Elasticsearch Version

main

Installed Plugins

No response

Java Version

bundled

OS Version

all

Problem Description

I discovered this while working on #106325.

Executing the following yaml test succeeds, which means requesting _seq_no via fields returns a 5xx error.

---
fetch _seq_no via fields:

  - do:
      catch: "request"
      search:
        index: test
        body:
          fields: [ _seq_no ]

  # This should be `unauthorized` (401) or `forbidden` (403) or at least `bad request` (400)
  # while instead it is mapped to an `internal_server_error (500)`
  - match: { status: 500 }
  - match: { error.root_cause.0.type: unsupported_operation_exception }
  - match: { error.root_cause.0.reason: "Cannot fetch values for internal field [_seq_no]." }

This happens because in the fetch phase (FetchFieldsPhase) we try to use the SeqNoFieldMapper ValueFetcher which throws an UnsupportedOperationException. I think this is the case for other metadata fields too...all those throwing such exception.

Notice that the same issue does not happen if we request _seq_no via stored_fields because StoredFieldPhase does not use ValueFetcher to retrieve field values.

Steps to Reproduce

Just execute the yaml test against main

Logs (if relevant)

No response