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Rewrite concat operation to not block on subscribe #271

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May 16, 2013
Merged

Rewrite concat operation to not block on subscribe #271

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167 changes: 88 additions & 79 deletions rxjava-core/src/main/java/rx/operators/OperationConcat.java
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Original file line number Diff line number Diff line change
Expand Up @@ -21,9 +21,11 @@
import java.util.Arrays;
import java.util.ArrayList;
import java.util.List;
import java.util.Queue;
import java.util.concurrent.CountDownLatch;
import java.util.concurrent.atomic.AtomicBoolean;
import java.util.concurrent.atomic.AtomicReference;
import java.util.concurrent.ConcurrentLinkedQueue;

import org.junit.Test;

Expand All @@ -41,23 +43,10 @@ public final class OperationConcat {

/**
* Combine the observable sequences from the list of Observables into one
* observable sequence without any transformation. If either the outer
* observable sequence without any transformation. If either the outer
* observable or an inner observable calls onError, we will call onError.
*
* <p/>
*
* The outer observable might run on a separate thread from (one of) the
* inner observables; in this case care must be taken to avoid a deadlock.
* The Concat operation may block the outer thread while servicing an inner
* thread in order to ensure a well-defined ordering of elements; therefore
* none of the inner threads must be implemented in a way that might wait on
* the outer thread.
*
* <p/>
*
* Beware that concat(o1,o2).subscribe() is a blocking call from
* which it is impossible to unsubscribe if observables are running on same thread.
*
* @param sequences An observable sequence of elements to project.
* @return An observable sequence whose elements are the result of combining the output from the list of Observables.
*/
Expand All @@ -70,73 +59,101 @@ public static <T> Func1<Observer<T>, Subscription> concat(final List<Observable<
}

public static <T> Func1<Observer<T>, Subscription> concat(final Observable<Observable<T>> sequences) {
return new Func1<Observer<T>, Subscription>() {

@Override
public Subscription call(Observer<T> observer) {
return new ConcatSubscription<T>(sequences, observer);
}
};
return new Concat<T>(sequences);
}

private static class ConcatSubscription<T> extends BooleanSubscription {
// Might be updated by an inner thread's onError during the outer
// thread's onNext, then read in the outer thread's onComplete.
final AtomicBoolean innerError = new AtomicBoolean(false);
private static class Concat<T> implements Func1<Observer<T>, Subscription> {
private Observable<Observable<T>> sequences;
private AtomicObservableSubscription innerSubscription = null;

public Concat(Observable<Observable<T>> sequences) {
this.sequences = sequences;
}

public ConcatSubscription(Observable<Observable<T>> sequences, final Observer<T> observer) {
public Subscription call(final Observer<T> observer) {
final AtomicBoolean completedOrErred = new AtomicBoolean(false);
final AtomicBoolean allSequencesReceived = new AtomicBoolean(false);
final Queue<Observable<T>> nextSequences = new ConcurrentLinkedQueue<Observable<T>>();
final AtomicObservableSubscription outerSubscription = new AtomicObservableSubscription();
outerSubscription.wrap(sequences.subscribe(new Observer<Observable<T>>() {

final Observer<T> reusableObserver = new Observer<T>() {
@Override
public void onNext(Observable<T> nextSequence) {
// We will not return from onNext until the inner observer completes.
// NB: while we are in onNext, the well-behaved outer observable will not call onError or onCompleted.
final CountDownLatch latch = new CountDownLatch(1);
final AtomicObservableSubscription innerSubscription = new AtomicObservableSubscription();
innerSubscription.wrap(nextSequence.subscribe(new Observer<T>() {
@Override
public void onNext(T item) {
// Make our best-effort to release resources in the face of unsubscribe.
if (isUnsubscribed()) {
innerSubscription.unsubscribe();
outerSubscription.unsubscribe();
} else {
observer.onNext(item);
public void onNext(T item) {
observer.onNext(item);
}
@Override
public void onError(Exception e) {
if (completedOrErred.compareAndSet(false, true)) {
outerSubscription.unsubscribe();
observer.onError(e);
}
}
@Override
public void onCompleted() {
synchronized (nextSequences) {
if (nextSequences.isEmpty()) {
// No new sequences available at the moment
innerSubscription = null;
if (allSequencesReceived.get()) {
// No new sequences are coming, we are finished
if (completedOrErred.compareAndSet(false, true)) {
observer.onCompleted();
}
}
}
@Override
public void onError(Exception e) {
outerSubscription.unsubscribe();
innerError.set(true);
observer.onError(e);
latch.countDown();
}
@Override
public void onCompleted() {
} else {
// Continue on to the next sequence
latch.countDown();
innerSubscription = new AtomicObservableSubscription();
innerSubscription.wrap(nextSequences.poll().subscribe(this));
}
}
}
};

