SI-8924 don't hold reference to list in iterator

Current implementation of LinearSeqLike.iterator holds
reference to complete sequence. This prevent garbage collecting
of List elements while we keep reference to iterator somewhere.

This commit removes reference from Iterator implementation. This
allow garbage collection of List while we iterating over its
elements (when there is no other references to List in our program).

1 files changed, 49 insertions, 0 deletions

diff --git a/test/junit/scala/collection/immutable/ListTest.scala b/test/junit/scala/collection/immutable/ListTest.scala
new file mode 100644
index 0000000000..1006801029
--- /dev/null
+++ b/test/junit/scala/collection/immutable/ListTest.scala
@@ -0,0 +1,49 @@
+package scala.collection.immutable
+
+import org.junit.{Assert, Test}
+import org.junit.runner.RunWith
+import org.junit.runners.JUnit4
+
+import scala.ref.WeakReference
+
+@RunWith(classOf[JUnit4])
+class ListTest {
+  /**
+   * Test that empty iterator does not hold reference
+   * to complete List
+   */
+  @Test
+  def testIteratorGC(): Unit = {
+    var num = 0
+    var emptyIterators = Seq.empty[(Iterator[Int], WeakReference[List[Int]])]
+
+    do {
+      val list = List.fill(10000)(num)
+      val ref = WeakReference(list)
+
+      val i = list.iterator
+
+      while (i.hasNext) i.next()
+
+      emptyIterators = (i, ref) +: emptyIterators
+
+      num+=1
+    } while (emptyIterators.forall(_._2.get.isDefined) && num<1000)
+
+    // check something is result to protect from JIT optimizations
+    for ((i, _) <- emptyIterators) {
+      Assert.assertTrue(i.isEmpty)
+    }
+
+    // await gc up to ~5 seconds
+    var forceLoops = 50
+    while (emptyIterators.forall(_._2.get.isDefined) && forceLoops>0) {
+      System.gc()
+      Thread.sleep(100)
+      forceLoops -= 1
+    }
+
+    // real assertion
+    Assert.assertTrue(emptyIterators.exists(_._2.get.isEmpty))
+  }
+}