Fix kotlinx.coroutines.main.delay being ignored (#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 #3044

Author: qwwdfsad

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)
+    }
 }