[REQ][Go Client] Support circular schema definitions

[grokify]

Is your feature request related to a problem? Please describe.

Valid OpenAPI specs can have circular definitions, however, this is not currently supported in generated Go client SDKs.

Validity of OpenAPI circular definitions is discussed here:

• Circular references in Definitions Object OAI/OpenAPI-Specification#822
• [openapi-spec] Circular dependencies in resource definitions  kubernetes/kubernetes#52163

Such definitions will also validate on the OAS3 validators by Mermade and APIDevTools.

Currently, when such a spec is used, the invalid recursive type compile error occurs.

# github.com/grokify/go-ringcentral/codegen/engagevoice
../../../../codegen/engagevoice/model_agent_account_access.go:12:6: invalid recursive type AgentAccountAccess
../../../../codegen/engagevoice/model_campaign.go:12:6: invalid recursive type Campaign
../../../../codegen/engagevoice/model_custom_dial_zone_groups.go:12:6: invalid recursive type CustomDialZoneGroups
../../../../codegen/engagevoice/model_quota_target.go:12:6: invalid recursive type QuotaTarget

Describe the solution you'd like

The problem with circular definitions occurs in Go because the compiler cannot determine the size of such a struct. The solution is to replace the nested struct with a pointer, who's size is known, as described here:

A pointer's size is known, but how big is something that contains itself? (And the inner struct contains itself as well, as does the inner inner struct, and so on.)

https://stackoverflow.com/a/8261789/1908967

This has been confirmed to compile by manually updating the generated model files.

Describe alternatives you've considered

None yet.

Additional context

1. The use of pointers for nested structs is also being discussed in various JSON null value threads.
   1. [Go] Optional values should be pointers #522
   2. [BUG][go-experimental] Explicit nulls support is broken #5278
2. Google's auto-generated Go client SDKs appear to always use pointers for nested structs - https://github.com/googleapis/google-api-go-client

Given how pointers are part of a long-running nullable discussion, one thought is to use a CLI parameter that will make all nested structs pointers as to make it an optional feature.

[wing328]

@grokify I wonder if you've tried the go-experimental generator to see whether it has the same issue.

[grokify]

@wing328 Not yet, but I'll check it out.

For interim use, I've created the following simple post processor. I'll look into this more after the current project ships.

• https://github.com/grokify/gotilla/blob/master/codegen/nestedstructtopointer.go
• https://github.com/grokify/gotilla/blob/master/codegen/apps/nestedstruct2pointer/main.go (CLI app)

[jonasknobloch]

Ran into this issue using the current go generator (go-experimental at the time this issue was created). Fixed it by manually updating the generated model files.

[tony-scio]

We've been hacking the generated files with a simple minded shell script to add pointers. @grokify thanks for the more robust hack than ours. Would be great to see this fixed in the generator though.