[Not For Review] [apache/helix] -- PreferenceList Ordering Changes during Maintenance Mode

[himanshukandwal]

Issues

• My PR addresses the following Helix issues and references them in the PR description:
  Demonstrates how preference list order changes during maintenance mode.

Description

• Here are some details about my PR, including screenshots of any UI changes:
  When entering maintenance-mode, we observed that instance order in preference list changes by PreferenceListNodeComparator as it uses the order in which the current-state is materialized in array list.
  For instance:

Lets say we have 3 hosts, and here's the Current state:

{
   A: LEADER,
   B: FOLLOWER
   C: FOLLOWER
 }

Now when we create a preference-list using new ArrayList(currentState.keySet), the order in array-list depends on the hash order the keys (A, B, C). With sorting using PreferenceListNodeComparator, we put Leader at the front and ordering among Followers can be anything and could either be [A, B, C] or [A, C, B ]

Tests

• The following tests are written for this issue:

• Added TestMaintenanceRebalancer.testComputeIdealState

There 4 tests here:

Test Case 1,2,3,4: Shows how ordering changes from existing one.
current: [A, B, C] -> updated: [A, C, B]

• The following is the result of the "mvn test" command on the appropriate module:

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Changes that Break Backward Compatibility (Optional)

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Documentation (Optional)

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Commits

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Code Quality

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