FVH Highlighter does not work with the inner hits properly

[babayotakun]

Elasticsearch version: 7.10.0
Running in docker from image docker.elastic.co/elasticsearch/elasticsearch:7.10.0

Fvh highligher does not highlight actual content of the inner hit. It seems to alway returning highlight for the first relevant inner hit but with the highlight offsets of the current inner hit. Other highlighters ("plain" and "unified") are working correctly.

Steps:

1. Create index with the nested field:

curl -H 'Content-type: application/json' -XPUT 'http://localhost:9200/test' -d '{
 "mappings": {
  "properties": {
   "obj": {
    "type": "nested",
    "properties": {
     "content": {
      "type": "text",
      "norms": true,
      "term_vector": "with_positions_offsets"
     }
    }
   }
  }
 }
}'

2. Put a document:

curl -H 'Content-type: application/json' -XPOST 'http://localhost:9200/test/_doc/' -d '{
 "obj": [
  {
   "content": "hello world"
  },
  {
   "content": "world hello"
  }
 ]
}'

3. Run a nested search query with the highlight:

curl -H 'Content-type: application/json' -XPOST 'http://localhost:9200/test/_search' -d '{
 "query": {
  "bool": {
   "should": [
    {
     "nested": {
      "query": {
       "match": {
        "obj.content": {
         "query": "hello",
         "operator": "OR",
         "prefix_length": 0,
         "max_expansions": 50,
         "minimum_should_match": "1<-1%",
         "fuzzy_transpositions": true,
         "lenient": true,
         "zero_terms_query": "NONE",
         "auto_generate_synonyms_phrase_query": true,
         "boost": 1
        }
       }
      },
      "path": "obj",
      "ignore_unmapped": false,
      "score_mode": "none",
      "boost": 1,
      "inner_hits": {
       "name": "obj|116088734",
       "ignore_unmapped": false,
       "from": 0,
       "size": 10,
       "version": false,
       "seq_no_primary_term": false,
       "explain": false,
       "track_scores": false,
       "highlight": {
        "fields": {
         "obj.content": {
          "fragment_size": 350,
          "number_of_fragments": 0,
          "type": "fvh",
          "order": "score",
          "no_match_size": 0,
          "matched_fields": [
           "obj.content"
          ]
         }
        }
       }
      }
     }
    }
   ],
   "adjust_pure_negative": true,
   "boost": 1
  }
 },
 "_source": false
}'

Result:

{ - 
  "took": 5,
  "timed_out": false,
  "_shards": { - 
    "total": 1,
    "successful": 1,
    "skipped": 0,
    "failed": 0
  },
  "hits": { - 
    "total": { - 
      "value": 1,
      "relation": "eq"
    },
    "max_score": 0,
    "hits": [ - 
      { - 
        "_index": "test",
        "_type": "_doc",
        "_id": "h4HGA3YBAHjK9WUfKSvs",
        "_score": 0,
        "inner_hits": { - 
          "obj|116088734": { - 
            "hits": { - 
              "total": { - 
                "value": 2,
                "relation": "eq"
              },
              "max_score": 0.18232156,
              "hits": [ - 
                { - 
                  "_index": "test",
                  "_type": "_doc",
                  "_id": "h4HGA3YBAHjK9WUfKSvs",
                  "_nested": { - 
                    "field": "obj",
                    "offset": 0
                  },
                  "_score": 0.18232156,
                  "_source": { - 
                    "content": "hello world"
                  },
                  "highlight": { - 
                    "obj.content": [ - 
                      "<em>hello</em> world"
                    ]
                  }
                },
                { - 
                  "_index": "test",
                  "_type": "_doc",
                  "_id": "h4HGA3YBAHjK9WUfKSvs",
                  "_nested": { - 
                    "field": "obj",
                    "offset": 1
                  },
                  "_score": 0.18232156,
                  "_source": { - 
                    "content": "world hello"
                  },
                  "highlight": { - 
                    "obj.content": [ - 
                      "hello <em>world</em>"
                    ]
                  }
                }
              ]
            }
          }
        }
      }
    ]
  }
}

Highlights are returned with the correct offsets and indents but with the wrong contents.
Should be world hello for the second inner hit.

[elasticmachine]

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

[jtibshirani]

Thanks a lot @babayotakun for raising this. A fix will be available in the upcoming releases 7.10.2 and 7.11.