[BUG][Go] UnmarshalJSON() does not allow unknown fields even when they are not forbidden in schema

[dyurovskykh-tivo]

Bug Report Checklist

• Have you provided a full/minimal spec to reproduce the issue?
• Have you validated the input using an OpenAPI validator (example)?
• Have you tested with the latest master to confirm the issue still exists?
• Have you searched for related issues/PRs?
• What's the actual output vs expected output?
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Description

JSON schema allows additional properties by default, see https://json-schema.org/understanding-json-schema/reference/object#additionalproperties

The additionalProperties keyword is used to control the handling of extra stuff, that is, properties whose names are not listed in the properties keyword or match any of the regular expressions in the patternProperties keyword. By default any additional properties are allowed.

The value of the additionalProperties keyword is a schema that will be used to validate any properties in the instance that are not matched by properties or patternProperties. Setting the additionalProperties schema to false means no additional properties will be allowed.

But Go generator produces UnmarshalJSON() with decoder.DisallowUnknownFields() regardless of the additionalProperties flag.

openapi-generator version

• latest

OpenAPI declaration file content or url

openapi: 3.0.0
info:
  title: Test
  version: 1.0.0
paths: {}
components:
  schemas:
    Object:
      required:
        - field1
      properties:
        field1:
          description: Something
          type: string

Generation Details

$ docker run --rm -u 1001:1001 -v "${PWD}:/local" openapitools/openapi-generator-cli generate -i /local/test.yaml -g go -o /local/out/go

Steps to reproduce

Run generator and check the generated code. UnmarshalJSON() must not contain decoder.DisallowUnknownFields() because additional properties are not forbidden.

Related issues/PRs

Introduced by #17575

Suggest a fix

Place the following LOC under the condition to be added only when additionalProperties = false for a given structure:
decoder.DisallowUnknownFields()

[dyurovskykh-tivo]

@wing328 @ctreatma
Please, check this bug.

[ctreatma]

The model template only generates the decoder.DisallowUnknownFields() code if the generator thinks that additional properties are not allowed: https://github.com/OpenAPITools/openapi-generator/blob/master/modules/openapi-generator/src/main/resources/go/model_simple.mustache#L493-L511

What config are you passing in when running the generator? There is a disallowAdditionalPropertiesIfNotPresent config option that controls the behavior of generated code for all languages: https://openapi-generator.tech/docs/generators/go/#config-options

For legacy reasons, the disallowAdditionalPropertiesIfNotPresent defaults to true. This means that by default, the generator will produce models that assume additional properties are not allowed if the additionalProperties key is not explicitly specified.

To configure the generator to produce code that complies with the additionalProperties behavior documented in OAS, you have to set disallowAdditionalPropertiesIfNotPresent to false.

[dyurovskykh-tivo]

@ctreatma
Thank you! I did not know about disallowAdditionalPropertiesIfNotPresent config.
When I run the generator like docker run --rm -u 1001:1001 -v "${PWD}:/local" openapitools/openapi-generator-cli generate --additional-properties=disallowAdditionalPropertiesIfNotPresent=false -i /local/test.yaml -g go -o /local/out/go, decoder.DisallowUnknownFields() is not generated