Fix a race condition

Author: dkhalanskyjb

kotlinx-coroutines-test/common/src/TestCoroutineScheduler.kt

@@ -64,12 +64,14 @@ public class TestCoroutineScheduler: AbstractCoroutineContextElement(TestCorouti
         isCancelled : (T) -> Boolean
     ): DisposableHandle {
         require(timeDeltaMillis >= 0) { "Attempted scheduling an event earlier in time (with the time delta $timeDeltaMillis)" }
-        sendDispatchEvent()
         val count = count.getAndIncrement()
         return synchronized(lock) {
             val time = addClamping(currentTime, timeDeltaMillis)
             val event = TestDispatchEvent(dispatcher, count, time, marker as Any) { isCancelled(marker) }
             events.addLast(event)
+            /** can't be moved above: otherwise, [onDispatchEvent] could consume the token sent here before there's
+             * actually anything in the event queue. */
+            sendDispatchEvent()
             DisposableHandle {
                 synchronized(lock) {
                     events.remove(event)

kotlinx-coroutines-test/jvm/test/MultithreadingTest.kt

@@ -4,12 +4,11 @@
 
 import kotlinx.coroutines.*
 import kotlinx.coroutines.test.*
-import java.util.concurrent.*
 import kotlin.concurrent.*
 import kotlin.coroutines.*
 import kotlin.test.*
 
-class MultithreadingTest : TestBase() {
+class MultithreadingTest {
 
     @Test
     fun incorrectlyCalledRunBlocking_doesNotHaveSameInterceptor() = runBlockingTest {
@@ -24,7 +23,7 @@ class MultithreadingTest : TestBase() {
     }
 
     @Test
-    fun testSingleThreadExecutor() = runTest {
+    fun testSingleThreadExecutor() = runBlocking {
         val mainThread = Thread.currentThread()
         Dispatchers.setMain(Dispatchers.Unconfined)
         newSingleThreadContext("testSingleThread").use { threadPool ->
@@ -98,4 +97,4 @@ class MultithreadingTest : TestBase() {
             }
         }
     }
-}
+}