[BUG] [Kotlin] [Retrofit 2] [openapi-generator-gradle-plugin] - Generating an absolute paths instead of relative.

Bug Report Checklist

• Have you provided a full/minimal spec to reproduce the issue?
• Have you validated the input using an OpenAPI validator (example)?
• What's the version of OpenAPI Generator used?
• Have you search for related issues/PRs?
• What's the actual output vs expected output?

Description

The openapi-generator-gradle-plugin is generating the endpoints as absolute paths, with leading / like /pets, instead of relative like pets.
And because the path is absolute - Retrofit seems to be ignoring any path information given to the builder in the .baseUrl(HttpUrl) function. It seems to just be taking the schema and host, and ignoring any base path given to it.

Info from Retrofit's site:

Endpoint values which contain a leading `/` are absolute.
Absolute values retain only the host from baseUrl and ignore any specified path components.
Base URL: http://example.com/api/
Endpoint: /foo/bar/
Result: http://example.com/foo/bar/

openapi-generator version

org.openapi.generator version "4.3.1"

OpenAPI declaration file content or url

https://raw.githubusercontent.com/OAI/OpenAPI-Specification/master/examples/v3.0/petstore.yaml

Steps to reproduce

This is the setup:
gradle task:

openApiGenerate {
    generatorName = "kotlin"
    library = "jvm-retrofit2"
    inputSpec = "https://raw.githubusercontent.com/OAI/OpenAPI-Specification/master/examples/v3.0/petstore.yaml"
    configOptions = [
            dateLibrary: "java8"
    ]
}

generated PetsApi interface:

interface PetsApi {
    @POST("/pets")
    fun createPets(): Call<Unit>

    @GET("/pets")
    fun listPets(@Query("limit") limit: kotlin.Int): Call<kotlin.Array<Pet>>

    @GET("/pets/{petId}")
    fun showPetById(@Path("petId") petId: kotlin.String): Call<Pet>

}

initialising the petsApi:

    private val apiClient = ApiClient(baseUrl = "http://petstore.swagger.io/v1/", okHttpClient = okHttpClient)
    private val petsApi: PetsApi = apiClient.createService(PetsApi::class.java)

Suggest a fix

Maybe to have the generating an absolute paths configurable.

Similar / related

Here is the same issue described on stackoverflow - https://stackoverflow.com/questions/32352159/retrofit-2-removes-characters-after-hostname-from-base-url
There is a presentation how to fix the issue manually, like removing the leading / to make the path relative. https://www.youtube.com/watch?v=KIAoQbAu3eA&feature=youtu.be&t=32m50s
Screenshot from the video:

Run the shown Gradle openApiGenerate setup with the petstore.yaml specification and inspect the generated Kotlin PetsApi interface from the openapi-generator-gradle-plugin. Compare the Retrofit annotations' endpoint paths with the configured base URL; done means generated paths preserve the base path rather than starting with a leading slash.