Fix Flowable.blockingSubscribe is unbounded and can lead to OOME

src/main/java/io/reactivex/Flowable.java

@@ -5853,6 +5853,38 @@ public final void blockingSubscribe(Consumer<? super T> onNext) {
         FlowableBlockingSubscribe.subscribe(this, onNext, Functions.ON_ERROR_MISSING, Functions.EMPTY_ACTION);
     }
 
+    /**
+     * Subscribes to the source and calls the given callbacks <strong>on the current thread</strong>.
+     * <p>
+     * If the Flowable emits an error, it is wrapped into an
+     * {@link io.reactivex.exceptions.OnErrorNotImplementedException OnErrorNotImplementedException}
+     * and routed to the RxJavaPlugins.onError handler.
+     * Using the overloads {@link #blockingSubscribe(Consumer, Consumer)}
+     * or {@link #blockingSubscribe(Consumer, Consumer, Action)} instead is recommended.
+     * <p>
+     * Note that calling this method will block the caller thread until the upstream terminates
+     * normally or with an error. Therefore, calling this method from special threads such as the
+     * Android Main Thread or the Swing Event Dispatch Thread is not recommended.
+     * <dl>
+     *  <dt><b>Backpressure:</b></dt>
+     *  <dd>The operator consumes the source {@code Flowable} in an bounded manner (up to bufferSize
+     *  outstanding request amount for items).</dd>
+     *  <dt><b>Scheduler:</b></dt>
+     *  <dd>{@code blockingSubscribe} does not operate by default on a particular {@link Scheduler}.</dd>
+     * </dl>
+     * @param onNext the callback action for each source value
+     * @param bufferSize the size of the buffer
+     * @since 2.1.15 - experimental
+     * @see #blockingSubscribe(Consumer, Consumer)
+     * @see #blockingSubscribe(Consumer, Consumer, Action)
+     */
+    @BackpressureSupport(BackpressureKind.FULL)
+    @SchedulerSupport(SchedulerSupport.NONE)
+    @Experimental
+    public final void blockingSubscribe(Consumer<? super T> onNext, int bufferSize) {
+        FlowableBlockingSubscribe.subscribe(this, onNext, Functions.ON_ERROR_MISSING, Functions.EMPTY_ACTION, bufferSize);
+    }
+
     /**
      * Subscribes to the source and calls the given callbacks <strong>on the current thread</strong>.
      * <p>
@@ -5877,6 +5909,32 @@ public final void blockingSubscribe(Consumer<? super T> onNext, Consumer<? super
         FlowableBlockingSubscribe.subscribe(this, onNext, onError, Functions.EMPTY_ACTION);
     }
 
+    /**
+     * Subscribes to the source and calls the given callbacks <strong>on the current thread</strong>.
+     * <p>
+     * Note that calling this method will block the caller thread until the upstream terminates
+     * normally or with an error. Therefore, calling this method from special threads such as the
+     * Android Main Thread or the Swing Event Dispatch Thread is not recommended.
+     * <dl>
+     *  <dt><b>Backpressure:</b></dt>
+     *  <dd>The operator consumes the source {@code Flowable} in an bounded manner (up to bufferSize
+     *  outstanding request amount for items).</dd>
+     *  <dt><b>Scheduler:</b></dt>
+     *  <dd>{@code blockingSubscribe} does not operate by default on a particular {@link Scheduler}.</dd>
+     * </dl>
+     * @param onNext the callback action for each source value
+     * @param onError the callback action for an error event
+     * @param bufferSize the size of the buffer
+     * @since 2.1.15 - experimental
+     * @see #blockingSubscribe(Consumer, Consumer, Action)
+     */
+    @BackpressureSupport(BackpressureKind.FULL)
+    @SchedulerSupport(SchedulerSupport.NONE)
+    @Experimental
+    public final void blockingSubscribe(Consumer<? super T> onNext, Consumer<? super Throwable> onError,
+        int bufferSize) {
+        FlowableBlockingSubscribe.subscribe(this, onNext, onError, Functions.EMPTY_ACTION, bufferSize);
+    }
 
     /**
      * Subscribes to the source and calls the given callbacks <strong>on the current thread</strong>.
@@ -5902,6 +5960,33 @@ public final void blockingSubscribe(Consumer<? super T> onNext, Consumer<? super
         FlowableBlockingSubscribe.subscribe(this, onNext, onError, onComplete);
     }
 
