[fix][client] Fix producer synchronous retry handling in failPendingMessages method

[sandeep-mst]

Fixes apache#25201

Motivation

Fix a re-entrancy bug in ProducerImpl.failPendingMessages. While executing on the timer, failPendingMessages invokes sendComplete(ex) on pending messages, which can synchronously trigger a retry from client code. The subsequent pendingMessages.clear() then removes the newly enqueued retry operation, leaving the retry’s CompletableFuture unresolved and the client in a limbo state.

Modifications

Updated failPendingMessages to first drain the pendingMessages queue into a local list and clear the queue before iterating. This prevents re-entrant retries triggered during sendComplete from being inadvertently cleared.

Verifying this change

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This change added tests and can be verified as follows:

• org.apache.pulsar.client.impl.ProducerImplTest#testFailPendingMessagesSyncRetry

Does this pull request potentially affect one of the following parts:

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• Dependencies (add or upgrade a dependency)
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Documentation

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Matching PR in forked repository

PR in forked repository:

[sandeep-mst]

Upstream PR is merged