Add optional bound to onBackpressureBuffer

src/main/java/rx/Observable.java

@@ -5034,6 +5034,44 @@ public final Observable<T> onBackpressureBuffer() {
         return lift(new OperatorOnBackpressureBuffer<T>());
     }
 
+    /**
+     * Instructs an Observable that is emitting items faster than its observer can consume them to buffer
+     * up to a given amount of items until they can be emitted. The resulting Observable will {@code onError} emitting a
+     * {@link java.nio.BufferOverflowException} as soon as the buffer's capacity is exceeded, dropping all
+     * undelivered items, and unsubscribing from the source.
+     * <p>
+     * <img width="640" height="300" src="https://raw.github.com/wiki/ReactiveX/RxJava/images/rx-operators/bp.obp.buffer.png" alt="">
+     * <dl>
+     *  <dt><b>Scheduler:</b></dt>
+     *  <dd>{@code onBackpressureBuffer} does not operate by default on a particular {@link Scheduler}.</dd>
+     * </dl>
+     *
+     * @return the source Observable modified to buffer items up to the given capacity.
+     * @see <a href="https://github.com/ReactiveX/RxJava/wiki/Backpressure">RxJava wiki: Backpressure</a>
+     */
+    public final Observable<T> onBackpressureBuffer(long capacity) {
+        return lift(new OperatorOnBackpressureBuffer<T>(capacity));
+    }
+
+    /**
+     * Instructs an Observable that is emitting items faster than its observer can consume them to buffer
+     * up to a given amount of items until they can be emitted. The resulting Observable will {@code onError} emitting a
+     * {@link java.nio.BufferOverflowException} as soon as the buffer's capacity is exceeded, dropping all
+     * undelivered items, unsubscribing from the source, and notifying the producer with {@code onOverflow}.
+     * <p>
+     * <img width="640" height="300" src="https://raw.github.com/wiki/ReactiveX/RxJava/images/rx-operators/bp.obp.buffer.png" alt="">
+     * <dl>
+     *  <dt><b>Scheduler:</b></dt>
+     *  <dd>{@code onBackpressureBuffer} does not operate by default on a particular {@link Scheduler}.</dd>
+     * </dl>
+     *
+     * @return the source Observable modified to buffer items up to the given capacity.
+     * @see <a href="https://github.com/ReactiveX/RxJava/wiki/Backpressure">RxJava wiki: Backpressure</a>
+     */
+    public final Observable<T> onBackpressureBuffer(long capacity, Func0<Void> onOverflow) {
+        return lift(new OperatorOnBackpressureBuffer<T>(capacity, onOverflow));
+    }
+
     /**
      * Instructs an Observable that is emitting items faster than its observer can consume them to discard,
      * rather than emit, those items that its observer is not prepared to observe.

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

@@ -15,23 +15,46 @@
  */
 package rx.internal.operators;
 
+import java.nio.BufferOverflowException;
 import java.util.Queue;
 import java.util.concurrent.ConcurrentLinkedQueue;
+import java.util.concurrent.atomic.AtomicBoolean;
 import java.util.concurrent.atomic.AtomicLong;
 
 import rx.Observable.Operator;
 import rx.Producer;
 import rx.Subscriber;
+import rx.functions.Func0;
 
 public class OperatorOnBackpressureBuffer<T> implements Operator<T, T> {
 
     private final NotificationLite<T> on = NotificationLite.instance();
 
+    private final Long capacity;
+    private final Func0 onOverflow;
+
+    public OperatorOnBackpressureBuffer() {
+        this.capacity = null;
+        this.onOverflow = null;
+    }
+
+    public OperatorOnBackpressureBuffer(long capacity) {
+        this(capacity, null);
+    }
+
+    public OperatorOnBackpressureBuffer(long capacity, Func0<Void> onOverflow) {
+        if (capacity <= 0) {
+            throw new IllegalArgumentException("Buffer capacity must be > 0");
+        }
+        this.capacity = capacity;
+        this.onOverflow = onOverflow;
+    }
+
     @Override
     public Subscriber<? super T> call(final Subscriber<? super T> child) {
         // TODO get a different queue implementation
-        // TODO start with size hint
         final ConcurrentLinkedQueue<Object> queue = new ConcurrentLinkedQueue<Object>();
+        final AtomicLong capacity = (this.capacity == null) ? null : new AtomicLong(this.capacity);
         final AtomicLong wip = new AtomicLong();
         final AtomicLong requested = new AtomicLong();
 
@@ -40,14 +63,17 @@ public Subscriber<? super T> call(final Subscriber<? super T> child) {
             @Override
             public void request(long n) {
                 if (requested.getAndAdd(n) == 0) {
-                    pollQueue(wip, requested, queue, child);
+                    pollQueue(wip, requested, capacity, queue, child);
                 }
             }
 
         });
         // don't pass through subscriber as we are async and doing queue draining
         // a parent being unsubscribed should not affect the children
         Subscriber<T> parent = new Subscriber<T>() {
+
+            private AtomicBoolean saturated = new AtomicBoolean(false);
+
             @Override
             public void onStart() {
                 request(Long.MAX_VALUE);
@@ -56,21 +82,47 @@ public void onStart() {
             @Override
             public void onCompleted() {
                 queue.offer(on.completed());
-                pollQueue(wip, requested, queue, child);
+                pollQueue(wip, requested, capacity, queue, child);
             }
 
