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Note in Javadocs that SubscribableListener fires once (#128131)

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Spell out that multiple completions of a `SubscribableListener` race to
be chosen as the winner that is passed to subscribed listeners, such
that the subscribed listeners all fire at most once.
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David Turner 2025-05-27 15:33:07 +01:00 • committed by GitHub
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server/src/main/java/org/elasticsearch/action/support/SubscribableListener.java
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@ -32,15 +32,17 @@ import java.util.concurrent.ExecutionException;
import java.util.concurrent.Executor; import java.util.concurrent.Executor;
/** /**
* An {@link ActionListener} to which other {@link ActionListener} instances can subscribe, such that when this listener is * An {@link ActionListener} to which other {@link ActionListener} instances can subscribe, such that when this listener is completed it
* completed it fans-out its result to the subscribed listeners. * fans-out its result to the subscribed listeners.
* <p> * <p>
* If this listener is complete, {@link #addListener} completes the subscribing listener immediately * If this listener is complete, {@link #addListener} completes the subscribing listener immediately with the result with which this
* with the result with which this listener was completed. Otherwise, the subscribing listener is retained * listener was completed. Otherwise, the subscribing listener is retained and completed when this listener is completed.
* and completed when this listener is completed.
* <p> * <p>
* Exceptions are passed to subscribed listeners without modification. {@link ListenableActionFuture} and {@link ListenableFuture} are child * Exceptions are passed to subscribed listeners without modification. {@link ListenableActionFuture} and {@link ListenableFuture} are
* classes that provide additional exception handling. * subclasses which modify the exceptions passed to subscribed listeners.
* <p>
* If this listener is completed more than once then all results other than the first (whether successful or otherwise) are silently
* discarded. All subscribed listeners will be notified of the same result, exactly once, even if several completions occur concurrently.
* <p> * <p>
* A sequence of async steps can be chained together using a series of {@link SubscribableListener}s, similar to {@link CompletionStage} * A sequence of async steps can be chained together using a series of {@link SubscribableListener}s, similar to {@link CompletionStage}
* (without the {@code catch (Throwable t)}). Listeners can be created for each step, where the next step subscribes to the result of the * (without the {@code catch (Throwable t)}). Listeners can be created for each step, where the next step subscribes to the result of the