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")

Also looks like compiler flattens out the nested thread dispatching

Or even more probable than that, is that URLSessionDataTasks are being added to OperationQueue.current with lower priority, but could not confirm. OperationQueue.current === queue inside URLSessionDataTask's completion block yields false (may be completion queue is different)

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.