NSOperationQueue + NSURLSession = no network activity

operationqueue_with_urlsession_no_activity.swift

import Dispatch
import Foundation

// If you run this playground, you will never see `Request feedback` message in console.
let queue = OperationQueue()

// but if you uncomment the line below, it suddenly starts working great
// queue.maxConcurrentOperationCount = 63
// but if you set maxConcurrentOperationCount to 64, it stops working again

for _ in 0...999 {
    queue.addOperation {
        let group = DispatchGroup()
        
        var request = URLRequest(url: URL(string: "http://example.com")!)
        request.timeoutInterval = 10
        
        let task = URLSession.shared.dataTask(with: request) { _,_,_ in
            print("\(Date()) Request feedback")
            group.leave()
        }
        
        print("\(Date()) Starting request")
        task.resume()
        group.enter()
        
        if group.wait(wallTimeout: .now() + 30) == .timedOut {
            task.cancel()
            print("\(Date()) Request timeout")
        }
    }
}

queue.waitUntilAllOperationsAreFinished()
print("\(Date()) Queue is empty")

Thanks Vakhid, I forgot to attach a link to the forum where Apple stuff has commented on this issue.

https://forums.swift.org/t/operationqueue-urlsession-no-network-activity-oob/33247

You were absolutely right, there is a shared thread pool in GCD for all queues, and that explains a lot.