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Fix directoryDownload cancel leak #6875

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Fix directoryDownload cancel leak #6875

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f1af8b7
Cancel AsyncBufferingSubscriber upstream subscription when future is …
RanVaknin Apr 17, 2026
6b3058a
Add changelog
RanVaknin Apr 17, 2026
9837072
Fix test
RanVaknin Apr 17, 2026
9e746e9
Update whenComplete to include a synchronization lock
RanVaknin Apr 17, 2026
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6 changes: 6 additions & 0 deletions .changes/next-release/bugfix-S3TransferManager-e28a164.json
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Original file line number Diff line number Diff line change
@@ -0,0 +1,6 @@
{
"type": "bugfix",
"category": "S3 Transfer Manager",
"contributor": "",
"description": "Fixed an issue where cancelling a directory transfer did not fully stop the operation."
}
5 changes: 5 additions & 0 deletions ...r/src/main/java/software/amazon/awssdk/transfer/s3/internal/AsyncBufferingSubscriber.java
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Expand Up @@ -56,6 +56,11 @@ public AsyncBufferingSubscriber(Function<T, CompletableFuture<?>> consumer,
returnFuture.whenComplete((r, t) -> {
if (t != null) {
requestsInFlight.forEach(f -> f.cancel(true));
synchronized (this) {
if (subscription != null) {
subscription.cancel();
}
}
}
});
}
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16 changes: 15 additions & 1 deletion ...c/test/java/software/amazon/awssdk/transfer/s3/internal/AsyncBufferingSubscriberTest.java
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Expand Up @@ -128,7 +128,21 @@ public void consumerFunctionThrows_shouldCancelSubscriptionAndCompleteFutureExce
subscriber.onSubscribe(mockSubscription);
subscriber.onNext("item");

verify(mockSubscription, times(1)).cancel();
verify(mockSubscription, times(2)).cancel();
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@RanVaknin RanVaknin Apr 17, 2026 •
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subscription.cancel() now exists in two codepaths:

  1. The existing codepath within onNext() catch block.
  2. The new addition in future.whenComplete()

Flow:

- onNext()
-- catch
-- subscription.cancel()
-- onError()

- onError()
-- completes exceptionally
-- future.whenCompleted()
-- subscription.cancel() // <-- new addition

I believe this is safe to call twice because:

5 Subscription.cancel MUST respect the responsivity of its caller by returning in a timely manner, MUST be idempotent and MUST be thread-safe.

Note: the codepath which the new addition solves is when the cancellation signal comes from outside of the pub/sub pipeline. If a user aborts the operation by cancelling the DirectoryDownload completion future directly, neither onNext nor onError are involved. The whenComplete handler is the only place that reacts to this external cancellation.

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@Fred1155 Fred1155 Apr 17, 2026

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Probably worth adding a quick comment here?

assertThatThrownBy(future::join).hasCause(exception);
}

@Test
void returnFutureCancelled_shouldCancelUpstreamSubscription() {
CompletableFuture<Void> future = new CompletableFuture<>();
AsyncBufferingSubscriber<String> subscriber = new AsyncBufferingSubscriber<>(
s -> new CompletableFuture<>(), future, 10);

Subscription mockSubscription = mock(Subscription.class);
subscriber.onSubscribe(mockSubscription);
subscriber.onNext("item");

future.cancel(true);
verify(mockSubscription, times(1)).cancel();
}
}
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