[BUG] Calls off multiple threads cannot run concurrently in Swift 4

Created by: DavePilks

Bug Report Checklist

• Have you provided a full/minimal spec to reproduce the issue?
• What's the version of OpenAPI Generator used?
• Have you search for related issues/PRs?

Description

When creating the ResponseData object in AlamofireImplementations.swift, no dispatch queue is passed into the init. This means that Alamofire defaults to DispatchQueue.main.

If you then have multiple threads trying to make service calls, they are all placed on the main queue and cannot run concurrently as there is only a single thread.

We noticed this as we were trying to break out of the callback pattern using a semaphore. This is when we noticed the blocking.

openapi-generator version

3.3.4

Steps to reproduce

Call Setup

• Use semaphore to block until call back is fire
• Return result from callback

Queuing

• Have multiple call running in the background queue

Expected Result The calls return once the call has been completed

Actual Result The calls block

Suggest a fix

The init for the Alamofire response takes in a queue parameter that is defaulted to nil. Suggested fix is to provide DispatchQueue.global(qos: .background) as the parameter to allow concurrency