Bulk REST API allows arbitrary fields after action

[pickypg]

While trying to refresh a specific index within a bulk request containing multiple indices, I came across an unexpected scenario where unrecognized field names were ignored:

POST /test/_bulk
{"index":{"_id":"1"}, "abc": true}
{"field":"value"}

In ES 7.2.0, at least, this results in the document being indexed.

However, if you put abc in front of the index action, then it triggers an expected error [from my perspective]:

POST /test/_bulk
{"abc": true, "index":{"_id":"1"}}
{"field":"value"}

results in

{
  "error": {
    "root_cause": [
      {
        "type": "illegal_argument_exception",
        "reason": "Malformed action/metadata line [1], expected START_OBJECT or END_OBJECT but found [VALUE_BOOLEAN]"
      }
    ],
    "type": "illegal_argument_exception",
    "reason": "Malformed action/metadata line [1], expected START_OBJECT or END_OBJECT but found [VALUE_BOOLEAN]"
  },
  "status": 400
}

We just need to update the parser to ensure that the JSON object is empty after finding the action. I can imagine that some people think some follow-on fields are taking effect as it is today.

[elasticmachine]

Pinging @elastic/es-distributed