r in rows
                }

                # We only want to resolve deferreds from the main thread
                def fire(lst, res):
                    for ids, d in lst:
                        if not d.called:
                            try:
                                with PreserveLoggingContext():
                                    d.callback([
                                        res[i]
                                        for i in ids
                                        if i in res
                                    ])
                            except:
                                logger.exception("Failed to callback")
                with PreserveLoggingContext():
                    reactor.callFromThread(fire, event_list, row_dict)
            except Exception as e:
                logger.exception("do_fetch")

                # We only want to resolve deferreds from the main thread
                def fire(evs):
                    for _, d in evs:
                        if not d.called:
                            with PreserveLoggingContext():
                                d.errback(e)

                if event_list:
                    with PreserveLoggingContext():
                        reactor.callFromThread(fire, event_list)
Takes a database connection and waits for requests for events from
dataset/ETHPy150Open matrix-org/synapse/synapse/storage/events.py/EventsStore._do_fetch