kafka sink can insert messages out-of-order

[jszwedko]

The kafka sink is implemented in such a way that messages could be sent to Kafka out-of-order. See:

https://github.com/timberio/vector/blob/29736f34150da640175dae4aeb4ff8638cd1c441/src/sinks/kafka.rs#L302-L324

Here we queue up a bunch of futures that are themselves inserting into Kafka. Depending on the ordering that those futures are resolved in, we could theoretically end up queueing up the inserts in rdkafka out-of-order. Notably, when the message is queued in rdkafka via send_result it will be sent in-order so I think we just need to move the send_result call out of the future that is being queued up for later resolution.

[zamazan4ik]

@jszwedko Is it still an issue for the current Vector version?

[jszwedko]

@jszwedko Is it still an issue for the current Vector version?

I believe so, yes.