Element is null, which is not allowed error when errors are added to context only

[arxeiss]

I'm getting error the requested element is null which the schema does not allow when adding errors to the GraphQL context and return nil, nil from resolver.

I have this query defined and resolver below. If I'm returning nil, err it is fine. I see proper GraphQL error response. Meaning {"data": null, "errors": ...}.

But in second case, I can get mutli error from payload.BuildURL. Based on error type inside I can differentiate which input was wrong.
However, that fails with the requested element is null which the schema does not allow. And to me that is strange.

I understand, that in schema I have non-nullable response. And if I would see that error in both cases (returning error directly and adding to context) it would be fine. But this is strange.

query {
  externalDataResolverReplacePlaceholders(
    config: ExternalDataResolverConfigInput!
    values: Map
  ): ExternalDataResolverReplacedPlaceholders!
}

Resolver:

func ErrorOnPath(ctx context.Context, err error, path ...string) error {
	gqlErr = gqlerror.WrapPath(graphql.GetPath(ctx), err)
	for _, v := range path {
		gqlErr.Path = append(gqlErr.Path, ast.PathName(v))
	}
	return gqlErr
}

func (*queryResolver) ExternalDataResolverReplacePlaceholders(
	ctx context.Context,
	cfg model.ExternalDataResolverConfigInput,
	values map[string]any,
) (*model.ExternalDataResolverReplacedPlaceholders, error) {
	if uri, err := url.Parse(cfg.URL); err != nil || !uri.IsAbs() {
		// Here I return nil as response together with error and it is fine
		return nil, ErrorOnPath(ctx, errors.New("URL must be valid and absolute"), "config", "URL")
	}

	resp := &model.ExternalDataResolverReplacedPlaceholders{}
	var err error

	resp.URL, err = payload.BuildURL(cfg.URL, values)
	if err != nil {
		if me, ok := err.(*payload.MultiError); ok {
			for _, e := range me.Errors {
				switch e := e.(type) {
				case *payload.MissingVariableError:
					graphql.AddError(ctx, ErrorOnPath(ctx, e, "values"))
				case *payload.InvalidDefaultValueError:
					graphql.AddError(ctx, ErrorOnPath(ctx, e, "config", "URL"))
				}
			}
			// Here if I return nil, and all errors are added to context, this is an issue
			return nil, nil
		}
		
		return nil, ErrorOnPath(ctx, err, "config", "URL")
	}
	return resp, nil
}

---

I was debugging that deeper into generated code, and I spot 1 place which causes the trouble I believe. But not sure if fixing that wouldn't break the rest of the code.

When I was debugging, I got here in generated code:

if v == nil {
	if !graphql.HasFieldError(ctx, graphql.GetFieldContext(ctx)) {
		ec.Errorf(ctx, "the requested element is null which the schema does not allow")
	}
	return graphql.Null
}

v in my case is nil so it is checking if field has error with this code. But when it goes to equalPath, input parameters are follow:

• path contains ["externalDataResolverReplacePlaceholders"]
• err.Path contains ["externalDataResolverReplacePlaceholders", "values"]

so equalPath returns false, which means that HasFieldError is false too and then I see the "the requested element is null..." error.

func HasFieldError(ctx context.Context, rctx *FieldContext) bool {
	...
	for _, err := range c.errors {
		if equalPath(err.Path, path) {
			return true
		}
	}
	...
}

[StevenACoffman]

Thanks for pursuing this! I'd be interested in a proposed PR for this, and I'd like to see how that responds in our existing test suites and examples.