Allow weak references to permanent objects. (#3781)

backend.native/tests/runtime/memory/weak1.kt

@@ -10,7 +10,7 @@ import kotlin.native.ref.*
 
 class Node(var next: Node?)
 
-@Test fun runTest() {
+@Test fun runTest1() {
     val node1 = Node(null)
     val node2 = Node(node1)
     node1.next = node2
@@ -18,3 +18,15 @@ class Node(var next: Node?)
     kotlin.native.ref.WeakReference(node1)
     println("OK")
 }
+
+@Test fun runTest2() {
+    val string = "Hello"
+    val refString = WeakReference(string)
+    assertEquals(string, refString.value)
+    val zero = 0
+    val refZero = WeakReference(zero)
+    assertEquals(0, refZero.value)
+    val long = Long.MAX_VALUE
+    val refLong = WeakReference(long)
+    assertEquals(Long.MAX_VALUE, refLong.value)
+}

runtime/src/main/cpp/Weak.cpp

@@ -49,10 +49,15 @@ extern "C" {
 
 OBJ_GETTER(makeWeakReferenceCounter, void*);
 OBJ_GETTER(makeObjCWeakReferenceImpl, void*);
+OBJ_GETTER(makePermanentWeakReferenceImpl, ObjHeader*);
 
 // See Weak.kt for implementation details.
 // Retrieve link on the counter object.
 OBJ_GETTER(Konan_getWeakReferenceImpl, ObjHeader* referred) {
+  if (referred->container() == nullptr) {
+    RETURN_RESULT_OF(makePermanentWeakReferenceImpl, referred);
+  }
+
   MetaObjHeader* meta = referred->meta_object();
 
 #if KONAN_OBJC_INTEROP

runtime/src/main/kotlin/kotlin/native/ref/Weak.kt

@@ -37,5 +37,11 @@ public class WeakReference<T : Any> {
      */
     @Suppress("UNCHECKED_CAST")
     public fun get(): T? = pointer?.get() as T?
+
+    /**
+     * Returns either reference to an object or null, if it was collected.
+     */
+    public val value: T?
+        get() = this.get()
 }
 

runtime/src/main/kotlin/kotlin/native/ref/WeakPrivate.kt

@@ -56,3 +56,10 @@ external internal fun getWeakReferenceImpl(referent: Any): WeakReferenceImpl
 @ExportForCppRuntime
 internal fun makeWeakReferenceCounter(referred: COpaquePointer) = WeakReferenceCounter(referred)
 
+internal class PermanentWeakReferenceImpl(val referred: Any): kotlin.native.ref.WeakReferenceImpl() {
+    override fun get(): Any? = referred
+}
+
+// Create a reference to the permanent object.
+@ExportForCppRuntime
+internal fun makePermanentWeakReferenceImpl(referred: Any) = PermanentWeakReferenceImpl(referred)