Revert "Allow single function in FunctionScoreQuery.functions"

[pquentin]

Reverts #5576

Turns out this is not supported by Elasticsearch! See elastic/elasticsearch#137421. Sorry.

[github-actions]

Following you can find the validation changes against the target branch for the API.

API	Status	Request	Response
search	🔴	2647/2659 → 2641/2659	2659/2659

You can validate this API yourself by using the make validate target.

[pquentin]

To clarify what is going on, there are two ways to use function_score, either with a shortcut (that we don't support):

GET /_search
{
  "query": {
    "function_score": {
      "query": { "match_all": {} },
      "boost": "5",
      "random_score": {},
      "boost_mode": "multiply"
    }
  }
}

Or with a functions array (that we support):

GET /_search
{
  "query": {
    "function_score": {
      "query": { "match_all": {} },
      "boost": "5",
      "functions": [
        {
          "filter": { "match": { "test": "bar" } },
          "random_score": {},
          "weight": 23
        },
        {
          "filter": { "match": { "test": "cat" } },
          "weight": 42
        }
      ],
      "max_boost": 42,
      "score_mode": "max",
      "boost_mode": "multiply",
      "min_score": 42
    }
  }
}

However, those are complex examples; in many YAML tests, we just see this:

GET /_search
{
  "query": {
    "function_score": {
      "random_score": {},
    }
  }
}

Which passes validation on the main branch, thanks to the current definition of FunctionScoreQuery. Here are the relevant bits:

/**
 * @shortcut_property functions
 */
export class FunctionScoreQuery extends QueryBase {
  /**
   * One or more functions that compute a new score for each document returned by the query.
   */
  functions?: FunctionScoreContainer | FunctionScoreContainer[]
}

The fact that 1/ we allow FunctionScoreContainer in functions and 2/ shortcut_property is still defined on functions makes all YAML tests about function_score pass, even though it is technically wrong. I think we have two options:

1. Remove the shortcut_property and define functions as FunctionScoreContainer[] - clarifying that we don't support the shortcut form at all.
2. Keep both the shortcut_property and keep defining functions as FunctionScoreContainer | FunctionScoreContainer[], which makes the easy case work while allowing wrong things.