Join field called "_type" is duplicated in the _search results

[AlexeySagin]

Elasticsearch Version

8.14.1

Installed Plugins

No response

Java Version

bundled

OS Version

Linux 5.19.0-2-amd64 #1 SMP PREEMPT_DYNAMIC Debian 5.19.11-1 (2022-09-24) x86_64 GNU/Linux

Problem Description

If join field called _type it's duplicated in _search request results.
Problem appears at 8.14 release.
8.9; 8.12; 8.13 is ok.

Steps to Reproduce

1. make index and docs as example in documentation for join field type:
   https://www.elastic.co/guide/en/elasticsearch/reference/8.14/parent-join.html

2. make search curl 'localhost:9200/my-index-000001/_search?pretty'
   got resutl like:

{
     "_index" : "my-index-000001",
     "_id" : "4",
     "_score" : 1.0,
     "_routing" : "1",
     "_source" : {
       "my_id" : "4",
       "text" : "This is another answer",
       "my_join_field" : {
         "name" : "answer",
         "parent" : "1"
       }
     }
   }

3. reproduce steps 1 and 2, but rename index name from my-index-000001 to my-index-000002 and join field my_join_field to _type

4. make search curl 'localhost:9200/my-index-000002/_search?pretty'

{
        "_index" : "my-index-000002",
        "_id" : "4",
        "_score" : 1.0,
        "_routing" : "1",
        "_type" : {
          "name" : "answer",
          "parent" : "1"
        },
        "_source" : {
          "my_id" : "4",
          "text" : "This is another answer",
          "_type" : {
            "name" : "answer",
            "parent" : "1"
          }
        }
      }

result: _type field is duplicated inside and outside _source
expected: _type field present only inside _source

Logs (if relevant)

No response

[elasticsearchmachine]

Pinging @elastic/es-search (Team:Search)

[elasticsearchmachine]

Pinging @elastic/es-storage-engine (Team:StorageEngine)

[benwtrent]

This is most likely caused by how we now fetch metadata fields (of which the ancient _type field is one), separately from regular fields.

#106325