Support partial responses

[stuebingerb]

According to the spec, there is a distinction between "request errors" and "field errors" (and it would most likely make sense to have that distinction in our code base as well).

Field errors are raised during execution from a particular field. This may occur due to an internal error during value resolution or failure to coerce the resulting value.

Field errors must result in a partial response, i.e. a response with data entry and all (unique) raised field errors in the errors entry. If a field error occurs on a nullable field, the value of that field will be coerced to null. Otherwise, the error propagates to the parent field where it will be handled similarly (if the parent is nullable, it will be coerced to null, otherwise the error propagates further).

Consider the following schema:

{
    type<Test> {
        property<String>("optional") {
            resolver { test: Test ->
                if (test.fail) {
                    null as String
                } else {
                    "success"
                }
            }
        }
    }
    query("test") {
        resolver { fail: Boolean ->
            Test(fail)
        }
    }
}

Happy case:

query {
  test(fail:false) {
    optional
    fail
  }
}

Result:

{
  "data": {
    "test": {
      "optional": "success",
      "fail": false
    }
  }
}

Error case:

query {
  test(fail:true) {
    optional
    fail
  }
}

Result:

{
  "errors": [
    {
      "message": "null cannot be cast to non-null type kotlin.String",
      "locations": [
        {
          "line": 3,
          "column": 5
        }
      ],
      "path": [],
      "extensions": {
        "type": "INTERNAL_SERVER_ERROR"
      }
    }
  ]
}

Expected would IMHO be a partial response along the lines of:

{
  "data": {
    "test": {
      "optional": null,
      "fail": true
    }
  },
  "errors": [
    {
      "message": "null cannot be cast to non-null type kotlin.String",
      "locations": [
        {
          "line": 3,
          "column": 5
        }
      ],
      "path": [],
      "extensions": {
        "type": "INTERNAL_SERVER_ERROR"
      }
    }
  ]
}

IMHO incorrect error handling should be considered a bug but there is definitely potential for related enhancement as well.

See

• https://spec.graphql.org/October2021/#sec-Handling-Field-Errors
• https://spec.graphql.org/October2021/#sec-Executing-Selection-Sets.Errors-and-Non-Null-Fields
• https://spec.graphql.org/October2021/#sec-Errors