BehaviorSubject concurrent subscription and sending is broken

BehaviorSubject should ensure that the last notification always reaches the subscriber. When the subscription and sending happens concurrently, there is a high probability that this property gets broken.

Test has been prepared that easily reproduces the error. https://gist.github.com/andrask/fc06abfd70daa6f91edb#file-behaviorsubjectsubscribeandsendconcurrently-java

The test involves two threads: 1) trying to subscribe 2) trying to send next. These are carefully coordinated to allow real concurrent execution. The test is that the subscriber must receive the sent value. The issue almost certainly happens in a few hundred retries.

Note that with a Thread.sleep(1) the issue goes away.

Note that the test is something I distilled from what I saw in my production code. There may be little issues with it but the concurrency problem certainly exists as it is reproducibly just by stepping through the code.

[JakeWharton]

Subscription is not an on* method and therefore this is not expected.

On Tue, May 24, 2016 at 7:38 PM Artem Zinnatullin [e-mail] wrote:

This is expected behavior.

If you you write to the subject from multiple threads and need consistency of on*() events -> you can convert your subject to SerializedSubject via subject.toSerialized().

Javadoc of Subject.toSerialized():

/** * Wraps a {@link Subject} so that it is safe to call its various {@code on} methods from different threads. *

* When you use an ordinary {@link Subject} as a {@link Subscriber}, you must take care not to call its * {@link Subscriber#onNext} method (or its other {@code on} methods) from multiple threads, as this could * lead to non-serialized calls, which violates * the Observable contract and creates an * ambiguity in the resulting Subject. *

* To protect a {@code Subject} from this danger, you can convert it into a {@code SerializedSubject} with * code like the following: *

{@code * mySafeSubject = myUnsafeSubject.toSerialized(); * }

* * @return SerializedSubject wrapping the current Subject */

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[sregg]

Seems that my idea of delaying the action in doOnSubscribe still doesn’t fix the issue. Now I’m thinking there’s something wrong between onNext and onComplete. Sometimes only onComplete is received…