DirectWriter only acquire buffer when need write.

[lifepuzzlefun]

Motivation

small fix: when ByteBuf size is zero. the acquired buffer is not released.

[codecov-commenter]

Codecov Report

Merging #3786 (e6b96f4) into master (2381d9b) will increase coverage by 0.08%.
The diff coverage is 100.00%.

@@             Coverage Diff              @@
##             master    #3786      +/-   ##
============================================
+ Coverage     68.08%   68.17%   +0.08%     
- Complexity     6712     6725      +13     
============================================
  Files           473      473              
  Lines         40952    40952              
  Branches       5236     5236              
============================================
+ Hits          27882    27918      +36     
+ Misses        10808    10782      -26     
+ Partials       2262     2252      -10     

Flag	Coverage Δ
bookie	39.73% <100.00%> (+0.03%)	⬆️
client	44.31% <0.00%> (+0.05%)	⬆️
remaining	29.70% <0.00%> (+0.07%)	⬆️
replication	41.36% <0.00%> (+0.03%)	⬆️
tls	21.09% <0.00%> (+0.09%)	⬆️

Flags with carried forward coverage won't be shown. Click here to find out more.

Impacted Files	Coverage Δ
...bookie/storage/directentrylogger/DirectWriter.java	75.29% <100.00%> (ø)
...ookkeeper/bookie/UncleanShutdownDetectionImpl.java	75.00% <0.00%> (-6.25%)	⬇️
...va/org/apache/bookkeeper/bookie/SkipListArena.java	73.01% <0.00%> (-4.77%)	⬇️
...keeper/meta/AbstractHierarchicalLedgerManager.java	72.97% <0.00%> (-4.06%)	⬇️
...keeper/replication/AuditorCheckAllLedgersTask.java	53.74% <0.00%> (-2.73%)	⬇️
...er/util/collections/ConcurrentLongLongHashMap.java	78.71% <0.00%> (-1.99%)	⬇️
...keeper/bookie/storage/ldb/TransientLedgerInfo.java	85.24% <0.00%> (-1.64%)	⬇️
...rg/apache/bookkeeper/bookie/IndexInMemPageMgr.java	86.87% <0.00%> (-0.39%)	⬇️
...okkeeper/meta/ZkLedgerUnderreplicationManager.java	60.85% <0.00%> (+0.34%)	⬆️
...in/java/org/apache/bookkeeper/bookie/FileInfo.java	76.75% <0.00%> (+0.36%)	⬆️
... and 12 more

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[StevenLuMT]

LGTM