[Swift] Alamofire client template does not support concurrent request

Description

The current Alamofire client template implementation does not support concurrent requests. The static AlamofireRequestBuilder.execute method adds an instance of Alamofire.SessionManager to a global variable (managerStore) without any thread safe mechanism to avoid concurrent accesses. Therefore this can result, for example, in the deallocation of a Alamofire.SessionManager that might cancel the request. As result, the requests affect will return Error Domain=NSURLErrorDomain Code=-999 "cancelled".

Swagger-codegen version

2.3.1, 3.0.5

Swagger declaration file content or url

Not needed, all applications making concurrent requests using the Alamofire implementation of these version should experience the same issues.

Command line used for generation

swagger-codegen-2.3.1 generate -i http://localhost:8083/api-docs/v1/swagger.json -l swift4 -o swagger-client

Steps to reproduce

Dispatch several concurrent operation to a OperationQueue performing different API requests. Check if any of these calls resulted in a cancelled request.

Suggest a fix/enhancement

I can think about two different solutions that could help in the resolution of this problem:

• Instead of using static methods, relying on global variables/state and create on each request a new session manager, we could instead use an API Client instance that would be injected/kept in memory and configurable as needed.
• Make the global variable thread safe. We could use something like this:

public final class Synchronized<T> {
    
    private let queue: DispatchQueue
    private var _value: T
    
    public init(_ value: T) {
        let queueLabel = "com.codegen.synchronized.\(type(of: value)).\(UUID().uuidString)"
        self.queue = DispatchQueue(label: queueLabel, attributes: .concurrent)
        self._value = value
    }
    
    public var value: T {
        var result: T? = nil
        queue.sync { result = self._value }
        return result!
    }
    
    public func write(using block: @escaping (inout T) -> Void) {
        queue.async(flags: .barrier) {
            block(&self._value)
        }
    }
}

Start at the AlamofireRequestBuilder.execute entry point and inspect how managerStore holds Alamofire.SessionManager instances. Reproduce the problem by dispatching several requests through an OperationQueue, then verify that concurrent requests complete without cancellation and add or update coverage if the relevant template tests are found.