[Bug] Compaction counters do not cover failures that happen before the raw reader is created

[lhotari]

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• I searched in the issues and found nothing similar.

Read release policy

• I understand that unsupported versions don't get bug fixes. I reproduced the issue on the current master branch.

User environment

• Broker version: 4.2.4
• Present on master (9ba61bd95de); line numbers below are master.

Issue Description

pulsar_compaction_succeed_count and pulsar_compaction_failed_count do not account for the whole compaction attempt. Compactor.compact() creates the raw reader outside the block that records the counters:

public CompletableFuture<Long> compact(String topic) {
    return RawReader.create(pulsar, topic, COMPACTION_SUBSCRIPTION, false, false)   // :58
            .thenComposeAsync(this::compactAndCloseReader, scheduler);
}

private CompletableFuture<Long> compactAndCloseReader(RawReader reader) {
    CompletableFuture<Long> promise = new CompletableFuture<>();
    mxBean.addCompactionStartOp(reader.getTopic());                                 // :64
    ...
                        mxBean.addCompactionEndOp(reader.getTopic(), false);        // :76

Compactor.java:58-80 → CompactorMXBeanImpl.addCompactionEndOp() → CompactionRecord.compactionFailedCount.

Any failure of RawReader.create() — lookup, connection, authorization, topic-not-found, a client-side error — happens before addCompactionStartOp, so it increments neither counter. The only trace is the Compaction failure. WARN in PersistentTopic's currentCompaction.whenComplete(...) (PersistentTopic.java:4921 on master).

Why it matters

pulsar_compaction_failed_count == 0 is routinely read as "no compaction attempt has failed", and it is used that way when triaging compaction problems. It does not mean that. A topic whose compaction never gets as far as opening a reader looks identical to a topic that is not being compacted at all — both show zero for both counters.

The same applies to any subclass that does work before delegating to Compactor.compact().

Error messages

[<topic>] Compaction failure.

(WARN, from PersistentTopic.java:4921; no counter moves)

Reproducing the issue

Analysis is from code:

1. Trigger compaction on a topic where RawReader.create() will fail — for example revoke the broker client's permission to subscribe, or point compaction at a topic that is being deleted concurrently.
2. Observe pulsar_compaction_failed_count for that topic stays at 0 while the run clearly failed.

Additional information

Suggested fix: move mxBean.addCompactionStartOp(topic) ahead of RawReader.create() (the topic name is already available in compact(String topic)), and record a failure if the reader cannot be created. Alternatively add a distinct counter for setup failures so they are not silently invisible.

Are you willing to submit a PR?

• I'm willing to submit a PR!