Fix 'request(0)' issue in Scan

Author: zsxwing

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

@@ -147,19 +147,17 @@ public void request(long n) {
                         if (once.compareAndSet(false, true)) {
                             if (initialValue == NO_INITIAL_VALUE || n == Long.MAX_VALUE) {
                                 producer.request(n);
+                            } else if (n == 1) {
+                                excessive.set(true);
+                                producer.request(1); // request at least 1
                             } else {
-                                if (n == Long.MAX_VALUE) {
-                                    producer.request(Long.MAX_VALUE);
-                                } else if (n == 1) {
-                                    excessive.set(true);
-                                    producer.request(1); // request at least 1
-                                } else {
-                                    producer.request(n - 1);
-                                }
+                                // n != Long.MAX_VALUE && n != 1
+                                producer.request(n - 1);
                             }
                         } else {
                             // pass-thru after first time
-                            if (excessive.compareAndSet(true, false) && n != Long.MAX_VALUE) {
+                            if (n > 1 // avoid to request 0
+                                    && excessive.compareAndSet(true, false) && n != Long.MAX_VALUE) {
                                 producer.request(n - 1);
                             } else {
                                 producer.request(n);

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

@@ -20,22 +20,22 @@
 import static org.mockito.Matchers.any;
 import static org.mockito.Matchers.anyInt;
 import static org.mockito.Matchers.anyString;
-import static org.mockito.Mockito.mock;
-import static org.mockito.Mockito.never;
-import static org.mockito.Mockito.times;
-import static org.mockito.Mockito.verify;
+import static org.mockito.Mockito.*;
 
 import java.util.ArrayList;
 import java.util.Arrays;
 import java.util.List;
+import java.util.concurrent.atomic.AtomicBoolean;
 import java.util.concurrent.atomic.AtomicInteger;
+import java.util.concurrent.atomic.AtomicReference;
 
 import org.junit.Before;
 import org.junit.Test;
 import org.mockito.MockitoAnnotations;
 
 import rx.Observable;
 import rx.Observer;
+import rx.Producer;
 import rx.Subscriber;
 import rx.functions.Action2;
 import rx.functions.Func0;
@@ -312,4 +312,52 @@ public Integer call(Integer t1, Integer t2) {
         subscriber.assertTerminalEvent();
         subscriber.assertNoErrors();
     }
+
+    @Test
+    public void testScanShouldNotRequestZero() {
+        final AtomicReference<Producer> producer = new AtomicReference<Producer>();
+        Observable<Integer> o = Observable.create(new Observable.OnSubscribe<Integer>() {
+            @Override
+            public void call(final Subscriber subscriber) {
+                Producer p = spy(new Producer() {
+
+                    private AtomicBoolean requested = new AtomicBoolean(false);
+
+                    @Override
+                    public void request(long n) {
+                        if (requested.compareAndSet(false, true)) {
+                            subscriber.onNext(1);
+                        } else {
+                            subscriber.onCompleted();
+                        }
+                    }
+                });
+                producer.set(p);
+                subscriber.setProducer(p);
+            }
+        }).scan(100, new Func2<Integer, Integer, Integer>() {
+
+            @Override
+            public Integer call(Integer t1, Integer t2) {
+                return t1 + t2;
+            }
+
+        });
+
+        o.subscribe(new TestSubscriber<Integer>() {
+
+            @Override
+            public void onStart() {
+                request(1);
+            }
+
+            @Override
+            public void onNext(Integer integer) {
+                request(1);
+            }
+        });
+
+        verify(producer.get(), never()).request(0);
+        verify(producer.get(), times(2)).request(1);
+    }
 }