Fix Stream.iterator memory leak

[htmldoug]

Stream.iterator kept a reference to the head of the stream which prevented any part of the Stream from getting garbage collected. I suspect this was the underlying cause of similar fixes like #8367. Scala 2.11 is unaffected.

This change prevents LinearSeqIterator from creating a private field and closing over the head of the Stream as a coll class field.

Before:

javap -p build/quick/classes/library/scala/collection/LinearSeqIterator.class
Compiled from "LinearSeq.scala"
public final class scala.collection.LinearSeqIterator<A> extends scala.collection.AbstractIterator<A> {
  private final scala.collection.LinearSeqOps<A, scala.collection.LinearSeq, scala.collection.LinearSeq<A>> coll;
  private scala.collection.LinearSeqIterator<A>.LazyCell these;
  public boolean hasNext();
  public A next();
...
}

After:

javap -p build/quick/classes/library/scala/collection/LinearSeqIterator.class
Compiled from "LinearSeq.scala"
public final class scala.collection.LinearSeqIterator<A> extends scala.collection.AbstractIterator<A> {
  private scala.collection.LinearSeqIterator<A>.LazyCell these;
  public boolean hasNext();
  public A next();
...
}

I didn't find any open issues on scala/bug for this.

[SethTisue]

review by @scala/collections ?

[johnynek]

did this test fail before the change?

Just making sure we have a test to catch any regression.

[htmldoug]

yes, fails without the change. OOME.

[lrytz]

👍 thank you!

[NthPortal]

late to the party, but why is this line repeated 3 times?

[htmldoug]

I wondered, too. git says two of them were added in https://github.com/scala/scala/pull/9324/files#diff-b0ab17467318c0ab594f748c21870010636257c36577157fcdd7f01836f4b272. The same PR also updated the number of deprecation warnings, so it seems somewhat intentional at least.

[SethTisue]

@dwijnand ?

[dwijnand]

That's funny - I have no recollection doing that. Was I trying to stress the GC? Did I do that before changing the java options and then committing? I'd assume some sanity and say it's related to using JDK 15 in some way... Maybe someone wants to try to retrace my steps. I also don't mind if someone wants to remove the extras.