Fields defined in _source return other fields too

[aliostad]

Elasticsearch version: 5.1.2
Plugins installed: x-pack
JVM version: 1.8.0_111
OS version: Linux myvmname 3.16.0-43-generic #58~14.04.1-Ubuntu SMP Mon Jun 22 10:21:20 UTC 2015 x86_64 x86_64 x86_64 GNU/Linux

Description of the problem including expected versus actual behavior:
If I define field foobar in the _source and my document has field foo, it will be returned. I find this happens if the content of the field is an array - but this is just an observation. Not sure how it could be related.

Steps to reproduce:

POST sisyphus/life
{
  "stone" : ["large"],
  "stoneWeight" : [500]
}

POST sisyphus/life
{
  "stone" : ["small"],
  "stoneWeight" : [200]
}

GET sisyphus/_search
{
  "_source": "stoneWeight", 
  "query": {
    "match_all": {}
  }
}

Returns stone as well as stoneWeight:

{
  "took": 4,
  "timed_out": false,
  "_shards": {
    "total": 5,
    "successful": 5,
    "failed": 0
  },
  "hits": {
    "total": 2,
    "max_score": 1,
    "hits": [
      {
        "_index": "sisyphus",
        "_type": "life",
        "_id": "AV32c1_JK9fID-KFUKqO",
        "_score": 1,
        "_source": {
          "stoneWeight": [
            500
          ],
          "stone": [
            "large"
          ]
        }
      },
      {
        "_index": "sisyphus",
        "_type": "life",
        "_id": "AV32c2hbK9fID-KFUKqP",
        "_score": 1,
        "_source": {
          "stoneWeight": [
            500
          ],
          "stone": [
            "large"
          ]
        }
      }
    ]
  }
}

[rjernst]

I cannot reproduce this on master. Can you try on a more recent version of elasticsearch (say 5.5)?

[aliostad]

I cannot reproduce on 5.4.

[jimczi]

This is a duplicate of #22557 which was fixed by #22593 in es 5.2.1

[aliostad]

Thank you!
Appreciate quick turn-around.