Invalid documentation for NewHashPartitioner

[AlekSi]

Sarama Version: v1.10

https://godoc.org/gopkg.in/Shopify/sarama.v1#NewHashPartitioner says:

If the message's key is nil, or fails to encode, then a random partition is chosen.

But code actually returns -1 in case of encoding error, which results in ErrInvalidPartition.

[eapache]

It actually returns -1 and the encoding error, so the producer should just pass along the encoding error rather than trying to treat the -1 as a partition (https://github.com/Shopify/sarama/blob/9bb4a68d57ff6f623363aa172f0a8297aa289ba7/async_producer.go#L346-L349). The behaviour makes sense as-is, I will fix the documentation.

Thanks for the report!