ConcurrentException/NullPointerException in GParsPool runForkJoin

For a little context, i'm trying to solve Project Euler problem 31 using excellent GParsPool Fork/Join support.

For that, i've written the foolowing code :

import groovyx.gpars.*
import groovy.util.GroovyCollections

@Grab(group="org.codehaus.gpars", module="gpars", version="0.11")
def getMatchingCombos(target) {
    combos = [200开发者_开发问答, 100 /*, 50, 20, 10, 5, 2, 1*/]
    GParsPool.withPool(1) { pool ->
        combos = combos.collectParallel { n ->  
            ((0..(target/n)).step(1) as TreeSet).collect { p -> p*n }
        }
        return GParsPool.runForkJoin(combos, 0, 0, target) { usableCombos, comboIndex, sum, targetSum ->
            def offset = "\t"*comboIndex
            def results = 0
            if(sum<=targetSum) {
                if(comboIndex<combos.size()) {
                    usableCombos[comboIndex].each { n ->
                        println offset+"now trying with $comboIndex element value $n (curent sum is $sum)"
                        results += forkOffChild(usableCombos, comboIndex+1, sum+n, targetSum)
                    }
                } else {
                    if(sum==targetSum) {
                        results +=1
                        println offset+"sum is target ! so we have $results"
                    }
                }
            }
            return results;
        }
    }
}

println getMatchingCombos(200)

Unfortunatly, each time I try to run this, I get the following stack trace :

now trying with 0 element value 0 (curent sum is 0). Known combos are [[0, 200], [0, 100, 200]] and target is 200
        now trying with 1 element value 0 (curent sum is 0). Known combos are [[0, 200], [0, 100, 200]] and target is 20
0
Caught: java.util.concurrent.ExecutionException: java.lang.NullPointerException
        at groovyx.gpars.GParsPool.runForkJoin(GParsPool.groovy:305)
        at probleme_31$_getMatchingCombos_closure1.doCall(probleme_31.groovy:18)
        at groovyx.gpars.GParsPool$_withExistingPool_closure1.doCall(GParsPool.groovy:170)
        at groovyx.gpars.GParsPool$_withExistingPool_closure1.doCall(GParsPool.groovy)
        at groovyx.gpars.GParsPool.withExistingPool(GParsPool.groovy:169)
        at groovyx.gpars.GParsPool.withPool(GParsPool.groovy:141)
        at groovyx.gpars.GParsPool.withPool(GParsPool.groovy:117)
        at probleme_31.getMatchingCombos(probleme_31.groovy:9)
        at probleme_31.run(probleme_31.groovy:41)

I understand it has something to do with the way i want to exploit Fork/Join as a recursion "flattening" mechanism, but what is the error I'm doing here ?

You're incorrectly trying to read the children results as a return value from the forkOffChild() method, while this should be done using getChildrenResults().

        return GParsPool.runForkJoin(combos, 0, 0, target) { usableCombos, comboIndex, sum, targetSum ->
        def offset = "\t"*comboIndex
        def results = 0
        if(sum<=targetSum) {
            if(comboIndex<combos.size()) {
                usableCombos[comboIndex].each { n ->
                    println offset+"now trying with $comboIndex element value $n (curent sum is $sum)"
                    forkOffChild(usableCombos, comboIndex+1, sum+n, targetSum)
                }
            } else {
                if(sum==targetSum) {
                    results +=1
                    println offset+"sum is target ! so we have $results"
                }
            }
        }
        results += getChildrenResults().sum(0)
        return results;
    }