[BUG] protobuf-schema generator wraps standalone enums in message types but does not update field references

Bug Report Checklist

• Have you provided a full/minimal spec to reproduce the issue?
• Have you validated the input using an OpenAPI validator?
• 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?
• [Optional] Sponsorship to speed up the bug fix or feature request (example)

Description

Since 7.20.0, in the protobuf-schema generator, standalone enum schemas (top-level $ref enums in OpenAPI) are unconditionally wrapped in a message type with a nested Enum:

v7.19.0:

enum ResourceType {
  RESOURCE_TYPE_UNSPECIFIED = 0;
  RESOURCE_TYPE_SYNC = 1;
  RESOURCE_TYPE_ASYNC = 2;
}

v7.20.0+:

message ResourceType {
  enum Enum {
    RESOURCE_TYPE_UNSPECIFIED = 0;
    RESOURCE_TYPE_SYNC = 1;
    RESOURCE_TYPE_ASYNC = 2;
  }
}

However, field references to that enum in other generated messages are not updated. Instead they still use the bare type name:

message BulkOperationMessage {
  ResourceType resource_type = 5;  // ResourceType is now a message, not an enum
}

The field should reference the nested enum type:

message BulkOperationMessage {
  ResourceType.Enum resource_type = 5;  // correct
}

The generated .proto files are syntactically valid but semantically broken. ResourceType is now an empty message, so the resource_type field carries no information. In Java:

• message.getResourceType() returns an empty ResourceType message object instead of an enum value
• ResourceType.RESOURCE_TYPE_SYNC no longer compiles (the constants are now on ResourceType.Enum)
• message.getResourceType().name() does not compile (no .name() method on a proto message)

Affected versions: 7.20.0, 7.21.0 (confirmed working: 7.19.0)

openapi-generator version

7.21.0
protobuf-schema generator

OpenAPI declaration file content or url

Minimal reproducer spec:

components:                                                                                                                                                                                 
  schemas:                                                
    ResourceType:
      type: string
      enum:
        - SYNC
        - ASYNC                                                                                                                                                                             
 
    BulkOperationMessage:                                                                                                                                                                   
      type: object                                        
      properties:
        resourceType:
          $ref: '#/components/schemas/ResourceType'

Generation Details

Generation through the gradle plugin, with the following additional options set:

generatorName = "protobuf-schema"
library = ""
configOptions.put("startEnumsWithUnspecified", "true")
configOptions.put("numberedFieldNumberList", "true")

Steps to reproduce

Related issues/PRs

Introduced by: #22740 (merged 2026-01-30)

Suggest a fix

enum.mustache was changed to always generate the message wrapper, but the field-reference was not updated to match. The two halves of the generated output are inconsistent with each other.
This should be fixed in ProtobufSchemaCodegen.java: when setting x-protobuf-data-type for a property whose $ref target is a wrapped enum (i.e. isEnum is true on the referenced model), it should set the value to ResourceType.Enum instead of ResourceType.

Workaround

There is a workaround possible, by reverting the enum.mustache file to its v7.19.0 state. This reverts the generated enums to a flat structure.
The gradle plugin provides a templateDir option to allow for that.

Start with ProtobufSchemaCodegen.java and compare its field-reference handling with the enum.mustache change introduced by #22740. Reproduce the issue using the minimal OpenAPI YAML and protobuf-schema generator options described in the report. Done means generated fields reference the nested enum type while standalone enums retain the expected values.