[BUG][Protobuf] Generates circular file imports that don't compile with protoc

Bug Report Checklist

• Have you provided a full/minimal spec to reproduce the issue?
• Have you validated the input using an OpenAPI validator (example)?
  • apidevtools.org/swagger-parser/online seems unable to cope with it and freezes indefinitely (?), but https://editor-next.swagger.io/ loads it fine
• 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

Converting Teamcity's swagger.json to Protocol Buffers results in protobuf specification that doesn't compile with protoc. Specifically because of circular/recursive file references/imports.

openapi-generator version

--version output:

openapi-generator-cli 6.6.0-SNAPSHOT
  commit : ff9b384
  built  : -999999999-01-01T00:00:00+18:00
  source : https://github.com/openapitools/openapi-generator
  docs   : https://openapi-generator.tech/

OpenAPI declaration file content or url

https://teamcity.jetbrains.com/app/rest/swagger.json

Generation Details

docker run -m 24g -it --rm -v "$(pwd):/local" openapitools/openapi-generator-cli java -Xmx20G -jar ./opt/openapi-generator/modules/openapi-generator-cli/target/openapi-generator-cli.jar generate -i https://teamcity.jetbrains.com/app/rest/swagger.json -g protobuf-schema -o /local/out/teamcity

Needs extra mem otherwise fails with Exception in thread "main" java.lang.OutOfMemoryError: Java heap space.

Steps to reproduce

1. Download/extract protobuf compiler v23.0 (the latest release) for win64 (link)
2. cd ./out/teamcity
3. .\protoc-23.0-win64\bin\protoc.exe --csharp_out=./csharp .\models/*
4. Note resulting errors:

models/agent.proto:15:1: File recursively imports itself: models/agent.proto -> models/agent_pool.proto -> models/agents.proto -> models/agent.proto
models/agents.proto:15:1: Import "models/agent.proto" was not found or had errors.
models/agents.proto:27:12: "Agent" is not defined.
<SNIP>
models/build_types.proto:15:1: Import "models/build_type.proto" was not found or had errors.
models/build_types.proto:27:12: "BuildType" is not defined.
models/cloud_image.proto:15:1: File recursively imports itself: models/cloud_image.proto -> models/cloud_instances.proto -> models/cloud_instance.proto -> models/cloud_image.proto
models/cloud_instance.proto:15:1: Import "models/agent.proto" was not found or had errors.
models/cloud_instance.proto:16:1: Import "models/cloud_image.proto" was not found or had errors.
models/cloud_instance.proto:32:3: "CloudImage" is not defined.
models/cloud_instance.proto:34:3: "Agent" is not defined.
models/cloud_instances.proto:15:1: Import "models/cloud_instance.proto" was not found or had errors.
models/cloud_instances.proto:27:12: "CloudInstance" is not defined.
<SNIP>

Related issues/PRs

Suggest a fix

Start by generating the protobuf-schema output from TeamCity's swagger.json with the documented openapi-generator command, then run protoc v23.0 against the generated models. Trace the imports shown in agent.proto, agent_pool.proto, agents.proto, cloud_image.proto, cloud_instances.proto, and cloud_instance.proto. Done means the generated protobuf files compile without recursive-import or undefined-symbol errors.