fail fast in mapAsync for completed failed future, #21318 (#21322)

* fail fast in mapAsync for completed failed future, #21318

* the callback was not run until todo == parallelism or the sync
  event limit was reached, and that can be a problem if upstream
  stages are slow (noticed this for a blocking Kafka producer stage)

* skip callback for completed futures

* fix callback allocation

akka-stream-tests/src/test/scala/akka/stream/scaladsl/FlowMapAsyncSpec.scala

@@ -48,10 +48,12 @@ class FlowMapAsyncSpec extends StreamSpec {
     "produce future elements in order" in {
       val c = TestSubscriber.manualProbe[Int]()
       implicit val ec = system.dispatcher
-      val p = Source(1 to 50).mapAsync(4)(n ⇒ Future {
-        Thread.sleep(ThreadLocalRandom.current().nextInt(1, 10))
-        n
-      }).to(Sink.fromSubscriber(c)).run()
+      val p = Source(1 to 50).mapAsync(4)(n ⇒
+        if (n % 3 == 0) Future.successful(n)
+        else Future {
+          Thread.sleep(ThreadLocalRandom.current().nextInt(1, 10))
+          n
+        }).to(Sink.fromSubscriber(c)).run()
       val sub = c.expectSubscription()
       sub.request(1000)
       for (n ← 1 to 50) c.expectNext(n)
@@ -99,6 +101,27 @@ class FlowMapAsyncSpec extends StreamSpec {
       latch.countDown()
     }
 
+    "signal future failure asap" in assertAllStagesStopped {
+      val latch = TestLatch(1)
+      val done = Source(1 to 5)
+        .map { n ⇒
+          if (n == 1) n
+          else {
+            // slow upstream should not block the error
+            Await.ready(latch, 10.seconds)
+            n
+          }
+        }
+        .mapAsync(4) { n ⇒
+          if (n == 1) Future.failed(new RuntimeException("err1") with NoStackTrace)
+          else Future.successful(n)
+        }.runWith(Sink.ignore)
+      intercept[RuntimeException] {
+        Await.result(done, remainingOrDefault)
+      }.getMessage should be("err1")
+      latch.countDown()
+    }
+
     "signal error from mapAsync" in assertAllStagesStopped {
       val latch = TestLatch(1)
       val c = TestSubscriber.manualProbe[Int]()