protoc-gen-openapiv2: oneof fields incorrectly marked as required with use_proto3_field_semantics=true

[sessa]

Bug Report

When using use_proto3_field_semantics=true, oneof member fields are incorrectly added to the required array of the parent message's OpenAPI schema. This is semantically wrong because a oneof means at most one of the fields can be set — they should never all be required simultaneously.

Steps to Reproduce

Given a proto message with a oneof:

message Filter {
  oneof filter_group_or_value {
    FilterList list = 1;
    FilterRule rule = 2;
  }
}

Run protoc-gen-openapiv2 with use_proto3_field_semantics=true.

Expected Output

oneof fields should be excluded from the required array, since only one can be set at a time:

"Filter": {
  "type": "object",
  "properties": {
    "list": {
      "$ref": "#/definitions/FilterList"
    },
    "rule": {
      "$ref": "#/definitions/FilterRule"
    }
  }
}

Actual Output

Both oneof members are marked as required:

"Filter": {
  "type": "object",
  "properties": {
    "list": {
      "$ref": "#/definitions/FilterList"
    },
    "rule": {
      "$ref": "#/definitions/FilterRule"
    }
  },
  "required": [
    "list",
    "rule"
  ]
}

Analysis

The use_proto3_field_semantics=true option marks all proto3 fields that are not optional as required. However, it does not check whether a field is part of a oneof group. Fields within a oneof should be treated as implicitly optional (only one can be present), so they should be excluded from the required list.

A potential fix would be to check field.OneofIndex in updateSwaggerObjectFromFieldBehavior and skip adding oneof members to the required array when use_proto3_field_semantics is enabled.

Environment

• Plugin: buf.build/grpc-ecosystem/openapiv2:v2.27.3
• Configuration:

  opt:
    - use_proto3_field_semantics=true
    - json_names_for_fields=true
    - output_format=json
  

Workaround

Currently the only workarounds are:

1. Remove use_proto3_field_semantics=true (makes all fields optional)
2. Post-process the generated swagger to strip oneof fields from required arrays
3. Use per-field google.api.field_behavior = REQUIRED annotations instead of the blanket flag

[johanbrandhorst]

Thanks for your issue, would you be interested in helping to fix this? Sounds like you have some idea of what would be required already. In addition to a logical fix, it'd be nice to get an example protobuf file in place that exhibits the problem, so we can see it fixed.