2.x: add subscribeOn overload to avoid same-pool deadlock with create (#5386)

Author: akarnokd

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

@@ -13045,6 +13045,44 @@ public final <E extends Subscriber<? super T>> E subscribeWith(E subscriber) {
     /**
      * Asynchronously subscribes Subscribers to this Publisher on the specified {@link Scheduler}.
      * <p>
+     * If there is a {@link #create(FlowableOnSubscribe, BackpressureStrategy)} type source up in the
+     * chain, it is recommended to use {@code subscribeOn(scheduler, false)} instead
+     * to avoid same-pool deadlock because requests may pile up behind a eager/blocking emitter.
+     * <p>
+     * <img width="640" height="305" src="https://raw.github.com/wiki/ReactiveX/RxJava/images/rx-operators/subscribeOn.png" alt="">
+     * <dl>
+     *  <dt><b>Backpressure:</b></dt>
+     *  <dd>The operator doesn't interfere with backpressure which is determined by the source {@code Publisher}'s backpressure
+     *  behavior.</dd>
+     *  <dt><b>Scheduler:</b></dt>
+     *  <dd>You specify which {@link Scheduler} this operator will use</dd>
+     * </dl>
+     *
+     * @param scheduler
+     *            the {@link Scheduler} to perform subscription actions on
+     * @return the source Publisher modified so that its subscriptions happen on the
+     *         specified {@link Scheduler}
+     * @see <a href="http://reactivex.io/documentation/operators/subscribeon.html">ReactiveX operators documentation: SubscribeOn</a>
+     * @see <a href="http://www.grahamlea.com/2014/07/rxjava-threading-examples/">RxJava Threading Examples</a>
+     * @see #observeOn
+     * @see #subscribeOn(Scheduler, boolean)
+     */
+    @CheckReturnValue
+    @BackpressureSupport(BackpressureKind.PASS_THROUGH)
+    @SchedulerSupport(SchedulerSupport.CUSTOM)
+    public final Flowable<T> subscribeOn(@NonNull Scheduler scheduler) {
+        ObjectHelper.requireNonNull(scheduler, "scheduler is null");
+        return subscribeOn(scheduler, !(this instanceof FlowableCreate));
+    }
+
+    /**
+     * Asynchronously subscribes Subscribers to this Publisher on the specified {@link Scheduler}
+     * optionally reroutes requests from other threads to the same {@link Scheduler} thread.
+     * <p>
+     * If there is a {@link #create(FlowableOnSubscribe, BackpressureStrategy)} type source up in the
+     * chain, it is recommended to have {@code requestOn} false to avoid same-pool deadlock
+     * because requests may pile up behind a eager/blocking emitter.
+     * <p>
      * <img width="640" height="305" src="https://raw.github.com/wiki/ReactiveX/RxJava/images/rx-operators/subscribeOn.png" alt="">
      * <dl>
      *  <dt><b>Backpressure:</b></dt>
@@ -13056,18 +13094,23 @@ public final <E extends Subscriber<? super T>> E subscribeWith(E subscriber) {
      *
      * @param scheduler
      *            the {@link Scheduler} to perform subscription actions on
+     * @param requestOn if true, requests are rerouted to the given Scheduler as well (strong pipelining)
+     *                  if false, requests coming from any thread are simply forwarded to
+     *                  the upstream on the same thread (weak pipelining)
      * @return the source Publisher modified so that its subscriptions happen on the
      *         specified {@link Scheduler}
      * @see <a href="http://reactivex.io/documentation/operators/subscribeon.html">ReactiveX operators documentation: SubscribeOn</a>
      * @see <a href="http://www.grahamlea.com/2014/07/rxjava-threading-examples/">RxJava Threading Examples</a>
      * @see #observeOn
+     * @since 2.1.1 - experimental
      */
     @CheckReturnValue
     @BackpressureSupport(BackpressureKind.PASS_THROUGH)
     @SchedulerSupport(SchedulerSupport.CUSTOM)
-    public final Flowable<T> subscribeOn(Scheduler scheduler) {
+    @Experimental
+    public final Flowable<T> subscribeOn(@NonNull Scheduler scheduler, boolean requestOn) {
         ObjectHelper.requireNonNull(scheduler, "scheduler is null");
-        return RxJavaPlugins.onAssembly(new FlowableSubscribeOn<T>(this, scheduler, this instanceof FlowableCreate));
+        return RxJavaPlugins.onAssembly(new FlowableSubscribeOn<T>(this, scheduler, requestOn));
     }
 
     /**

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

@@ -65,13 +65,13 @@ static final class SubscribeOnSubscriber<T> extends AtomicReference<Thread>
 
         Publisher<T> source;
 
-        SubscribeOnSubscriber(Subscriber<? super T> actual, Scheduler.Worker worker, Publisher<T> source, boolean nonScheduledRequests) {
+        SubscribeOnSubscriber(Subscriber<? super T> actual, Scheduler.Worker worker, Publisher<T> source, boolean requestOn) {
             this.actual = actual;
             this.worker = worker;
             this.source = source;
             this.s = new AtomicReference<Subscription>();
             this.requested = new AtomicLong();
-            this.nonScheduledRequests = nonScheduledRequests;
+            this.nonScheduledRequests = !requestOn;
         }
 
         @Override

src/test/java/io/reactivex/internal/operators/flowable/FlowableSubscribeOnTest.java

@@ -372,4 +372,51 @@ public void subscribe(FlowableEmitter<Object> s) throws Exception {
         .assertNoErrors()
         .assertComplete();
     }
+
+    @Test
+    public void nonScheduledRequestsNotSubsequentSubscribeOn() {
+        TestSubscriber<Object> ts = Flowable.create(new FlowableOnSubscribe<Object>() {
+            @Override
+            public void subscribe(FlowableEmitter<Object> s) throws Exception {
+                for (int i = 1; i < 1001; i++) {
+                    s.onNext(i);
+                    Thread.sleep(1);
+                }
+                s.onComplete();
+            }
+        }, BackpressureStrategy.DROP)
+        .map(Functions.identity())
+        .subscribeOn(Schedulers.single(), false)
+        .observeOn(Schedulers.computation())
+        .test()
+        .awaitDone(5, TimeUnit.SECONDS)
+        .assertNoErrors()
+        .assertComplete();
+
+        int c = ts.valueCount();
+
+        assertTrue("" + c, c > Flowable.bufferSize());
+    }
+
+    @Test
+    public void scheduledRequestsNotSubsequentSubscribeOn() {
+        Flowable.create(new FlowableOnSubscribe<Object>() {
+            @Override
+            public void subscribe(FlowableEmitter<Object> s) throws Exception {
+                for (int i = 1; i < 1001; i++) {
+                    s.onNext(i);
+                    Thread.sleep(1);
+                }
+                s.onComplete();
+            }
+        }, BackpressureStrategy.DROP)
+        .map(Functions.identity())
+        .subscribeOn(Schedulers.single(), true)
+        .observeOn(Schedulers.computation())
+        .test()
+        .awaitDone(5, TimeUnit.SECONDS)
+        .assertValueCount(Flowable.bufferSize())
+        .assertNoErrors()
+        .assertComplete();
+    }
 }