Example app Swift4, crashes when run on Linux environment with concurrency > 1

main.swift

import Foundation
import Dispatch

guard CommandLine.arguments.count > 1 else {
    print("Please add one argument for the number of lanes")
    exit(0)
}
var maxConcurrency = Int(CommandLine.arguments[1]) ?? 1

print("Starting to fetch with max \(maxConcurrency) simultaneous fetches")

let session = URLSession(configuration: .default)
let lanes: [DispatchSemaphore] = (0..<maxConcurrency).map({ _ in DispatchSemaphore(value: 1) })

var laneCtr = 0
func load(url: URL, completion: @escaping (String) -> ()) {
    let lane = lanes[laneCtr]
    let currentLaneNr = laneCtr
    laneCtr = (laneCtr + 1) % maxConcurrency
    lane.wait()
    completion("[\(currentLaneNr)]Fetched: Starting to fetch \(url)")
    let dataTask = session.dataTask(with: url) { (data,response, error) in
        defer { lane.signal() }
        if let error = error {
            completion("[\(currentLaneNr)]Encountered error: \(error.localizedDescription)")
        } else {
            completion("[\(currentLaneNr)]Fetched: \(data?.count ?? 0) bytes of data")
        }
    }
    dataTask.resume()
}

func fetch() {
    let urlString = "https://github.com/search?q=\(randomInt())"
    guard let url = URL(string: urlString) else { return }
    load(url: url) { print($0) }
}

func randomInt() -> UInt32 {
    #if os(Linux)
        return UInt32(random())
    #else
        return arc4random()
    #endif
}

func randomTime() -> Double {
    #if os(Linux)
        return Double(random()) / Double(Int.max)
    #else
        return Double(arc4random()) / Double(UInt32.max)
    #endif
}

func delayFetch() {
    let delay = randomTime()
    DispatchQueue.main.asyncAfter(deadline: .now() + delay) {
        //print("delay \(delay)")
        fetch()
        delayFetch()
    }
}

delayFetch()
delayFetch()

RunLoop.main.run()

Example illustrating the bug discussed in https://bugs.swift.org/browse/SR-5936?page=com.atlassian.jira.plugin.system.issuetabpanels%3Aall-tabpanel