consumer.Seek(...) throws KafkaException "Local: Errorneous state" when executed immediately

Description

IConsumer.Seek(TopicPartitionOffset tpo) throws an exception, if immediately executed after IConsumer.Assign(TopicPartition). With a delay in between, it works. This should work immediately and suggest a non-completed task in Assign. I suggest creating async APIs where the state is sane when the API is awaited:

await c.AssignAsync(topic);
await c.SeekAsync(topic_partition_offset);

Nuget package version: 1.2.0 Kafka version: 2.1.1cp1 OS: Windows (kafka + zookeeper in docker confluentinc/cp-…)

How to reproduce

async Task Test()
{
   var config = new ConsumerConfig { BootstrapServers="localhost:1234", GroupId = "test",  };
   using (var c = new ConsumerBuilder<Ignore, string>(config).Build())
   {
      var tp = new TopicPartition("test-topic", new Partition(0));
      c.Assign(tp);
      //await Task.Delay(TimeSpan.FromSeconds(1)); // activate this to "fix" it / perform exponential backoff
      c.Seek(new TopicPartitionOffset(tp, 0)); // throws, if not delayed!
   }
}

Checklist

Please provide the following information:

• A complete (i.e. we can run it), minimal program demonstrating the problem. No need to supply a project file.
• Confluent.Kafka nuget version.
• [ X] Apache Kafka version.
• [X ] Client configuration.
• [ X] Operating system.
• [N/A] Provide logs (with “debug” : “…” as necessary in configuration).
• [N/A] Provide broker log excerpts.
• [Well, you decide…] Critical issue.

[nikoudel]

I was trying to seek to the beginning of all partitions after restart (like told here): and was getting the same error as in this issue, however, I suspect seeking is not the right way to do it - the consumer example suggests a different way:

...
.SetPartitionsAssignedHandler((c, partitions) =>
{
	// This will fail:
	//foreach (var p in partitions)
	//{
	//    c.Seek(new TopicPartitionOffset(p, Offset.Beginning));
	//}

	// This works:
	return partitions.Select(tp => new TopicPartitionOffset(tp, Offset.Beginning));
})
...

[edenhill]

Seek() may only be used on partitions that have been Assign()ed. Assign() is called after SetPartitionsAssignedHandler() returns.