[Bug] Shared subscription stalls forever on a chunked message whose first chunk is already acked #26395

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• I searched in the issues and found nothing similar.

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• I understand that unsupported versions don't get bug fixes. I will attempt to reproduce the issue on a supported version of Pulsar client and Pulsar broker.

User environment

Broker 4.0.13, Go client consumer on a Shared subscription. The code looks the same on master.

Issue Description

Producer has chunking enabled (messages can be tens of MB, so most of them get chunked),
consumed by a Shared subscription.

1. Publish a chunked message, chunks 0..2 land on entries 100, 101, 102.
2. Consumer gets chunks 0 and 1 and acks them, mark-delete moves to 101.
3. Consumer restarts. A topic unload or broker restart has the same effect.
4. Entry 102 (chunk 2) is next in line.

From that point the subscription never delivers anything again. Backlog grows, no error, no
warning. pulsar-admin topics skip on that one entry and everything resumes instantly.

What did you expect to see

The leftover chunk either delivered (clients already know how to discard an incomplete chunked
message) or skipped, and the rest of the topic dispatched normally.

What did you see instead

Nothing in the broker logs at all, which is why this took us so long to find.

SharedConsumerAssignor.getConsumerForUuid() looks the uuid up in uuidToConsumer. That map is
in-memory per dispatcher and only ever written when chunk 0 is assigned, so after the restart it
is empty and chunk 2 hits:

if (metadata.getChunkId() != 0) {
    // Not the first chunk, skip it
    return null;
}

null makes assign() hand the entry to unassignedMessageProcessor, which the dispatcher wires
to addEntryToReplay, so it goes into redeliveryMessages. Next round the exact same thing
happens, because chunk 0 is acked and will never be read again to recreate the mapping. The entry stays in the replay queue.

Error messages

Reproducing the issue

I created a a unittest and patch.

Additional information

Please backport to 4.0 LTS

Are you willing to submit a PR?

• I'm willing to submit a PR!