Fixes #25277

Motivation

DrainingHashesTracker.ConsumerDrainingHashesStats#updateConsumerStats currently logs a WARN every time the hash iterator observes an entry that was concurrently removed:

https://github.com/apache/pulsar/blob/master/pulsar-broker/src/main/java/org/apache/pulsar/broker/service/DrainingHashesTracker.java#L188-L196

if (entry == null) {
    log.warn()
            .attr("dispatcher", dispatcherName)
            .attr("hash", hash)
            .attr("consumer", consumer)
            .log("Draining hash not found in the tracker for consumer");
    continue;
}

In the original PIP-379 design the draining-hash stats were strongly consistent with the tracker state, so a missing entry on this read path would indeed have been unexpected. That strong-consistency guarantee was intentionally dropped in #23854 to break a deadlock — the stats read path is now allowed to race with entry removal.

A null entry is therefore a benign, expected outcome during normal broker operation, not an alertable condition. Logging at WARN floods broker logs and drowns out actionable warnings.

The issue author (@lhotari) explicitly notes: "The warning log no longer makes sense and should be changed to debug logging."

Modifications

• DrainingHashesTracker.java: change the not-found branch from log.warn() to log.debug() and add an inline comment explaining why the log is deliberately quiet (pointing at the [fix][broker] Fix deadlock in Key_Shared PIP-379 implementation #23854 context), so future readers don't bump it back up.

No behavioural change — the continue and surrounding logic are untouched.

Verifying this change

• Make sure that the change passes the CI checks.

This change is a trivial fix without any test coverage: it only lowers the level of an existing log statement. No functional behaviour changes, so no new tests are required (existing DrainingHashesTracker tests still cover the race path).

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

None of the listed areas are affected. Log level only.

Documentation

• doc-not-needed

No user-facing or config documentation references this log line.