Bulker swallows errors from elasticsearch when the error is specific to the document

[blakerouse]

Overview

Inside of the BulkIndexerResponseItem there is an error field that could have possible been set but is ignored in here https://github.com/elastic/fleet-server/blob/main/internal/pkg/bulk/opBulk.go#L77.

What should be done?

Any code path that reads a BulkIndexerResponseItem should check the Error field and ensure that an error has not occurred. If an error has occurred then it should return an error instead of continuing on.

Relates to elastic/elastic-agent#760