2.x: Fix Flowable.concatMap backpressure w/ scalars (#7091)

* 2.x: Fix Flowable.concatMap backpressure w/ scalars

* Replace Java 8 constructs

Author: akarnokd

src/main/java/io/reactivex/internal/operators/flowable/FlowableConcatMap.java

@@ -13,7 +13,7 @@
 package io.reactivex.internal.operators.flowable;
 
 import java.util.concurrent.Callable;
-import java.util.concurrent.atomic.AtomicInteger;
+import java.util.concurrent.atomic.*;
 
 import org.reactivestreams.*;
 
@@ -332,7 +332,7 @@ void drain() {
                                     continue;
                                 } else {
                                     active = true;
-                                    inner.setSubscription(new WeakScalarSubscription<R>(vr, inner));
+                                    inner.setSubscription(new SimpleScalarSubscription<R>(vr, inner));
                                 }
 
                             } else {
@@ -349,20 +349,20 @@ void drain() {
         }
     }
 
-    static final class WeakScalarSubscription<T> implements Subscription {
+    static final class SimpleScalarSubscription<T>
+    extends AtomicBoolean
+    implements Subscription {
         final Subscriber<? super T> downstream;
         final T value;
-        boolean once;
 
-        WeakScalarSubscription(T value, Subscriber<? super T> downstream) {
+        SimpleScalarSubscription(T value, Subscriber<? super T> downstream) {
             this.value = value;
             this.downstream = downstream;
         }
 
         @Override
         public void request(long n) {
-            if (n > 0 && !once) {
-                once = true;
+            if (n > 0 && compareAndSet(false, true)) {
                 Subscriber<? super T> a = downstream;
                 a.onNext(value);
                 a.onComplete();
@@ -538,7 +538,7 @@ void drain() {
                                     continue;
                                 } else {
                                     active = true;
-                                    inner.setSubscription(new WeakScalarSubscription<R>(vr, inner));
+                                    inner.setSubscription(new SimpleScalarSubscription<R>(vr, inner));
                                 }
                             } else {
                                 active = true;

src/test/java/io/reactivex/internal/operators/flowable/FlowableConcatMapTest.java

@@ -24,7 +24,9 @@
 import io.reactivex.*;
 import io.reactivex.exceptions.*;
 import io.reactivex.functions.*;
-import io.reactivex.internal.operators.flowable.FlowableConcatMap.WeakScalarSubscription;
+import io.reactivex.internal.functions.Functions;
+import io.reactivex.internal.operators.flowable.FlowableConcatMap.SimpleScalarSubscription;
+import io.reactivex.processors.PublishProcessor;
 import io.reactivex.schedulers.Schedulers;
 import io.reactivex.subscribers.TestSubscriber;
 
@@ -33,7 +35,7 @@ public class FlowableConcatMapTest {
     @Test
     public void weakSubscriptionRequest() {
         TestSubscriber<Integer> ts = new TestSubscriber<Integer>(0);
-        WeakScalarSubscription<Integer> ws = new WeakScalarSubscription<Integer>(1, ts);
+        SimpleScalarSubscription<Integer> ws = new SimpleScalarSubscription<Integer>(1, ts);
         ts.onSubscribe(ws);
 
         ws.request(0);
@@ -105,6 +107,68 @@ public Publisher<? extends Object> apply(String v)
         .assertResult("RxSingleScheduler");
     }
 
+    @Test
+    public void innerScalarRequestRace() {
+        final Flowable<Integer> just = Flowable.just(1);
+        final int n = 1000;
+        for (int i = 0; i < TestHelper.RACE_DEFAULT_LOOPS; i++) {
+            final PublishProcessor<Flowable<Integer>> source = PublishProcessor.create();
+
+            final TestSubscriber<Integer> ts = source
+                    .concatMap(Functions.<Flowable<Integer>>identity(), n + 1)
+                    .test(1L);
+
+            TestHelper.race(new Runnable() {
+                @Override
+                public void run() {
+                    for (int j = 0; j < n; j++) {
+                        source.onNext(just);
+                    }
+                }
+            }, new Runnable() {
+                @Override
+                public void run() {
+                    for (int j = 0; j < n; j++) {
+                        ts.request(1);
+                    }
+                }
+            });
+
+            ts.assertValueCount(n);
+        }
+    }
+
+    @Test
+    public void innerScalarRequestRaceDelayError() {
+        final Flowable<Integer> just = Flowable.just(1);
+        final int n = 1000;
+        for (int i = 0; i < TestHelper.RACE_DEFAULT_LOOPS; i++) {
+            final PublishProcessor<Flowable<Integer>> source = PublishProcessor.create();
+
+            final TestSubscriber<Integer> ts = source
+                    .concatMapDelayError(Functions.<Flowable<Integer>>identity(), n + 1, true)
+                    .test(1L);
+
+            TestHelper.race(new Runnable() {
+                @Override
+                public void run() {
+                    for (int j = 0; j < n; j++) {
+                        source.onNext(just);
+                    }
+                }
+            }, new Runnable() {
+                @Override
+                public void run() {
+                    for (int j = 0; j < n; j++) {
+                        ts.request(1);
+                    }
+                }
+            });
+
+            ts.assertValueCount(n);
+        }
+    }
+
     @Test
     public void pollThrows() {
         Flowable.just(1)