WeakReference does not work if subclassed

[armanbilge]

It seems that if WeakReference is sub-classed then it is not correctly handled by the GC. Probably due to the logic here?

scala-native/nativelib/src/main/resources/scala-native/gc/immix_commix/headers/ObjectHeader.h

Lines 88 to 90 in e5ee79a

	static inline bool Object_IsWeakReference(Object *object) {
	return object->rtti->rt.id == __weak_ref_id;
	}

Consider the following program, which allocates large arrays and stores them in weak references. It terminates when it detects the that a weak reference has been GCed.

import java.lang.ref.WeakReference

class MyWeakRef[A](a: A) extends WeakReference(a)

@main def main =
  val refs = new Array[MyWeakRef[Array[Long]]](Int.MaxValue / 8)

  var i = 0
  while (i < refs.length) {
    refs(i) = new MyWeakRef(new Array[Long](Int.MaxValue / 64))
    var j = 0
    while (j < i) {
      if (refs(j).get == null) {
        println((i, j))
        System.exit(0)
      }
      j += 1
    }
    i += 1
  }

As written, that program never prints or terminates for me. This suggests that none of the arrays are being GCed.

However, if I replace MyWeakRef with WeakReference then it terminates quickly with (3,0). If I run the program on the JVM, it also terminates quickly.

Motivation

In Cats Effect we sub-class WeakReference for a custom data structure. We use this data structure to power a "fiber dump" feature (similar to thread dump).

https://github.com/typelevel/cats-effect/blob/e9aeb8ccb53016a37588256f4c8086f8ef1612bc/core/shared/src/main/scala/cats/effect/unsafe/WeakBag.scala#L108-L110

[WojciechMazur]

We should replace current direct comparison of class id with a port of Class.is(Class[_], Class[_]) https://github.com/scala-native/scala-native/blob/main/nativelib/src/main/scala/java/lang/Class.scala#L74-L96

For the WeakReference detection we can use simplified algorithm, because WeakReference is a class, and trait would never be instantiated, we might get:

  val WeakReferenceId = ???
  private def isWeakReference(cls: _Class[_]): Boolean =
    cls.id >= WeakReferenceId && cls.idRangeUnti <= WeakReferenceId

This should be translated to C using ObjectHeader.h datastructres https://github.com/scala-native/scala-native/blob/main/nativelib/src/main/resources/scala-native/gc/immix_commix/headers/ObjectHeader.h#L28-L42

[mox692]

I will work on this.
To me, it looks good issue to delve into scala-native internals and gc stuff.