+    /**
+     * Subscribes to the source and calls the given callbacks <strong>on the current thread</strong>.
+     * <p>
+     * Note that calling this method will block the caller thread until the upstream terminates
+     * normally or with an error. Therefore, calling this method from special threads such as the
+     * Android Main Thread or the Swing Event Dispatch Thread is not recommended.
+     * <dl>
+     *  <dt><b>Backpressure:</b></dt>
+     *  <dd>The operator consumes the source {@code Flowable} in an bounded manner (up to bufferSize
+     *  outstanding request amount for items).</dd>
+     *  <dt><b>Scheduler:</b></dt>
+     *  <dd>{@code blockingSubscribe} does not operate by default on a particular {@link Scheduler}.</dd>
+     * </dl>
+     * @param onNext the callback action for each source value
+     * @param onError the callback action for an error event
+     * @param onComplete the callback action for the completion event.
+     * @param bufferSize the size of the buffer
+     * @since 2.1.15 - experimental
+     */
+    @BackpressureSupport(BackpressureKind.FULL)
+    @SchedulerSupport(SchedulerSupport.NONE)
+    @Experimental
+    public final void blockingSubscribe(Consumer<? super T> onNext, Consumer<? super Throwable> onError, Action onComplete,
+        int bufferSize) {
+        FlowableBlockingSubscribe.subscribe(this, onNext, onError, onComplete, bufferSize);
+    }
+
     /**
      * Subscribes to the source and calls the {@link Subscriber} methods <strong>on the current thread</strong>.
      * <p>

src/main/java/io/reactivex/internal/functions/Functions.java

@@ -745,4 +745,23 @@ public void accept(Subscription t) throws Exception {
             t.request(Long.MAX_VALUE);
         }
     }
+
+    @SuppressWarnings("unchecked")
+    public static <T> Consumer<T> boundedConsumer(int bufferSize) {
+        return (Consumer<T>) new BoundedConsumer(bufferSize);
+    }
+
+    public static class BoundedConsumer implements Consumer<Subscription> {
+
+        final int bufferSize;
+
+        BoundedConsumer(int bufferSize) {
+            this.bufferSize = bufferSize;
+        }
+
+        @Override
+        public void accept(Subscription s) throws Exception {
+            s.request(bufferSize);
+        }
+    }
 }

src/main/java/io/reactivex/internal/operators/flowable/FlowableBlockingSubscribe.java

@@ -108,4 +108,23 @@ public static <T> void subscribe(Publisher<? extends T> o, final Consumer<? supe
         ObjectHelper.requireNonNull(onComplete, "onComplete is null");
         subscribe(o, new LambdaSubscriber<T>(onNext, onError, onComplete, Functions.REQUEST_MAX));
     }
+
+    /**
+     * Subscribes to the source and calls the given actions on the current thread.
+     * @param o the source publisher
+     * @param onNext the callback action for each source value
+     * @param onError the callback action for an error event
+     * @param onComplete the callback action for the completion event.
+     * @param bufferSize the number of elements to prefetch from the source Publisher
+     * @param <T> the value type
+     */
+    public static <T> void subscribe(Publisher<? extends T> o, final Consumer<? super T> onNext,
+        final Consumer<? super Throwable> onError, final Action onComplete, int bufferSize) {
+        ObjectHelper.requireNonNull(onNext, "onNext is null");
+        ObjectHelper.requireNonNull(onError, "onError is null");
+        ObjectHelper.requireNonNull(onComplete, "onComplete is null");
+        ObjectHelper.verifyPositive(bufferSize, "number > 0 required");
+        subscribe(o, new BoundedSubscriber<T>(onNext, onError, onComplete, Functions.boundedConsumer(bufferSize),
+                bufferSize));
+    }
 }

