[Go] Parse int arrays and respect the 'required' property

Created by: aliakseiz

*Affects the Go-server only.

Integer arrays support in a form

Adds a code generation to parse form parameter fields of int32 array and int64 array types.

post:
  summary: Test operation
  operationId: postTest
  requestBody:
    description: New array
    content:
      multipart/form-data:
        schema:
          type: object
          properties:
            Test:
              type: array
              items:
                type: integer
                format: int64

Before:

test, err := parseInt64Parameter( r.FormValue("Test"))

After:

// "," - delimiter to parse an array
// false - state of `required` property
test, err := parseInt64ArrayParameter(r.FormValue("Test"), ",", false)

Required integer fields

Also parameter's required property is not ignored anymore, meaning that missing optional parameters of int32, int64, []int32 and []int64 types are initialized with 0 and empty slice respectively, instead of failing the whole operation. Before/after below assume, that mentioned optional and required fields are missing in the payload. Before:

// Optional field
id, err := parseInt64Parameter(params["id"])
if err != nil { // <-- err is not nil, because empty string cannot be parsed as int64. Operation fails
    w.WriteHeader(http.StatusBadRequest)
    return
}

After:

// Optional field
id, err := parseInt64Parameter(params["id"], false)
if err != nil { // <-- err is nil, id is 0. Operation continues
    w.WriteHeader(http.StatusBadRequest)
    return
}

// Required field
idReq, err := parseInt64Parameter(params["idReq"], true)
if err != nil { // <-- err is not nil, contains `errMsgRequiredMissing` message. Operation fails, as expected
    w.WriteHeader(http.StatusBadRequest)
    return
}

Multipart/form-data parsing

Replaces form parsing with more specific call for multipart/form-data. ParseForm itself doesn't initialize the Files parameters.

FileParam:
  type: object
  properties:
    Files:
      description: Some files
      type: array
      items:
        type: string
        format: binary

Before:

err := r.ParseForm()
if err != nil {
    w.WriteHeader(http.StatusBadRequest)
    return
}

After:

if err := r.ParseMultipartForm(32 << 20); err != nil {
    w.WriteHeader(http.StatusBadRequest)
    return
}

Other

Fixes minor typos and FIXMEs, which caught my eye during debugging.

Technical committee

@antihax @grokify @kemokemo @jirikuncar

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