The checkpoint synchronization is a problem of synchronizing multiple tasks. Consider a workshop where several workers (tasks) assembly details of some mechanism. When each of them completes his work they put the details together. There is no store, so a worker who finished its part first must wait for others before starting another one. Putting details together is the checkpoint at which tasks synchronize themselves before going their paths apart.

The task

Implement checkpoint synchronization in your language.

Make sure that the solution is race condition-free. Note that a straightforward solution based on events is exposed to race condition. Let two tasks A and B need to be synchronized at a checkpoint. Each signals its event (EA and EB correspondingly), then waits for the AND-combination of the events (EA&EB) and resets its event. Consider the following scenario: A signals EA first and gets blocked waiting for EA&EB. Then B signals EB and loses the processor. Then A is released (both events are signaled) and resets EA. Now if B returns and enters waiting for EA&EB, it gets lost.

When a worker is ready it shall not continue before others finish. A typical implementation bug is when a worker is counted twice within one working cycle causing its premature completion. This happens when the quickest worker serves its cycle two times while the laziest one is lagging behind.

If you can, implement workers joining and leaving.

// Version 1.2.41
 
import java.util.Random
 
val rgen = Random()
var nWorkers = 0
var nTasks = 0
 
class Worker(private val threadID: Int) : Runnable {
 
    @Synchronized
    override fun run() {
        try {
            val workTime = rgen.nextInt(900) + 100L  // 100..999 msec.
            println("Worker $threadID will work for $workTime msec.")
            Thread.sleep(workTime)
            nFinished++
            println("Worker $threadID is ready")
        }
        catch (e: InterruptedException) {
            println("Error: thread execution interrupted")
            e.printStackTrace()
        }
    }
 
    companion object {
        private var nFinished = 0
 
        @Synchronized
        fun checkPoint() {
            while (nFinished != nWorkers) {
                try {
                    Thread.sleep(10)
                }
                catch (e: InterruptedException) {
                    println("Error: thread execution interrupted")
                    e.printStackTrace()
                }
            }
            nFinished = 0  // reset
        } 
    }
}
 
fun runTasks() {
    for (i in 1..nTasks) {
        println("\nStarting task number $i.")
        // Create a thread for each worker and run it.
        for (j in 1..nWorkers) Thread(Worker(j)).start()
        Worker.checkPoint()  // wait for all workers to finish the task
    }
}
 
fun main(args: Array<String>) {
    print("Enter number of workers to use: ")
    nWorkers = readLine()!!.toInt()
    print("Enter number of tasks to complete: ")
    nTasks = readLine()!!.toInt()
    runTasks()
}