[BUG] Wrong parameter for multipart-form-data file upload when using java reactive

Created by: bit-factor

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?
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Description

I want to build a rest-service which needs to be able to support large-ish file uploads. I am using spring-boot 2.2.7.RELEASE with webflux.

openapi-generator version

openapi-generator-maven-plugin v4.3.1.

OpenAPI declaration file content or url

  /upload/{parentId}/doc:
    post:
      tags:
        - docs
      summary: uploads a new document
      operationId: upload.doc
      security:
        - bearerAuth: []
      parameters:
        - in: path
          name: parentId
          schema:
            type: string
          required: true
      requestBody:
        content:
          multipart/form-data:
            schema:
              type: object
              properties:
                file:
                  type: string
                  format: binary
      responses:
        201:
          description: upload was succesfull

Command line used for generation

I am using it directly in my pom.xml file as follows:

            <plugin>
                <groupId>org.openapitools</groupId>
                <artifactId>openapi-generator-maven-plugin</artifactId>
                <version>${openapi-generator.version}</version>
                <executions>
                    <execution>
                        <goals>
                            <goal>generate</goal>
                        </goals>
                        <configuration>
                            <inputSpec>${project.basedir}/swagger.yaml</inputSpec>
                            <modelPackage>com.application.rest.models</modelPackage>
                            <apiPackage>com.application.rest</apiPackage>
                            <library>spring-boot</library>
                            <generatorName>spring</generatorName>
                            <modelNamePrefix>Rest</modelNamePrefix>
                            <generateSupportingFiles>true</generateSupportingFiles>

                            <configOptions>
                                <interfaceOnly>true</interfaceOnly>
                                <serializableModel>true</serializableModel>
                                <invokerPackage>com.application</invokerPackage>
                                <basePackage>com.application.backend</basePackage>
                                <configPackage>com.application.conf</configPackage>
                                <dateLibrary>java8</dateLibrary>
                                <delegatePattern>false</delegatePattern>
                                <useOptional>true</useOptional>
                                <sourceFolder>src/main/java</sourceFolder>
                                <reactive>true</reactive>
                            </configOptions>
                        </configuration>
                    </execution>
                </executions>
            </plugin>

Steps to reproduce

In the maven-java directory, simply run mvn clean compile.

Related issues/PRs

https://github.com/OpenAPITools/openapi-generator/issues/6677 -- uses react/java, but doesn't refer to file-upload, it looks for a way to make the parameter optional;

Suggest a fix

The correct parameter should be Flux<Part> or Flux<FilePart>. There are several examples:

Simply using MultipartFile forces us to go through InputStream and then handle blocking calls (like close, for instance) from non-blocking code.