Fix problem with ensureNeverFrozen(). (#2292)

backend.native/tests/build.gradle

@@ -737,6 +737,12 @@ task freeze5(type: RunKonanTest) {
     source = "runtime/workers/freeze5.kt"
 }
 
+task freeze6(type: RunKonanTest) {
+    disabled = (project.testTarget == 'wasm32') // No exceptions on WASM.
+    goldValue = "OK\nOK\n"
+    source = "runtime/workers/freeze6.kt"
+}
+
 task atomic0(type: RunKonanTest) {
     disabled = (project.testTarget == 'wasm32') // Workers need pthreads.
     goldValue = "35\n" + "20\n" + "OK\n"

backend.native/tests/runtime/workers/freeze6.kt

@@ -0,0 +1,34 @@
+/*
+ * Copyright 2010-2018 JetBrains s.r.o. Use of this source code is governed by the Apache 2.0 license
+ * that can be found in the LICENSE file.
+ */
+
+package runtime.workers.freeze6
+
+import kotlin.test.*
+import kotlin.native.concurrent.*
+
+@Test
+fun ensureNeverFrozenNoFreezeChild(){
+    val noFreeze = Hi("qwert")
+    noFreeze.ensureNeverFrozen()
+
+    val nested = Nested(noFreeze)
+    assertFails { nested.freeze() }
+
+    println("OK")
+}
+
+@Test
+fun ensureNeverFrozenFailsTarget(){
+    val noFreeze = Hi("qwert")
+    noFreeze.ensureNeverFrozen()
+
+    assertFalse(noFreeze.isFrozen)
+    assertFails { noFreeze.freeze() }
+    assertFalse(noFreeze.isFrozen)
+    println("OK")
+}
+
+data class Hi(val s:String)
+data class Nested(val hi:Hi)

runtime/src/main/cpp/Memory.cpp

@@ -1725,7 +1725,8 @@ void FreezeSubgraph(ObjHeader* root) {
 
   // Do DFS cycle detection.
   bool hasCycles = false;
-  KRef firstBlocker = nullptr;
+  KRef firstBlocker = root->has_meta_object() && ((root->meta_object()->flags_ & MF_NEVER_FROZEN) != 0) ?
+    root : nullptr;
   KStdVector<ContainerHeader*> order;
   depthFirstTraversal(rootContainer, &hasCycles, &firstBlocker, order);
   if (firstBlocker != nullptr) {