[BUG][golang] 'isCollectionFormatMulti' flag not being set to true on multiple file uploads

Bug Report Checklist

• Have you provided a full/minimal spec to reproduce the issue?
• Have you validated the input using an OpenAPI validator (example)?
• 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

A generated Golang client for an OpenAPI 3 spec that includes multi-file uploads fails to build because the array of files is generated as a single file. For example, the following snippet is from the generated api_default.go:

	localVarFormFileName = "files"
	var localVarFile []*os.File
	if r.files != nil {
		localVarFile = *r.files
	}
	if localVarFile != nil {
		fbs, _ := _ioutil.ReadAll(localVarFile)
		localVarFileBytes = fbs
		localVarFileName = localVarFile.Name()
		localVarFile.Close()
	}

openapi-generator version

I have tried with the following versions:

Version	Install Method
v0.5.0	Homebrew
	Docker
v4.3.1	Docker
HEAD	Homebrew

OpenAPI declaration file content or url

openapi: 3.0.1
info:
  title: API
  version: "1.0"
servers:
- url: https://example.com
tags:
- name: default
  description: Default namespace
paths:
  /bug/{bug_id}/attachment:
    post:
      tags:
      - default
      summary: Upload a file
      operationId: post_res_attach
      parameters:
      - name: bug_id
        in: path
        required: true
        schema:
          type: integer
      requestBody:
        content:
          multipart/form-data:
            schema:
              type: object
              properties:
                files:
                  type: array
                  description: One or more files to upload
                  items:
                    type: string
                    format: binary
      responses:
        200:
          description: Success
          content: {}
        401:
          description: Unauthenticated
          content: {}

Generation Details

openapi-generator generate --input-spec spec.yaml \
  --generator-name go \
  --template-dir ./generator-template \
  --output ./openapi

The generator-template directory contains a single file, partial_header.mustache:

// Copyright (c) 2020 VMware, Inc. All Rights Reserved.
// SPDX-License-Identifier: Apache-2.0

/*
 {{#appName}}
 * {{{appName}}}
 *
 {{/appName}}
 {{#appDescription}}
 * {{{appDescription}}}
 *
 {{/appDescription}}
 {{#version}}
 * API version: {{{version}}}
 {{/version}}
 {{#infoEmail}}
 * Contact: {{{infoEmail}}}
 {{/infoEmail}}
{{^withGoCodegenComment}}
 * Generated by: OpenAPI Generator (https://openapi-generator.tech)
{{/withGoCodegenComment}}
 */
{{#withGoCodegenComment}}

// Code generated by OpenAPI Generator (https://openapi-generator.tech); DO NOT EDIT.
{{/withGoCodegenComment}}

Steps to reproduce

1. Switch to a temporary directory:

   cd $(mktemp -d)
   

2. Create a spec file:

   cat <<EOF >spec.yaml
   openapi: 3.0.1
   info:
     title: API
     version: "1.0"
   servers:
   - url: https://example.com
   tags:
   - name: default
     description: Default namespace
   paths:
     /bug/{bug_id}/attachment:
       post:
         tags:
         - default
         summary: Upload a file
         operationId: post_res_attach
         parameters:
         - name: bug_id
           in: path
           required: true
           schema:
             type: integer
         requestBody:
           content:
             multipart/form-data:
               schema:
                 type: object
                 properties:
                   files:
                     type: array
                     description: One or more files to upload
                     items:
                       type: string
                       format: binary
         responses:
           200:
             description: Success
             content: {}
           401:
             description: Unauthenticated
             content: {}
   EOF
   

3. Create a generator template:

   cat <<EOF >partial_header.mustache
   // Copyright (c) 2020 Company, Inc. All Rights Reserved.
   // SPDX-License-Identifier: Apache-2.0
   
   /*
    {{#appName}}
    * {{{appName}}}
    *
    {{/appName}}
    {{#appDescription}}
    * {{{appDescription}}}
    *
    {{/appDescription}}
    {{#version}}
    * API version: {{{version}}}
    {{/version}}
    {{#infoEmail}}
    * Contact: {{{infoEmail}}}
    {{/infoEmail}}
   {{^withGoCodegenComment}}
    * Generated by: OpenAPI Generator (https://openapi-generator.tech)
   {{/withGoCodegenComment}}
    */
   {{#withGoCodegenComment}}
   
   // Code generated by OpenAPI Generator (https://openapi-generator.tech); DO NOT EDIT.
   {{/withGoCodegenComment}}
   EOF
   

4. Generate the Golang code using the Open API Docker image:

   docker run --rm \
     -v $(pwd)/spec.yaml:/openapi/spec.yaml:ro \
     -v $(pwd):/openapi/output \
     -v $(pwd):/openapi/generator-template \
     openapitools/openapi-generator-cli:v5.0.0 \
     generate --input-spec /openapi/spec.yaml \
       --generator-name go \
       --template-dir /openapi/generator-template \
       --output /openapi/output
   

5. Print lines 106-116 of api_default.go to see the incorrectly generated client:

   $ sed -n '106,116p' api_default.go
   	localVarFormFileName = "files"
   	var localVarFile []*os.File
   	if r.files != nil {
   		localVarFile = *r.files
   	}
   	if localVarFile != nil {
   		fbs, _ := _ioutil.ReadAll(localVarFile)
   		localVarFileBytes = fbs
   		localVarFileName = localVarFile.Name()
   		localVarFile.Close()
   	}
   

I imagine the same lines should treat the property as a slice/collection similar to the related issue in the next section.

Related issues/PRs

The following issue and PR seemed promising:

• https://github.com/OpenAPITools/openapi-generator/issues/8093
• https://github.com/OpenAPITools/openapi-generator/pull/8103

However, it seems the fix was for the server-side handling only.

Suggest a fix

Issue https://github.com/OpenAPITools/openapi-generator/issues/8104 seems to be the same as this one, but for typescript. I believe the same fix, but for Go, should work.

Sponsorship

I'm willing to pay $100 for a quick fix on this and $150 if there's a subsequent v5.0.1 patch release with the fix (both for home-brew and the Docker image).

Reproduce the issue with the supplied spec.yaml and Go generator command, then inspect the generated api_default.go and the Go generator templates or entry points responsible for multipart file parameters. Done means the generated client treats the files property as a collection, builds successfully, and handles multiple uploaded files rather than one file.