[improve][broker] Enhance compaction triggering with message check to avoid unnecessary operations

[coderzc]

Motivation

If no more message need to compact, then we should skip compaction task

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Matching PR in forked repository

PR in forked repository:

[3pacccccc]

I think lastDispatchablePosition might be null, so need to add null check to prevent NPE

[coderzc]

fix

[3pacccccc]

I'm wondering does lastDispatchablePosition.compareTo(lastCompactedPosition) > 0 really means need to triggering compaction? For example, I publish a message with no key present after last compaction, there's actually no need to triggering compaction as well.

[coderzc]

I'm wondering does lastDispatchablePosition.compareTo(lastCompactedPosition) > 0 really means need to triggering compaction? For example, I publish a message with no key present after last compaction, there's actually no need to triggering compaction as well.

if topicCompactionRetainNullKey=false, then a message with no key may be deleted during compaction, so trigger compaction task is meaningful

[3pacccccc]

I'm wondering does lastDispatchablePosition.compareTo(lastCompactedPosition) > 0 really means need to triggering compaction? For example, I publish a message with no key present after last compaction, there's actually no need to triggering compaction as well.

if topicCompactionRetainNullKey=false, then a message with no key may be deleted during compaction, so trigger compaction task is meaningful

OK, I get it, thank you, now LGTM