src/main/java/io/reactivex/internal/subscribers/BoundedSubscriber.java

@@ -0,0 +1,140 @@
+/**
+ * Copyright (c) 2016-present, RxJava Contributors.
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License"); you may not use this file except in
+ * compliance with the License. You may obtain a copy of the License at
+ *
+ * http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software distributed under the License is
+ * distributed on an "AS IS" BASIS, WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied. See
+ * the License for the specific language governing permissions and limitations under the License.
+ */
+
+package io.reactivex.internal.subscribers;
+
+import io.reactivex.FlowableSubscriber;
+import io.reactivex.disposables.Disposable;
+import io.reactivex.exceptions.CompositeException;
+import io.reactivex.exceptions.Exceptions;
+import io.reactivex.functions.Action;
+import io.reactivex.functions.Consumer;
+import io.reactivex.internal.functions.Functions;
+import io.reactivex.internal.subscriptions.SubscriptionHelper;
+import io.reactivex.observers.LambdaConsumerIntrospection;
+import io.reactivex.plugins.RxJavaPlugins;
+import org.reactivestreams.Subscription;
+
+import java.util.concurrent.atomic.AtomicReference;
+
+public final class BoundedSubscriber<T> extends AtomicReference<Subscription>
+        implements FlowableSubscriber<T>, Subscription, Disposable, LambdaConsumerIntrospection {
+
+    private static final long serialVersionUID = -7251123623727029452L;
+    final Consumer<? super T> onNext;
+    final Consumer<? super Throwable> onError;
+    final Action onComplete;
+    final Consumer<? super Subscription> onSubscribe;
+
+    final int bufferSize;
+    int consumed;
+    final int limit;
+
+    public BoundedSubscriber(Consumer<? super T> onNext, Consumer<? super Throwable> onError,
+                            Action onComplete, Consumer<? super Subscription> onSubscribe, int bufferSize) {
+        super();
+        this.onNext = onNext;
+        this.onError = onError;
+        this.onComplete = onComplete;
+        this.onSubscribe = onSubscribe;
+        this.bufferSize = bufferSize;
+        this.limit = bufferSize - (bufferSize >> 2);
+    }
+
+    @Override
+    public void onSubscribe(Subscription s) {
+        if (SubscriptionHelper.setOnce(this, s)) {
+            try {
+                onSubscribe.accept(this);
+            } catch (Throwable e) {
+                Exceptions.throwIfFatal(e);
+                s.cancel();
+                onError(e);
+            }
+        }
+    }
+
+    @Override
+    public void onNext(T t) {
+        if (!isDisposed()) {
+            try {
+                onNext.accept(t);
+
+                int c = consumed + 1;
+                if (c == limit) {
+                    consumed = 0;
+                    get().request(limit);
+                } else {
+                    consumed = c;
+                }
+            } catch (Throwable e) {
+                Exceptions.throwIfFatal(e);
+                get().cancel();
+                onError(e);
+            }
+        }
+    }
+
+    @Override
+    public void onError(Throwable t) {
+        if (get() != SubscriptionHelper.CANCELLED) {
+            lazySet(SubscriptionHelper.CANCELLED);
+            try {
+                onError.accept(t);
+            } catch (Throwable e) {
+                Exceptions.throwIfFatal(e);
+                RxJavaPlugins.onError(new CompositeException(t, e));
+            }
+        } else {
+            RxJavaPlugins.onError(t);
+        }
+    }
+
+    @Override
+    public void onComplete() {
+        if (get() != SubscriptionHelper.CANCELLED) {
+            lazySet(SubscriptionHelper.CANCELLED);
+            try {
+                onComplete.run();
+            } catch (Throwable e) {
+                Exceptions.throwIfFatal(e);
+                RxJavaPlugins.onError(e);
+            }
+        }
+    }
+
+    @Override
+    public void dispose() {
+        cancel();
+    }
+
+    @Override
+    public boolean isDisposed() {
+        return get() == SubscriptionHelper.CANCELLED;
+    }
+
+    @Override
+    public void request(long n) {
+        get().request(n);
+    }
+
+    @Override
+    public void cancel() {
+        SubscriptionHelper.cancel(this);
+    }
+
+    @Override
+    public boolean hasCustomOnError() {
+        return onError != Functions.ON_ERROR_MISSING;
+    }
+}

src/test/java/io/reactivex/exceptions/OnErrorNotImplementedExceptionTest.java

@@ -66,6 +66,12 @@ public void flowableBlockingSubscribe1() {
         .blockingSubscribe(Functions.emptyConsumer());
     }
 
+    @Test
+    public void flowableBoundedBlockingSubscribe1() {
+        Flowable.error(new TestException())
+                .blockingSubscribe(Functions.emptyConsumer(), 128);
+    }
+
     @Test
     public void observableSubscribe0() {
         Observable.error(new TestException())