2.x: Fix Observable.concatMapSingle dropping upstream items (#5972)

Author: akarnokd

src/main/java/io/reactivex/internal/operators/mixed/ObservableConcatMapSingle.java

@@ -18,12 +18,12 @@
 import io.reactivex.*;
 import io.reactivex.annotations.Experimental;
 import io.reactivex.disposables.Disposable;
-import io.reactivex.exceptions.*;
+import io.reactivex.exceptions.Exceptions;
 import io.reactivex.functions.Function;
 import io.reactivex.internal.disposables.DisposableHelper;
 import io.reactivex.internal.functions.ObjectHelper;
 import io.reactivex.internal.fuseable.SimplePlainQueue;
-import io.reactivex.internal.queue.SpscArrayQueue;
+import io.reactivex.internal.queue.SpscLinkedArrayQueue;
 import io.reactivex.internal.util.*;
 import io.reactivex.plugins.RxJavaPlugins;
 
@@ -107,7 +107,7 @@ static final class ConcatMapSingleMainObserver<T, R>
             this.errorMode = errorMode;
             this.errors = new AtomicThrowable();
             this.inner = new ConcatMapSingleObserver<R>(this);
-            this.queue = new SpscArrayQueue<T>(prefetch);
+            this.queue = new SpscLinkedArrayQueue<T>(prefetch);
         }
 
         @Override

src/test/java/io/reactivex/internal/operators/mixed/ObservableConcatMapMaybeTest.java

@@ -399,4 +399,21 @@ public void cancelNoConcurrentClean() {
 
         assertTrue(operator.queue.isEmpty());
     }
+
+    @Test
+    public void checkUnboundedInnerQueue() {
+        MaybeSubject<Integer> ms = MaybeSubject.create();
+        
+        @SuppressWarnings("unchecked")
+        TestObserver<Integer> to = Observable
+                .fromArray(ms, Maybe.just(2), Maybe.just(3), Maybe.just(4))
+                .concatMapMaybe(Functions.<Maybe<Integer>>identity(), 2)
+                .test();
+
+        to.assertEmpty();
+
+        ms.onSuccess(1);
+
+        to.assertResult(1, 2, 3, 4);
+    }
 }

src/test/java/io/reactivex/internal/operators/mixed/ObservableConcatMapSingleTest.java

@@ -336,4 +336,21 @@ public void cancelNoConcurrentClean() {
 
         assertTrue(operator.queue.isEmpty());
     }
+
+    @Test
+    public void checkUnboundedInnerQueue() {
+        SingleSubject<Integer> ss = SingleSubject.create();
+        
+        @SuppressWarnings("unchecked")
+        TestObserver<Integer> to = Observable
+                .fromArray(ss, Single.just(2), Single.just(3), Single.just(4))
+                .concatMapSingle(Functions.<Single<Integer>>identity(), 2)
+                .test();
+
+        to.assertEmpty();
+
+        ss.onSuccess(1);
+
+        to.assertResult(1, 2, 3, 4);
+    }
 }