Fix a race condition in metaclient leader election

[xyuanlu]

Issues

• My PR addresses the following Helix issues and references them in the PR description:
  idle node in Meta client leader election #3059

Description

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  Fix a race condition in metaclient leader election.
  When the client gets an expired event, it need to re-register all user registered listener and its own re-election listeners before rectreate the leader node.

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Tests

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• new test added
• testClientLeadershipChangeListenersAfterExpire

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Code Quality

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[junkaixue]

Please give the a comprehensive description on PR for what to fix.

[LZD-PratyushBhatt]

Hey @xyuanlu thanks for this!
I have a question here.
what happens if listener re-subscription succeeds but participant node creation fails? What i mean is, currently, the flow would re-subscribe all listeners successfully, then if it fail to create the participant node (due to network issues, parent path missing, whatever), but still proceed to attempt leader node creation and touch the leader node. could this result in a participant becoming leader without being a valid member of the participant pool, leading to an invalid leader state that other participants don't recognize as legitimate?

[xyuanlu]

In handleConnectStateChanged, where we recreate participant node, re-subscribe listeners and touch leader node. I think if recreate participant node failed in line 425 and if it is not nodeAlreadyExist error, it would through exception out and exit before register listeners and touch leader node.

[LZD-PratyushBhatt]

That makes sense, Thanks!

[xyuanlu]

This change is ready to be merged. Approved by @junkaixue
Commit message "Fix a race condition in metaclient leader election."