Dispatcher may receive duplicate event from event collector

[lidezhu]

What did you do?

Event collector may reset dispatcher when it detects event loss and the resetTs is hard to determine.
Because multiple transactions may have the same commitTs, after we receive a event at commitTs1, it is hard to know whether there will be more events at the same commitTs1. So we cannot use a resetTs larger or equal to commitTs1, and this cause duplicate events.

What's more, after we support big transaction split, this problem will become worse.

What did you expect to see?

No response

What did you see instead?

Nan

Versions of the cluster

Upstream TiDB cluster version (execute SELECT tidb_version(); in a MySQL client):

(paste TiDB cluster version here)

Upstream TiKV version (execute tikv-server --version):

(paste TiKV version here)

TiCDC version (execute cdc version):

(paste TiCDC version here)