Remote echo arriving before /send returns can cause stuck message

[bwindels]

See event id $Gf2vrm-QNmQy0Z9uuSF5jxTFqifbcFsACFzpIcrcfHM in bwindels-stuck-message-hydrogen-logs-1616578957490.json

The pending event seems to be removed by the remote echo, but then added again when /send returns? Perhaps PendingEventStore.exists has a bug?

[bwindels]

the stuck message had queueIndex:1, and when sending another message some many minutes later, the stuck message got replaced by the new message (who would also have queueIndex:1, so the count was wrong, perhaps because in-memory wasn't in sync with storage?)

[bwindels]

Hit this again, see http://localhost:3000/scripts/logviewer/#2933/4/1/0 and http://localhost:3000/scripts/logviewer/#2935/0 in log bwindels-hydrogen-logs-1617642134956.json

[bwindels]

this is the pending event emitting an update for status after send completes, which calls SortedArray.set which adds the pending event again.