Canal error: "invalid sequence 6 != 1" on first authentication

[tcd93]

System:

• MySQL 8.0 (authentication plugin: caching_sha2_password)

How to reproduce:

• Stop any running Canal instance
• Restart MySQL instance, or execute FLUSH PRIVILEGES;
• Restart Canal
  • The error will appear
  • After seeing first error, if we start the Canal again, Canal would work normally

Investigation:
After checking I found the method packet.WritePublicKeyAuthPacket leaves dirty buffer after writing, causing sequence mismatch in the next read op
https://github.com/siddontang/go-mysql/blob/0c5789dd0bd378b4b84f99b320a2d35a80d8858f/packet/conn.go#L250
I added an blank read op to clear the buffer and it works properly:

	// return errors.Wrap(c.WritePacket(data), "WritePacket failed")
	err = c.WritePacket(data)
	if err != nil {
		return errors.Wrap(err, "WritePacket failed")
	}
	_, err = c.reader.Read(data) // clear dirty buffer
	return err

I think the same would happen to packet.WriteClearAuthPacket too, but I have not tested it yet

I'm not sure this is the correct way to fix this as I'm not very experienced with Go, so I just open an issue here