Today we wrap exceptions that occur while executing an ingest processor
in an ElasticsearchException. Today, in ExceptionsHelper#unwrapCause we
only unwrap causes for exceptions that implement
ElasticsearchWrapperException, which the top-level
ElasticsearchException does not. Ultimately, this means that any
exception that occurs during processor execution does not have its cause
unwrapped, and so its status is blanket treated as a 500. This means
that while executing a bulk request with an ingest pipeline,
document-level failures that occur during a processor will cause the
status for that document to be treated as 500. Since that does not give
the client any indication that they made a mistake, it means some
clients will enter infinite retries, thinking that there is some
server-side problem that merely needs to clear. This commit addresses
this by introducing a dedicated ingest processor exception, so that its
causes can be unwrapped. While we could consider a broader change to
unwrap causes for more than just ElasticsearchWrapperExceptions, that is
a broad change with unclear implications. Since the problem of reporting
500s on client errors is a user-facing bug, we take the conservative
approach for now, and we can revisit the unwrapping in a future change.
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