Fix kotlinx.coroutines.main.delay being ignored (Kotlin#3046)

Fix reading an uninitialized value of defaultMainDelayOptIn due to
the initialization cycle and add a test that ensures this behaviour in the future

Fixes Kotlin#3044

kotlinx-coroutines-core/jvm/src/DefaultExecutor.kt

@@ -8,10 +8,10 @@ import kotlinx.coroutines.internal.*
 import java.util.concurrent.*
 import kotlin.coroutines.*
 
-internal actual val DefaultDelay: Delay = initializeDefaultDelay()
-
 private val defaultMainDelayOptIn = systemProp("kotlinx.coroutines.main.delay", true)
 
+internal actual val DefaultDelay: Delay = initializeDefaultDelay()
+
 private fun initializeDefaultDelay(): Delay {
  // Opt-out flag
  if (!defaultMainDelayOptIn) return DefaultExecutor

kotlinx-coroutines-core/jvm/src/internal/MainDispatchers.kt

@@ -61,7 +61,9 @@ public fun MainDispatcherFactory.tryCreateDispatcher(factories: List<MainDispatc
 
 /** @suppress */
 @InternalCoroutinesApi
-public fun MainCoroutineDispatcher.isMissing(): Boolean = this is MissingMainCoroutineDispatcher
+public fun MainCoroutineDispatcher.isMissing(): Boolean =
+ // not checking `this`, as it may be wrapped in a `TestMainDispatcher`, whereas `immediate` never is.
+ this.immediate is MissingMainCoroutineDispatcher
 
 // R8 optimization hook, not const on purpose to enable R8 optimizations via "assumenosideeffects"
 @Suppress("MayBeConstant")

ui/kotlinx-coroutines-android/test/HandlerDispatcherTest.kt

@@ -161,4 +161,22 @@ class HandlerDispatcherTest : TestBase() {
  assertEquals("Dispatchers.Main", Dispatchers.Main.toString())
  assertEquals("Dispatchers.Main.immediate", Dispatchers.Main.immediate.toString())
  }
+
+ @Test
+ fun testDelayIsDelegatedToMain() = runTest {
+ val mainLooper = shadowOf(Looper.getMainLooper())
+ mainLooper.pause()
+ val mainMessageQueue = shadowOf(Looper.getMainLooper().queue)
+ assertNull(mainMessageQueue.head)
+ val job = launch(Dispatchers.Default, start = CoroutineStart.UNDISPATCHED) {
+ expect(1)
+ delay(10_000_000)
+ expect(3)
+ }
+ expect(2)
+ assertNotNull(mainMessageQueue.head)
+ mainLooper.runOneTask()
+ job.join()
+ finish(4)
+ }
 }