[BUG] [KOTLIN] [CLIENT] Optional query parameters handled wrong

Created by: ghost

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?
• [Optional] Bounty to sponsor the fix (example)

Description

The code generation does not work properly for optional / nullable query parameters. The example below contains a REST resource with an optional query parameter. The generator produces a nullable type in the function signature - in this case here kotlin.Int? - which is correct. But when building the localVariableQuery the nullable type is not handled correctly:

    fun resourcesGet(page: kotlin.Int?) : Unit {
        val localVariableBody: kotlin.Any? = null
        val localVariableQuery: MultiValueMap = mapOf("page" to listOf("$page"))

        [...]
    }

As you can see in the code snippet, the function arguments are directly embedded into a String. When calling the method with page=null, the String "null" is sent to the server instead of simply omitting the query parameter. In this case the server will try to parse "null" into a number which will probably not work and let the request fail.

openapi-generator version

I am using v4.0.3. Current code from master seems to have same issue.

OpenAPI declaration file content or url

{
  "swagger": "2.0",
  "info": {
    "version": "1.0.0",
    "title": "MyService"
  },
  "paths": {
    "/resources": {
      "get": {
        "produces": [
          "application/json",
          "application/json;charset=UTF-8"
        ],
        "parameters": [
          {
            "name": "page",
            "in": "query",
            "description": "Zero-based page number for paging (minimum 0).",
            "required": false,
            "type": "integer",
            "default": 0,
            "minimum": 0,
            "exclusiveMinimum": false,
            "format": "int32",
            "allowEmptyValue": true,
            "x-example": 0
          }
        ],
        "responses": {
          "200": {
            "description": "Success"
          }
        }
      }
    }
  }
}

Command line used for generation

generate --generator-name kotlin --additional-properties enumPropertyNaming=original --additional-properties dateLibrary=java8

Steps to reproduce

• Use above JSON to generate a client (and a server)
• Use the generated client code to perform a request with function parameter page=null

Related issues/PRs

Suggest a fix

As a workaround, I do a post-processing of the generated code: file.text.replaceAll('"([^"]*)" to listOf\\("[^"]*"\\)', '"$1" to if ($1 != null) listOf($1.toString()) else emptyList()')

This transforms mapOf("page" to listOf("$page")) into mapOf("page" to if (page != null) listOf(page.toString()) else emptyList())

This is more a hack than a real solution. It would be better to put the optional/nullable parameters into the localVariableQuery variable only if they are not null.