fix: multicast event handler silently drops events when no aggregates found

[Copilot]

In pub/sub systems, messages from different topics have no guaranteed delivery order. This caused ChannelSummary.onUserSubscribed to silently discard subscription events that arrived before the corresponding ChannelCreated had been processed — findByChannel returned empty, the handler did nothing, and the message was ACK'd and lost.

Root cause

MulticastEventHandlerWriter generated code that treated an empty query result as a no-op:

// Before — empty result silently dropped
var aggregates = channelSummaryRepository.findByChannel(event.channel()).stream()
    .map(aggregate -> { aggregate = aggregate.onUserSubscribed(event); return aggregate; })
    .toList();
channelSummaryRepository.saveAll(aggregates); // saveAll([]) = no-op, message ACK'd

This is inconsistent with ByReferenceEventHandlerWriter, which uses .orElseThrow() and therefore triggers the retry mechanism on missing entities.

Fix

Fail fast on empty results so PubSubUtil's retry template re-delivers the message until the target aggregate exists:

// After — throws when empty, triggering retry
var aggregates = channelSummaryRepository.findByChannel(event.channel());
if (aggregates.isEmpty()) {
    throw new IllegalStateException("No aggregates found for event: " + event);
}
channelSummaryRepository.saveAll(aggregates.stream()
    .map(aggregate -> { aggregate = aggregate.onUserSubscribed(event); return aggregate; })
    .toList());

With default config (5 retries, 1 s initial backoff), the out-of-order event is retried within the first second — well within the test's 30 s window — and succeeds once ChannelCreated has been processed.

Warning

Firewall rules blocked me from connecting to one or more addresses (expand for details)

I tried to connect to the following addresses, but was blocked by firewall rules:

• packages.confluent.io
  • Triggering command: /usr/lib/jvm/temurin-17-jdk-amd64/bin/java /usr/lib/jvm/temurin-17-jdk-amd64/bin/java --enable-native-access=ALL-UNNAMED -classpath /usr/share/apache-maven-3.9.14/boot/plexus-classworlds-2.9.0.jar -Dclassworlds.conf=/usr/share/apache-maven-3.9.14/bin/m2.conf -Dmaven.home=/usr/share/apache-maven-3.9.14 -Dlibrary.jansi.path=/usr/share/apache-maven-3.9.14/lib/jansi-native -Dmaven.multiModuleProjectDirectory=/home/REDACTED/work/prefab/prefab org.codehaus.plexus.classworlds.launcher.Launcher compile -pl annotation-processor -am --no-transfer-progress -q resources/pubsub/single/User.jav/home/REDACTED/work/prefab/prefab/pubsub/src/test/resources/pubsub--noprofile resources/pubsubgit resources/pubsubpush resources/pubsub-v resources/pubsuborigin (dns block)
  • Triggering command: /usr/lib/jvm/temurin-17-jdk-amd64/bin/java /usr/lib/jvm/temurin-17-jdk-amd64/bin/java --enable-native-access=ALL-UNNAMED -classpath /usr/share/apache-maven-3.9.14/boot/plexus-classworlds-2.9.0.jar -Dclassworlds.conf=/usr/share/apache-maven-3.9.14/bin/m2.conf -Dmaven.home=/usr/share/apache-maven-3.9.14 -Dlibrary.jansi.path=/usr/share/apache-maven-3.9.14/lib/jansi-native -Dmaven.multiModuleProjectDirectory=/home/REDACTED/work/prefab/prefab org.codehaus.plexus.classworlds.launcher.Launcher compile -pl annotation-processor --no-transfer-progress -q --global nverter.java er.java nverter.java (dns block)
  • Triggering command: /usr/lib/jvm/temurin-17-jdk-amd64/bin/java /usr/lib/jvm/temurin-17-jdk-amd64/bin/java --enable-native-access=ALL-UNNAMED -classpath /usr/share/apache-maven-3.9.14/boot/plexus-classworlds-2.9.0.jar -Dclassworlds.conf=/usr/share/apache-maven-3.9.14/bin/m2.conf -Dmaven.home=/usr/share/apache-maven-3.9.14 -Dlibrary.jansi.path=/usr/share/apache-maven-3.9.14/lib/jansi-native -Dmaven.multiModuleProjectDirectory=/home/REDACTED/work/prefab/prefab org.codehaus.plexus.classworlds.launcher.Launcher -f pom.xml -B -V -e -Dfindbugs.skip -Dcheckstyle.skip -Dpmd.skip=true -Dspotbugs.skip -Denforcer.skip -Dmaven.javadoc.skip (dns block)

If you need me to access, download, or install something from one of these locations, you can either:

• Configure Actions setup steps to set up my environment, which run before the firewall is enabled
• Add the appropriate URLs or hosts to the custom allowlist in this repository's Copilot coding agent settings (admins only)