More tests and fix empty() case as well

Author: akarnokd

src/main/java/rx/internal/operators/OnSubscribeConcatMap.java

@@ -220,7 +220,7 @@ void drain() {
 
             final int delayErrorMode = this.delayErrorMode;
 
-            do {
+            for (;;) {
                 if (actual.isUnsubscribed()) {
                     return;
                 }
@@ -288,11 +288,17 @@ void drain() {
                                     return;
                                 }
                             }
+                            request(1);
+                        } else {
+                            request(1);
+                            continue;
                         }
-                        request(1);
                     }
                 }
-            } while (wip.decrementAndGet() != 0);
+                if (wip.decrementAndGet() == 0) {
+                    break;
+                }
+            }
         }
 
         void drainError(Throwable mapperError) {

src/test/java/rx/internal/operators/OperatorConcatTest.java

@@ -864,4 +864,50 @@ public void scalarAndRangeBackpressured() {
         ts.assertCompleted();
         ts.assertNoErrors();
     }
+    
+    @Test
+    public void scalarAndEmptyBackpressured() {
+        TestSubscriber<Integer> ts = TestSubscriber.create(0);
+        
+        Observable.just(1).concatWith(Observable.<Integer>empty()).subscribe(ts);
+        
+        ts.assertNoValues();
+        
+        ts.requestMore(5);
+        
+        ts.assertValue(1);
+        ts.assertCompleted();
+        ts.assertNoErrors();
+    }
+
+    @Test
+    public void rangeAndEmptyBackpressured() {
+        TestSubscriber<Integer> ts = TestSubscriber.create(0);
+        
+        Observable.range(1, 2).concatWith(Observable.<Integer>empty()).subscribe(ts);
+        
+        ts.assertNoValues();
+        
+        ts.requestMore(5);
+        
+        ts.assertValues(1, 2);
+        ts.assertCompleted();
+        ts.assertNoErrors();
+    }
+
+    @Test
+    public void emptyAndScalarBackpressured() {
+        TestSubscriber<Integer> ts = TestSubscriber.create(0);
+        
+        Observable.<Integer>empty().concatWith(Observable.just(1)).subscribe(ts);
+        
+        ts.assertNoValues();
+        
+        ts.requestMore(5);
+        
+        ts.assertValue(1);
+        ts.assertCompleted();
+        ts.assertNoErrors();
+    }
+
 }