ensure iterator.hasNext is not called unnecessarily as per #3006

Author: davidmoten

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

@@ -71,42 +71,52 @@ public void request(long n) {
             }
             if (n == Long.MAX_VALUE && REQUESTED_UPDATER.compareAndSet(this, 0, Long.MAX_VALUE)) {
                 // fast-path without backpressure
-                while (it.hasNext()) {
+
+                while (true) {
                     if (o.isUnsubscribed()) {
                         return;
+                    } else if (it.hasNext()) {
+                        o.onNext(it.next());
+                    } else if (!o.isUnsubscribed()) {
+                        o.onCompleted();
+                        return;
+                    } else {
+                        // is unsubscribed
+                        return;
                     }
-                    o.onNext(it.next());
-                }
-                if (!o.isUnsubscribed()) {
-                    o.onCompleted();
                 }
             } else if (n > 0) {
                 // backpressure is requested
                 long _c = BackpressureUtils.getAndAddRequest(REQUESTED_UPDATER, this, n);
                 if (_c == 0) {
                     while (true) {
                         /*
-                         * This complicated logic is done to avoid touching the volatile `requested` value
-                         * during the loop itself. If it is touched during the loop the performance is impacted significantly.
+                         * This complicated logic is done to avoid touching the
+                         * volatile `requested` value during the loop itself. If
+                         * it is touched during the loop the performance is
+                         * impacted significantly.
                          */
                         long r = requested;
                         long numToEmit = r;
-                        while (it.hasNext() && --numToEmit >= 0) {
+                        while (true) {
                             if (o.isUnsubscribed()) {
                                 return;
-                            }
-                            o.onNext(it.next());
-
-                        }
-
-                        if (!it.hasNext()) {
-                            if (!o.isUnsubscribed()) {
+                            } else if (it.hasNext()) {
+                                if (--numToEmit >= 0) {
+                                    o.onNext(it.next());
+                                } else
+                                    break;
+                            } else if (!o.isUnsubscribed()) {
                                 o.onCompleted();
+                                return;
+                            } else {
+                                // is unsubscribed
+                                return;
                             }
-                            return;
                         }
                         if (REQUESTED_UPDATER.addAndGet(this, -r) == 0) {
-                            // we're done emitting the number requested so return
+                            // we're done emitting the number requested so
+                            // return
                             return;
                         }
 

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

@@ -15,6 +15,7 @@
  */
 package rx.internal.operators;
 
+import static org.junit.Assert.assertFalse;
 import static org.junit.Assert.assertTrue;
 import static org.mockito.Matchers.any;
 import static org.mockito.Mockito.mock;
@@ -29,6 +30,7 @@
 import java.util.concurrent.TimeUnit;
 import java.util.concurrent.atomic.AtomicBoolean;
 
+import org.junit.Assert;
 import org.junit.Test;
 import org.mockito.Mockito;
 
@@ -221,4 +223,90 @@ public void onNext(Object t) {
         assertTrue(completed.get());
     }
 
+    @Test
+    public void testDoesNotCallIteratorHasNextMoreThanRequiredWithBackpressure() {
+        final AtomicBoolean called = new AtomicBoolean(false);
+        Iterable<Integer> iterable = new Iterable<Integer>() {
+
+            @Override
+            public Iterator<Integer> iterator() {
+                return new Iterator<Integer>() {
+
+                    int count = 1;
+
+                    @Override
+                    public boolean hasNext() {
+                        if (count > 1) {
+                            called.set(true);
+                            return false;
+                        } else
+                            return true;
+                    }
+
+                    @Override
+                    public Integer next() {
+                        return count++;
+                    }
+
+                };
+            }
+        };
+        Observable.from(iterable).take(1).subscribe();
+        assertFalse(called.get());
+    }
+
+    @Test
+    public void testDoesNotCallIteratorHasNextMoreThanRequiredFastPath() {
+        final AtomicBoolean called = new AtomicBoolean(false);
+        Iterable<Integer> iterable = new Iterable<Integer>() {
+
+            @Override
+            public Iterator<Integer> iterator() {
+                return new Iterator<Integer>() {
+
+                    @Override
+                    public void remove() {
+                        // ignore
+                    }
+
+                    int count = 1;
+
+                    @Override
+                    public boolean hasNext() {
+                        if (count > 1) {
+                            called.set(true);
+                            return false;
+                        } else
+                            return true;
+                    }
+
+                    @Override
+                    public Integer next() {
+                        return count++;
+                    }
+
+                };
+            }
+        };
+        Observable.from(iterable).subscribe(new Subscriber<Integer>() {
+
+            @Override
+            public void onCompleted() {
+
+            }
+
+            @Override
+            public void onError(Throwable e) {
+
+            }
+
+            @Override
+            public void onNext(Integer t) {
+                // unsubscribe on first emission
+                unsubscribe();
+            }
+        });
+        assertFalse(called.get());
+    }
+    
 }