             @Override
             public void onError(Throwable e) {
                 queue.offer(on.error(e));
-                pollQueue(wip, requested, queue, child);
+                pollQueue(wip, requested, capacity, queue, child);
             }
 
             @Override
             public void onNext(T t) {
+                if (!ensureCapacity()) {
+                    return;
+                }
                 queue.offer(on.next(t));
-                pollQueue(wip, requested, queue, child);
+                pollQueue(wip, requested, capacity, queue, child);
             }
 
+            private boolean ensureCapacity() {
+                if (capacity == null) {
+                    return true;
+                }
+
+                long currCapacity;
+                do {
+                    currCapacity = capacity.get();
+                    if (currCapacity <= 0) {
+                        if (saturated.compareAndSet(false, true)) {
+                            // ensure single completion contract
+                            child.onError(new BufferOverflowException());
+                            unsubscribe();
+                            if (onOverflow != null) {
+                                onOverflow.call();
+                            }
+                        }
+                        return false;
+                    }
+                // ensure no other thread stole our slot, or retry
+                } while (!capacity.compareAndSet(currCapacity, currCapacity - 1));
+                return true;
+            }
         };
 
         // if child unsubscribes it should unsubscribe the parent, but not the other way around
@@ -79,7 +131,7 @@ public void onNext(T t) {
         return parent;
     }
 
-    private void pollQueue(AtomicLong wip, AtomicLong requested, Queue<Object> queue, Subscriber<? super T> child) {
+    private void pollQueue(AtomicLong wip, AtomicLong requested, AtomicLong capacity, Queue<Object> queue, Subscriber<? super T> child) {
         // TODO can we do this without putting everything in the queue first so we can fast-path the case when we don't need to queue?
         if (requested.get() > 0) {
             // only one draining at a time
@@ -96,6 +148,9 @@ private void pollQueue(AtomicLong wip, AtomicLong requested, Queue<Object> queue
                                 requested.incrementAndGet();
                                 return;
                             }
+                            if (capacity != null) { // it's bounded
+                                capacity.incrementAndGet();
+                            }
                             on.accept(child, o);
                         } else {
                             // we hit the end ... so increment back to 0 again

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

@@ -15,8 +15,7 @@
  */
 package rx.internal.operators;
 
-import static org.junit.Assert.assertEquals;
-
+import java.nio.BufferOverflowException;
 import java.util.concurrent.CountDownLatch;
 
 import org.junit.Test;
@@ -25,9 +24,15 @@
 import rx.Observable.OnSubscribe;
 import rx.Observer;
 import rx.Subscriber;
+import rx.Subscription;
+import rx.functions.Func0;
+import rx.observables.ConnectableObservable;
 import rx.observers.TestSubscriber;
 import rx.schedulers.Schedulers;
 
+import static org.junit.Assert.assertEquals;
+import static org.junit.Assert.assertTrue;
+
 public class OperatorOnBackpressureBufferTest {
 
     @Test
@@ -81,6 +86,68 @@ public void onNext(Long t) {
         assertEquals(499, ts.getOnNextEvents().get(499).intValue());
     }
 
+    @Test(expected = IllegalArgumentException.class)
+    public void testFixBackpressureBufferNegativeCapacity() throws InterruptedException {
+        Observable.empty().onBackpressureBuffer(-1);
+    }
+
+    @Test(expected = IllegalArgumentException.class)
+    public void testFixBackpressureBufferZeroCapacity() throws InterruptedException {
+        Observable.empty().onBackpressureBuffer(-1);
+    }
+
+    @Test(timeout = 500)
+    public void testFixBackpressureBoundedBuffer() throws InterruptedException {
+        final CountDownLatch l1 = new CountDownLatch(250);
+        final CountDownLatch l2 = new CountDownLatch(500);
+        final CountDownLatch l3 = new CountDownLatch(1);
+        TestSubscriber<Long> ts = new TestSubscriber<Long>(new Observer<Long>() {
+
+            @Override
+            public void onCompleted() {
+            }
+
+            @Override
+            public void onError(Throwable e) {
+            }
+
+            @Override
+            public void onNext(Long t) {
+                l1.countDown();
+                l2.countDown();
+            }
+
+        });
+
+        ts.requestMore(500);
+
+        final ConnectableObservable<Long> flood =
+            infinite.subscribeOn(Schedulers.computation())
+                    .publish();
+        final ConnectableObservable<Long> batch =
+            infinite.subscribeOn(Schedulers.computation())
+                    .onBackpressureBuffer(100, new Func0<Void>() {
+                        @Override
+                        public Void call() {
+                            l3.countDown();
+                            return null;
+                        }
+                    }).publish();
+        Subscription s = batch.subscribe(ts);
+        batch.connect();          // first controlled batch
+
+        l1.await();
+        flood.connect();          // open flood
+        l2.await();               // ts can only swallow 250 more
+        l3.await();               // hold until it chokes
+
+        assertEquals(500, ts.getOnNextEvents().size());
+        assertEquals(0, ts.getOnNextEvents().get(0).intValue());
+        assertTrue(ts.getOnErrorEvents().get(0) instanceof BufferOverflowException);
+        assertTrue(s.isUnsubscribed());
+
+    }
+
     static final Observable<Long> infinite = Observable.create(new OnSubscribe<Long>() {
 
         @Override
@@ -92,4 +159,5 @@ public void call(Subscriber<? super Long> s) {
         }
 
     });
+
 }