mapZIOParUnordered does not respect parallelism

[adrian-salajan]

I am surprised of what this program using mapZIOParUnordered outputs

  override def run: ZIO[Any with ZIOAppArgs with Scope, Any, Any] = for {
    _ <- ZIO.debug("start")
    _ <- ZStream
      .iterate[Int](0)(n => n +1)
      .mapZIOParUnordered(2){ _ =>
        ZIO.debug("tick").delay(1.second)
      }.runDrain.zipParLeft(ZIO.debug("----").delay(2.seconds).forever)

  } yield ()

the number of ticks printed between the ----- is: 2, 7, 11, 15, 19, 23 ....
while if instead of mapZIOParUnordered(2) i use mapZioPar(2) ticks printed is: 2, 6, 6, 6, 6. Here i would expect it to be 4, 4, 4, 4 but at least it does not keep increasing

[jdegoes]

/bounty $100

[algora-pbc]

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Attempt	Started (GMT+0)	Solution
🟢 @varshith257	May 20, 2024, 2:57:18 PM	WIP

[varshith257]

/attempt #8753