Remove duplicate subscribe in CallBackHandler

[xyuanlu]

Issues

• My PR addresses the following Helix issues and references them in the PR description:
  Remove duplicate subscribeForChanges in CallBackHandler #1503

Description

• Here are some details about my PR, including screenshots of any UI changes:

During code inspection, We found there are multiple subscribeForChanges being called in CallBackHandler.handelChildChange. This leads to longer time spend when process callbacks in zkClient, witch eventually leads to increased PendingCallback queue size.
This PR removes duplicate subscribeForChanges in CallBackHandler to improve performance.

Tests

• The following tests are written for this issue:

NA

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

[INFO] Tests run: 1244, Failures: 0, Errors: 0, Skipped: 0, Time elapsed: 5,043.226 s - in TestSuite
[INFO] 
[INFO] Results:
[INFO] 
[INFO] Tests run: 1244, Failures: 0, Errors: 0, Skipped: 0
[INFO] 
[INFO] ------------------------------------------------------------------------
[INFO] BUILD SUCCESS
[INFO] ------------------------------------------------------------------------
[INFO] Total time:  01:24 h
[INFO] Finished at: 2020-11-04T14:47:44-08:00
[INFO] ------------------------------------------------------------------------

Documentation (Optional)

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(Link the GitHub wiki you added)

Commits

• My commits all reference appropriate Apache Helix GitHub issues in their subject lines. In addition, my commits follow the guidelines from "How to write a good git commit message":
  1. Subject is separated from body by a blank line
  2. Subject is limited to 50 characters (not including Jira issue reference)
  3. Subject does not end with a period
  4. Subject uses the imperative mood ("add", not "adding")
  5. Body wraps at 72 characters
  6. Body explains "what" and "why", not "how"

Code Quality

• My diff has been formatted using helix-style.xml
  (helix-style-intellij.xml if IntelliJ IDE is used)

[junkaixue]

Can we add an unit test? For example, we invoke the with different changes, we can make sure all changes we subscribe back?

[zhangmeng916]

Did we try to get rid of async subscribe in this PR and do all subscription synchronously? I thought we only remove the extra one.

[xyuanlu]

TFTR. The duplicated subscribeForChanges in HandleChildChange and do resubscribe for child change in line 361. For data changes, this async subscribe is also duplicated since the path is already resubscribed in zkClient.

[xyuanlu]

Can we add an unit test? For example, we invoke the with different changes, we can make sure all changes we subscribe back?

TFTR. Let me try.

[zhangmeng916]

I think we try to get rid of this kind of thread sleep, and use verifier. But as this is legacy test, I think it's fine. You can let @kaisun2000 know.

[xyuanlu]

I am not sure if we have watcher or listener installation verifier. If not, may be larger change is needed.

[xyuanlu]

Thanks for offline discuss. Updated.

[junkaixue]

I dont have any concerns. But just make sure resolved other comments.

[xyuanlu]

This PR is ready to be merged. Approved by @dasahcc

Commit message:
Remove duplicate subscribe in CallBackHandler.handleChildChange()

Duplicate subscribes lead to longer time spend when process callbacks in zkClient, witch eventually leads to increased PendingCallback queue size. This PR removes duplicate subscribeForChanges in CallBackHandler to improve performance.