Document potentially cyclic cases in ownership transfer and add test. (#3207)

backend.native/tests/runtime/workers/worker9.kt

@@ -58,5 +58,27 @@ fun withLock(op: () -> Unit) {
         println("frozen OK")
     }.freeze())
 
+    worker.requestTermination().result
+}
+
+class Node(var node: Node?, var outher: Node?)
+
+fun makeCyclic(): Node {
+    val inner = Node(null, null)
+    inner.node = inner
+    val outer = Node(null, null)
+    inner.outher = outer
+    return outer
+}
+
+@Test fun runTest4() {
+    val worker = Worker.start()
+
+    val future = worker.execute(TransferMode.SAFE, { }) {
+        makeCyclic().also {
+            kotlin.native.internal.GC.collect()
+        }
+    }
+    assert(future.result != null)
     worker.requestTermination().result
 }

runtime/src/main/kotlin/kotlin/native/concurrent/Worker.kt

@@ -76,6 +76,9 @@ public inline class Worker @PublishedApi internal constructor(val id: Int) {
      * explicitly stored in object produced by [producer]. Scheduled job is being executed by the worker,
      * and result of such a execution is being disconnected from worker's object graph. Whoever will consume
      * the future, can use result of worker's computations.
+     * Note, that some technically disjoint subgraphs may lead to `kotlin.IllegalStateException`
+     * so `kotlin.native.internal.GC.collect()` could be called in the end of `producer` and `job`
+     * if garbage cyclic structures or other uncollected objects refer to the value being transferred.
      *
      * @return the future with the computation result of [job]
      */