OS-Lib subprocess tests fail on Scala-Native 0.5.0

[lihaoyi]

They previously weren't supported at all on 0.4.x due to lack of multithreading. Now that multithreading exists, I would assume they should work, but they don't.

To repro,

1. Check out this branch:Support os.proc on Scala Native com-lihaoyi/os-lib#257
2. Run ./mill -i "os.jvm[2.12.17].test" test.os.SubprocessTests.lines to see the code passing on Scala-JVM
3. Run ./mill -i "os.native[2.12.17].test" test.os.SubprocessTests.lines to see the exact same Scala code failing on Scala-Native 0.5.0

This gives me the following error:

----------------- Running Tests test.os.SubprocessTests.lines -----------------
X test.os.SubprocessTests.lines 70176ms
  java.lang.IllegalArgumentException
    scala.collection.immutable.VectorPointer.gotoNextBlockStartWritable(Unknown)
    scala.collection.immutable.VectorBuilder.gotoNextBlockStartWritable(Unknown)
    scala.collection.immutable.VectorBuilder.$plus$eq(Unknown)
    scala.collection.immutable.VectorBuilder.$plus$eq(Unknown)
    scala.collection.generic.Growable.$anonfun$$plus$plus$eq$1(Unknown)
    scala.collection.generic.Growable$$Lambda$1.apply(Unknown)
    scala.collection.Iterator.foreach(Unknown)
    scala.collection.AbstractIterator.foreach(Unknown)
    scala.collection.generic.Growable.$plus$plus$eq(Unknown)
    scala.collection.immutable.VectorBuilder.$plus$plus$eq(Unknown)
    scala.collection.immutable.VectorBuilder.$plus$plus$eq(Unknown)
    scala.collection.TraversableOnce.to(Unknown)
    scala.collection.AbstractIterator.to(Unknown)
    scala.collection.TraversableOnce.toIndexedSeq(Unknown)
    scala.collection.AbstractIterator.toIndexedSeq(Unknown)
    os.proc.call(Unknown)
    test.os.SubprocessTests$.$anonfun$tests$2(Unknown)

I'm not sure how to go about begin debugging this on Scala-Native, but presumably if the same code is passing on Scala-JVM and failing on Scala-Native, it is a Scala-Native bug.

At a first glance, it seems it might be some kind of multithreading bug, where we are trying to convert a java.util.concurrent.ConcurrentLinkedQueue into a Vector via a VectorBuilder, and someone along the chain is having its internal invariants violated

[lihaoyi]

Here's a minimal repro that seems to hang in Scala Native 0.5.0

      val chunks = new java.util.concurrent.ConcurrentLinkedQueue[Int]

      chunks.add(1)

      println("spawn chunks " + chunks.size())
      println("spawn toIndexedSeq")
      import collection.JavaConverters._
      val chunksSeq = chunks.iterator.asScala.toIndexedSeq
      println(chunksSeq)

It hangs at the line val chunksSeq = .... On Scala-JVM it runs successfully