DList ops being executed more than once

[blever]

DList ops should only be executed once. This is not only important for performance but also necessary if the operation is side-effecting, e.g. making a calling to Random.

An example of when this doesn't seem to be the case is exercised in the following code:

package com.nicta.scoobi
package acceptance

import Scoobi._
import testing.mutable.NictaSimpleJobs

class SideEffectsAreBadSpec extends NictaSimpleJobs {

  "Filtering with side effectss" >> {
    "Filtering random" >> { implicit c: SC =>
      val xs = (1 to 5000).toDList
      val zs = (xs filter (x => { if (x == 100) println("running"); scala.util.Random.nextDouble() > 0.5})) 
      val as = (zs.size join zs.map(x => "" + x)).materialise
      val ys = as.run
      println(" total: " + ys.size + " vs " + ys.head._1 )
      true
    }
  }
}

// running
// running
// total: 2538 vs 2479

[markhibberd]

Concurrent bug reporting. See #282.

[blever]

Duplicate of #282.