Fixed sporadic test crashes related to withTimeout and children

- The bug only manifested when withTimeout coroutine had children working
  in a different dispatcher
- A separate stress-test that reproduces this problem with much higher
  probability

Author: elizarov

common/kotlinx-coroutines-core-common/src/JobSupport.kt

@@ -707,7 +707,7 @@ internal open class JobSupport constructor(active: Boolean) : Job, SelectClause0
                 if (state !is Finishing) onFinishingInternal(proposedUpdate)
                 if (child != null && tryWaitForChild(child, proposedUpdate))
                     return COMPLETING_WAITING_CHILDREN
-                if (tryFinalizeState(completing, proposedUpdate, mode = MODE_ATOMIC_DEFAULT))
+                if (tryFinalizeState(completing, proposedUpdate, mode))
                     return COMPLETING_COMPLETED
             }
         }

core/kotlinx-coroutines-core/test/WithTimeoutChildDipspatchStressTest.kt

@@ -0,0 +1,32 @@
+/*
+ * Copyright 2016-2018 JetBrains s.r.o. Use of this source code is governed by the Apache 2.0 license.
+ */
+
+package kotlinx.coroutines.experimental
+
+import org.junit.Test
+import kotlin.test.*
+
+class WithTimeoutChildDispatchStressTest : TestBase() {
+    private val N_REPEATS = 10_000 * stressTestMultiplier
+
+    /**
+     * This stress-test makes sure that dispatching resumption from within withTimeout
+     * works appropriately (without additional dispatch) despite the presence of
+     * children coroutine in a different dispatcher.
+     */
+    @Test
+    fun testChildDispatch() = runBlocking {
+        repeat(N_REPEATS) {
+            val result = withTimeout(5000) {
+                // child in different dispatcher
+                val job = launch(Dispatchers.Default) {
+                    // done nothing, but dispatches to join from another thread
+                }
+                job.join()
+                "DONE"
+            }
+            assertEquals("DONE", result)
+        }
+    }
+}