outerSubscription.wrap(sequences.subscribe(new Observer<Observable<T>>() {
@Override
public void onNext(Observable<T> nextSequence) {
synchronized (nextSequences) {
if (innerSubscription == null) {
// We are currently not subscribed to any sequence
innerSubscription = new AtomicObservableSubscription();
innerSubscription.wrap(nextSequence.subscribe(reusableObserver));
} else {
// Put this sequence at the end of the queue
nextSequences.add(nextSequence);
}
}));
try {
latch.await();
} catch (InterruptedException e) {
Thread.currentThread().interrupt();
throw Exceptions.propagate(e);
}
}
@Override
public void onError(Exception e) {
// NB: a well-behaved observable will not interleave on{Next,Error,Completed} calls.
observer.onError(e);
if (completedOrErred.compareAndSet(false, true)) {
if (innerSubscription != null) {
innerSubscription.unsubscribe();
}
observer.onError(e);
}
}
@Override
public void onCompleted() {
// NB: a well-behaved observable will not interleave on{Next,Error,Completed} calls.
if (!innerError.get()) {
observer.onCompleted();
allSequencesReceived.set(true);
if (innerSubscription == null) {
// We are not subscribed to any sequence, and none are coming anymore
if (completedOrErred.compareAndSet(false, true)) {
observer.onCompleted();
}
}
}
}));

return new Subscription() {
@Override
public void unsubscribe() {
synchronized (nextSequences) {
if (innerSubscription != null)
innerSubscription.unsubscribe();
outerSubscription.unsubscribe();
}
}
};
}
}

Expand Down Expand Up @@ -445,7 +462,7 @@ public void testConcatConcurrentWithInfinity() {


/**
* The outer observable is running on the same thread and subscribe() in this case is a blocking call. Calling unsubscribe() is no-op because the sequence is complete.
* Test unsubscribing the concatenated Observable in a single thread.
*/
@Test
public void testConcatUnsubscribe() {
Expand All @@ -459,20 +476,13 @@ public void testConcatUnsubscribe() {
@SuppressWarnings("unchecked")
final Observable<String> concat = Observable.create(concat(w1, w2));
final AtomicObservableSubscription s1 = new AtomicObservableSubscription();
Thread t = new Thread() {
@Override
public void run() {
// NB: this statement does not complete until after "six" has been delivered.
s1.wrap(concat.subscribe(aObserver));
}
};
t.start();

try {
// Subscribe
s1.wrap(concat.subscribe(aObserver));
//Block main thread to allow observable "w1" to complete and observable "w2" to call onNext once.
callOnce.await();
// NB: This statement has no effect, since s1 cannot possibly
// wrap anything until "six" has been delivered, which cannot
// happen until we okToContinue.countDown()
// Unsubcribe
s1.unsubscribe();
//Unblock the observable to continue.
okToContinue.countDown();
Expand All @@ -488,10 +498,9 @@ public void run() {
inOrder.verify(aObserver, times(1)).onNext("two");
inOrder.verify(aObserver, times(1)).onNext("three");
inOrder.verify(aObserver, times(1)).onNext("four");
// NB: you might hope that five and six are not delivered, but see above.
inOrder.verify(aObserver, times(1)).onNext("five");
inOrder.verify(aObserver, times(1)).onNext("six");
inOrder.verify(aObserver, times(1)).onCompleted();
inOrder.verify(aObserver, never()).onNext("five");
inOrder.verify(aObserver, never()).onNext("six");
inOrder.verify(aObserver, never()).onCompleted();

}

Expand Down