[ISSUE #S2] Guard against a missing ext unit in ConsumeQueue.estimateMessageCount

[zjncs]

Motivation

ConsumeQueue.estimateMessageCount dereferences the ext unit unconditionally:

ConsumeQueueExt.CqExtUnit ext = null;
if (isExtWriteEnable()) {
    ext = consumeQueueExt.get(tagCode);
    tagCode = ext.getTagsCode();   // NPE when get() returns null
}

The stored tagsCode can legitimately be a raw (positive) value rather than an ext address:

• entries written while the ext was disabled and the config was enabled later, or
• entries persisted by the save-tagsCode-only fallback in putMessagePositionInfoWrapper when consumeQueueExt.put fails (logged as "Save consume queue extend fail, So just save tagsCode").

For a raw code ConsumeQueueExt.get returns null (it rejects non-ext addresses), so estimateMessageCount throws an NPE. This path is live in production: DefaultMessageStore.estimateMessageCount → ConsumerLagCalculator (broker consumer-lag metrics with a filter). MessageFilter.isMatchedByConsumeQueue already accepts a null ext unit ("message is before consumer"), so only this caller assumes non-null.

Modifications

• Null-guard the ext unit: keep the raw tagCode when get returns null and pass the null ext to the filter, matching how the rest of the code treats a missing ext.

Verification

Fail-before (new test on unpatched code):

ConsumeQueueTest.testEstimateMessageCountWhenExtUnitMissing:500 » NullPointer
  Cannot invoke "ConsumeQueueExt.getTagsCode()" because "ext" is null

The test writes one entry with the ext disabled (raw tagsCode persisted), re-enables the ext, then estimates the count — exactly the mixed-content state described above.

Pass-after:

mvn -pl store test -Dtest='ConsumeQueueTest#testEstimateMessageCountWhenExtUnitMissing+testCorrectMinOffset'
Tests run: 2, Failures: 0, Errors: 0, Skipped: 0

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[RockteMQ-AI]

Summary

Correct NPE fix for a real production path — estimateMessageCount dereferences the ext unit unconditionally, but ConsumeQueueExt.get() returns null for raw tagsCodes in mixed-content states. The null-guard preserves the raw tagCode and passes null ext to the filter, matching how the rest of the codebase handles missing ext units. Test reproduces the exact mixed-content scenario.

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