Fix await/asDeferred for MinimalState implementations (#2457)

Fixes #2456

Author: elizarov

integration/kotlinx-coroutines-jdk8/src/future/Future.kt

@@ -105,16 +105,19 @@ private fun Job.setupCancellation(future: CompletableFuture<*>) {
 }
 
 /**
- * Converts this completion stage to an instance of [Deferred].
- * When this completion stage is an instance of [Future], then it is cancelled when
- * the resulting deferred is cancelled.
+ * Converts this [CompletionStage] to an instance of [Deferred].
+ *
+ * The [CompletableFuture] that corresponds to this [CompletionStage] (see [CompletionStage.toCompletableFuture])
+ * is cancelled when the resulting deferred is cancelled.
  */
+@Suppress("DeferredIsResult")
 public fun <T> CompletionStage<T>.asDeferred(): Deferred<T> {
+    val future = toCompletableFuture() // retrieve the future
     // Fast path if already completed
-    if (this is Future<*> && isDone()){
+    if (future.isDone) {
         return try {
             @Suppress("UNCHECKED_CAST")
-            CompletableDeferred(get() as T)
+            CompletableDeferred(future.get() as T)
         } catch (e: Throwable) {
             // unwrap original cause from ExecutionException
             val original = (e as? ExecutionException)?.cause ?: e
@@ -132,25 +135,28 @@ public fun <T> CompletionStage<T>.asDeferred(): Deferred<T> {
             result.completeExceptionally((exception as? CompletionException)?.cause ?: exception)
         }
     }
-    if (this is Future<*>) result.cancelFutureOnCompletion(this)
+    result.cancelFutureOnCompletion(future)
     return result
 }
 
 /**
- * Awaits for completion of the completion stage without blocking a thread.
+ * Awaits for completion of [CompletionStage] without blocking a thread.
  *
  * This suspending function is cancellable.
  * If the [Job] of the current coroutine is cancelled or completed while this suspending function is waiting, this function
  * stops waiting for the completion stage and immediately resumes with [CancellationException][kotlinx.coroutines.CancellationException].
- * This method is intended to be used with one-shot futures, so on coroutine cancellation completion stage is cancelled as well if it is instance of [CompletableFuture].
- * If cancelling given stage is undesired, `stage.asDeferred().await()` should be used instead.
+ *
+ * This method is intended to be used with one-shot futures, so on coroutine cancellation the [CompletableFuture] that
+ * corresponds to this [CompletionStage] (see [CompletionStage.toCompletableFuture])
+ * is cancelled. If cancelling the given stage is undesired, `stage.asDeferred().await()` should be used instead.
  */
 public suspend fun <T> CompletionStage<T>.await(): T {
+    val future = toCompletableFuture() // retrieve the future
     // fast path when CompletableFuture is already done (does not suspend)
-    if (this is Future<*> && isDone()) {
+    if (future.isDone) {
         try {
-            @Suppress("UNCHECKED_CAST")
-            return get() as T
+            @Suppress("UNCHECKED_CAST", "BlockingMethodInNonBlockingContext")
+            return future.get() as T
         } catch (e: ExecutionException) {
             throw e.cause ?: e // unwrap original cause from ExecutionException
         }
@@ -160,8 +166,7 @@ public suspend fun <T> CompletionStage<T>.await(): T {
         val consumer = ContinuationConsumer(cont)
         whenComplete(consumer)
         cont.invokeOnCancellation {
-            // mayInterruptIfRunning is not used
-            (this as? CompletableFuture<T>)?.cancel(false)
+            future.cancel(false)
             consumer.cont = null // shall clear reference to continuation to aid GC
         }
     }

integration/kotlinx-coroutines-jdk8/test/future/FutureTest.kt

@@ -490,4 +490,81 @@ class FutureTest : TestBase() {
             }
         }
     }
+
+    /**
+     * https://github.com/Kotlin/kotlinx.coroutines/issues/2456
+     */
+    @Test
+    fun testCompletedStageAwait() = runTest {
+        val stage = CompletableFuture.completedStage("OK")
+        assertEquals("OK", stage.await())
+    }
+
+    /**
+     * https://github.com/Kotlin/kotlinx.coroutines/issues/2456
+     */
+    @Test
+    fun testCompletedStageAsDeferredAwait() = runTest {
+        val stage = CompletableFuture.completedStage("OK")
+        val deferred = stage.asDeferred()
+        assertEquals("OK", deferred.await())
+    }
+
+    @Test
+    fun testCompletedStateThenApplyAwait() = runTest {
+        expect(1)
+        val cf = CompletableFuture<String>()
+        launch {
+            expect(3)
+            cf.complete("O")
+        }
+        expect(2)
+        val stage = cf.thenApply { it + "K" }
+        assertEquals("OK", stage.await())
+        finish(4)
+    }
+
+    @Test
+    fun testCompletedStateThenApplyAwaitCancel() = runTest {
+        expect(1)
+        val cf = CompletableFuture<String>()
+        launch {
+            expect(3)
+            cf.cancel(false)
+        }
+        expect(2)
+        val stage = cf.thenApply { it + "K" }
+        assertFailsWith<CancellationException> { stage.await() }
+        finish(4)
+    }
+
+    @Test
+    fun testCompletedStateThenApplyAsDeferredAwait() = runTest {
+        expect(1)
+        val cf = CompletableFuture<String>()
+        launch {
+            expect(3)
+            cf.complete("O")
+        }
+        expect(2)
+        val stage = cf.thenApply { it + "K" }
+        val deferred = stage.asDeferred()
+        assertEquals("OK", deferred.await())
+        finish(4)
+    }
+
+    @Test
+    fun testCompletedStateThenApplyAsDeferredAwaitCancel() = runTest {
+        expect(1)
+        val cf = CompletableFuture<String>()
+        expect(2)
+        val stage = cf.thenApply { it + "K" }
+        val deferred = stage.asDeferred()
+        launch {
+            expect(3)
+            deferred.cancel() // cancel the deferred!
+        }
+        assertFailsWith<CancellationException> { stage.await() }
+        finish(4)
+    }
 }