Fix bug preventing return from worker value passed in it. (#2135)

backend.native/tests/build.gradle

@@ -663,6 +663,12 @@ task worker10(type: RunKonanTest) {
     source = "runtime/workers/worker10.kt"
 }
 
+task worker11(type: RunKonanTest) {
+    disabled = (project.testTarget == 'wasm32') // Workers need pthreads.
+    goldValue = "OK\n"
+    source = "runtime/workers/worker11.kt"
+}
+
 task freeze0(type: RunKonanTest) {
     disabled = (project.testTarget == 'wasm32') // No workers on WASM.
     goldValue = "frozen bit is true\n" +

backend.native/tests/runtime/workers/worker11.kt

@@ -0,0 +1,48 @@
+/*
+ * 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.worker11
+
+import kotlin.test.*
+
+import kotlin.native.concurrent.*
+import kotlinx.cinterop.convert
+
+data class Job(val index: Int, var input: Int, var counter: Int)
+
+fun initJobs(count: Int) = Array<Job?>(count) { i -> Job(i, i * 2, i)}
+
+@Test fun runTest() {
+    val COUNT = 100
+    val workers = Array(COUNT, { _ -> Worker.start() })
+    val jobs = initJobs(COUNT)
+    val futures = Array(workers.size, { workerIndex ->
+        workers[workerIndex].execute(TransferMode.SAFE, {
+            val job = jobs[workerIndex]
+            jobs[workerIndex] = null
+            job!!
+        }) { job ->
+            job.counter += job.input
+            job
+        }
+    })
+    val futureSet = futures.toSet()
+    var consumed = 0
+    while (consumed < futureSet.size) {
+        val ready = futureSet.waitForMultipleFutures(10000)
+        ready.forEach {
+            it.consume { job ->
+                assertEquals(job.index * 3, job.counter)
+                jobs[job.index] = job
+            }
+            consumed++
+        }
+    }
+    assertEquals(consumed, COUNT)
+    workers.forEach {
+        it.requestTermination().result
+    }
+    println("OK")
+}

runtime/src/main/cpp/Memory.h

@@ -496,6 +496,7 @@ class ObjHolder {
    ObjHeader* obj() { return obj_; }
    const ObjHeader* obj() const { return obj_; }
    ObjHeader** slot() { return &obj_; }
+   void clear() { ::UpdateRef(&obj_, nullptr); }
 
   private:
    ObjHeader* obj_;

runtime/src/main/cpp/Worker.cpp

@@ -379,6 +379,7 @@ void* workerRoutine(void* argument) {
     KNativePtr result = nullptr;
     try {
         job.function(argument, &resultRef);
+        argumentHolder.clear();
         // Transfer the result.
         result = transfer(resultRef, job.transferMode);
     } catch (ObjHolder& e) {