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

Bug Report Checklist

[x] Have you provided a full/minimal spec to reproduce the issue?
[x] Have you validated the input using an OpenAPI validator (example)?
[x] Have you tested with the latest master to confirm the issue still exists?
[x] Have you searched for related issues/PRs?
[x] What's the actual output vs expected output?
[x] [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

Switch to a temporary directory:
```
cd $(mktemp -d)
```

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

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

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

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:

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).

OpenAPITools / openapi-generator