[fix][ml] Avoid NPE when getCurrentLedger() returns null

[lhotari]

Motivation

In branch-3.0, tests are failing with logs about NPEs such as

  java.lang.NullPointerException: Cannot invoke "org.apache.bookkeeper.client.LedgerHandle.getId()" because the return value of "org.apache.bookkeeper.mledger.impl.ManagedLedgerImpl.getCurrentLedger()" is null
  	at org.apache.bookkeeper.mledger.impl.ManagedCursorImpl.estimateEntryCountBySize(ManagedCursorImpl.java:3705)
  	at org.apache.bookkeeper.mledger.impl.ManagedCursorImpl.applyMaxSizeCap(ManagedCursorImpl.java:3691)
  	at org.apache.bookkeeper.mledger.impl.ManagedCursorImpl.asyncReadEntriesWithSkip(ManagedCursorImpl.java:853)
  	at org.apache.bookkeeper.mledger.impl.ManagedCursorImpl.asyncReadEntries(ManagedCursorImpl.java:840)

This is due to #23931 changes.

Modifications

Handle the case when getCurrentLedger() returns null

Documentation

• doc
• doc-required
• doc-not-needed
• doc-complete

[poorbarcode]

I am considering why managedLedger.currentLedger will be null, could you explain the test's name?

[lhotari]

@poorbarcode In branch-3.0, there are at least 2 tests where this can be seen in the output.
org.apache.pulsar.broker.service.ClusterMigrationTest#testClusterMigration is one. The issue is #24038

[lhotari]

It seems that currentLedger could be null immediately after the ledger has been created, but not after that.

[codecov-commenter]

Codecov Report

Attention: Patch coverage is 50.00000% with 1 line in your changes missing coverage. Please review.

Project coverage is 74.19%. Comparing base (bbc6224) to head (09c29bc).
Report is 948 commits behind head on master.

Files with missing lines	Patch %	Lines
...che/bookkeeper/mledger/impl/ManagedCursorImpl.java	50.00%	0 Missing and 1 partial ⚠️

Additional details and impacted files

@@             Coverage Diff              @@
##             master   #24058      +/-   ##
============================================
+ Coverage     73.57%   74.19%   +0.62%     
+ Complexity    32624    32405     -219     
============================================
  Files          1877     1861      -16     
  Lines        139502   144210    +4708     
  Branches      15299    16433    +1134     
============================================
+ Hits         102638   107003    +4365     
+ Misses        28908    28748     -160     
- Partials       7956     8459     +503     

Flag	Coverage Δ
inttests	26.62% <50.00%> (+2.03%)	⬆️
systests	23.11% <50.00%> (-1.21%)	⬇️
unittests	73.70% <50.00%> (+0.85%)	⬆️

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

Files with missing lines	Coverage Δ
...che/bookkeeper/mledger/impl/ManagedCursorImpl.java	79.41% <50.00%> (+0.11%)	⬆️

... and 1055 files with indirect coverage changes

[lhotari]

Closing this PR since @poorbarcode will create a new PR addressing the root cause.