BarrierTests.swift

import XCTest

final class BarrierTests: XCTestCase {
    func test() async {
        let subsystem = Subsystem()
        await withTaskGroup(of: Void.self) { group in
            for index in 0 ..< 100 {
                group.addTask { subsystem.performWork(id: index) }
            }

            await group.waitForAll()
        }
    }
}

final class Subsystem: Sendable {
    let queue = DispatchQueue(label: "my concurrent queue", attributes: .concurrent)

    func performWork(id: Int) {
        if id % 10 == 0 { write(id: id) }
        else { read(id: id) }
    }

    func write(id: Int) {
        print("schedule exclusive write \(id)")
        queue.async(flags: .barrier) { print(" execute exclusive write \(id)") }
        print("schedule exclusive write \(id) done")
    }

    func read(id: Int) {
        print("schedule read \(id)")
        queue.sync { print(" execute read \(id)") }
        print("schedule read \(id) done")
    }
}