Integrated: 8348108: Race condition in AggregatePublisher.AggregateSubscription

[Daniel Fuchs]

On Mon, 20 Jan 2025 17:55:49 GMT, Daniel Fuchs <dfuchs at openjdk.org> wrote:

> There's a subtle race condition in AggregatePublisher.AggregateSubscription. The AggregatePublisher will subscribe to downstream publishers in turn, subscribing to the next publisher when the previous publisher onComplete() has been called.
> 
> The request() method passed to the upstream subscriber detects that all subscribers have completed if the current downstream publisher is null and the queue of publisher yet to subscribe to is empty.
> 
> The event loop is responsible for subscribing to the next publisher, and does so by polling the queue and assigning the returned value to this.publisher, and then subscribe to that publisher. However, when subscribing to the last publisher in the queue, there is a small time window where the queue can be observed to be empty, before this.publisher is assigned.
> 
> The fix adds synchronization to make sure a consistent state is observed when it matters. Typically - setting `this.publisher` or taking a snapshot of a publisher and its subscription, should be done within synchronized.

This pull request has now been integrated.

Changeset: b720517c
Author:    Daniel Fuchs <dfuchs at openjdk.org>
URL:       https://git.openjdk.org/jdk/commit/b720517cb33c2119ec6ed85504bce321de748228
Stats:     125 lines in 2 files changed: 81 ins; 13 del; 31 mod

8348108: Race condition in AggregatePublisher.AggregateSubscription

Reviewed-by: jpai

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PR: https://git.openjdk.org/jdk/pull/23204