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Extend docs on double-completion using SubscribableListener #133391

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DaveCTurner merged 1 commit into from
Aug 22, 2025
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Extend docs on double-completion using SubscribableListener #133391

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12 changes: 12 additions & 0 deletions server/src/main/java/org/elasticsearch/action/support/SubscribableListener.java
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Original file line number Diff line number Diff line change
Expand Up @@ -129,6 +129,9 @@ public static <T> SubscribableListener<T> newFailed(Exception exception) {
/**
* Create a {@link SubscribableListener}, fork a computation to complete it, and return the listener. If the forking itself throws an
* exception then the exception is caught and fed to the returned listener.
* <p>
* The listener passed to {@code fork} is the returned {@link SubscribableListener}. In particular, it is valid to complete this
* listener more than once, but all results after the first completion will be silently ignored.
*/
public static <T> SubscribableListener<T> newForked(CheckedConsumer<ActionListener<T>, ? extends Exception> fork) {
final var listener = new SubscribableListener<T>();
Expand Down Expand Up @@ -448,6 +451,9 @@ public void complete(ActionListener<?> listener) {
* <li>Ensure that this {@link SubscribableListener} is always completed using that executor, and</li>
* <li>Invoke {@link #andThen} using that executor.</li>
* </ul>
* <p>
* The listener passed to {@code nextStep} is the returned {@link SubscribableListener}. In particular, it is valid to complete this
* listener more than once, but all results after the first completion will be silently ignored.
*/
public <U> SubscribableListener<U> andThen(CheckedConsumer<ActionListener<U>, ? extends Exception> nextStep) {
return newForked(l -> addListener(l.delegateFailureIgnoreResponseAndWrap(nextStep)));
Expand Down Expand Up @@ -475,6 +481,9 @@ public <U> SubscribableListener<U> andThen(CheckedConsumer<ActionListener<U>, ?
* <li>Ensure that this {@link SubscribableListener} is always completed using that executor, and</li>
* <li>Invoke {@link #andThen} using that executor.</li>
* </ul>
* <p>
* The listener passed to {@code nextStep} is the returned {@link SubscribableListener}. In particular, it is valid to complete this
* listener more than once, but all results after the first completion will be silently ignored.
*/
public <U> SubscribableListener<U> andThen(CheckedBiConsumer<ActionListener<U>, T, ? extends Exception> nextStep) {
return andThen(EsExecutors.DIRECT_EXECUTOR_SERVICE, null, nextStep);
Expand Down Expand Up @@ -513,6 +522,9 @@ public <U> SubscribableListener<U> andThen(CheckedBiConsumer<ActionListener<U>,
* with a rejection exception on the thread which completes this listener. Likewise if this listener is completed exceptionally but
* {@code executor} rejects the execution of the completion of the returned listener then the returned listener is completed with a
* rejection exception on the thread which completes this listener.
* <p>
* The listener passed to {@code nextStep} is the returned {@link SubscribableListener}. In particular, it is valid to complete this
* listener more than once, but all results after the first completion will be silently ignored.
*/
public <U> SubscribableListener<U> andThen(
Executor executor,
Expand Down