Consumer paused indefinitely when using `AsyncOffsetTracker` with lot of null values

[jpthomasset]

When using the default AsyncOffsetTracker (flush.synchronously=false), if the topic contains a lot of null values, the consumer get paused indefinitely.

Logs:

[2023-10-10 13:20:52,259] TRACE Ignoring record from topic=real-time-analytics.package-tree.state partition=5 offset=2866040 with null value. (io.confluent.connect.elasticsearch.ElasticsearchSinkTask)
[2023-10-10 13:20:52,259] DEBUG Pausing all partitions (io.confluent.connect.elasticsearch.ElasticsearchSinkTask)
[2023-10-10 13:20:52,361] DEBUG Putting 0 records to Elasticsearch. (io.confluent.connect.elasticsearch.ElasticsearchSinkTask)
[2023-10-10 13:20:52,467] DEBUG Putting 0 records to Elasticsearch. (io.confluent.connect.elasticsearch.ElasticsearchSinkTask)
[2023-10-10 13:20:52,571] DEBUG Putting 0 records to Elasticsearch. (io.confluent.connect.elasticsearch.ElasticsearchSinkTask)

Looking at the code, I think it's because the AsyncOffsetTracker.updateOffsets() is never called. It's supposed to be called at the end of a bulk but as no data is sent to Elastic, the bulk is never complete and so we never resume the consumption. This behavior is hard to track as from Kafka point of view there is a consumer and no error is shown in kafka-connect logs without setting log level to TRACE.

As a workaround, changing to SyncOffsetTracker (flush.synchronously=true) solves the issue.