fix patching WorkloadRebalancer failed due to misuse of shared slices

[chaosi-zju]

What type of PR is this?

/kind bug

What this PR does / why we need it:

fix patching WorkloadRebalancer failed due to misuse of shared slices.

in previous code:

// get previous status and update basing on it
newStatus = rebalancer.Status
if len(newStatus.ObservedWorkloads) == 0 {
	newStatus = c.buildWorkloadRebalancerStatus(rebalancer)
}

actually newStatus and rebalancer.Status shared the slice of rebalancer.Status.ObservedWorkloads.

if this slice didn't change from nil to non-nil or occurs expansion, modification to newStatus will also be applied to the rebalancer.Status, which will lead to patch status failed.

Which issue(s) this PR fixes:

Fixes part of #4840

Special notes for your reviewer:

Does this PR introduce a user-facing change?:

[chaosi-zju]

@XiShanYongYe-Chang @RainbowMango

Hello, sorry for this bug, please help review once more.

In previous code:

// get previous status and update basing on it
newStatus = rebalancer.Status
if len(newStatus.ObservedWorkloads) == 0 {
	newStatus = c.buildWorkloadRebalancerStatus(rebalancer)
}

Actually newStatus and rebalancer.Status shared the slice of rebalancer.Status.ObservedWorkloads.

If this slice didn't change from nil to non-nil or occurs expansion, modification to newStatus will also be applied to the rebalancer.Status, which will lead to patch status ignored.

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 53.06%. Comparing base (6e5a602) to head (f672ffa).
Report is 20 commits behind head on master.

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Additional details and impacted files

@@            Coverage Diff             @@
##           master    #4875      +/-   ##
==========================================
+ Coverage   52.98%   53.06%   +0.08%     
==========================================
  Files         250      251       +1     
  Lines       20421    20387      -34     
==========================================
- Hits        10820    10819       -1     
+ Misses       8881     8854      -27     
+ Partials      720      714       -6     

Flag	Coverage Δ
unittests	53.06% <ø> (+0.08%)	⬆️

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[XiShanYongYe-Chang]

/lgtm

[chaosi-zju]

Is there any further comments? If any other suggestions, please let me know.

[RainbowMango]

@XiShanYongYe-Chang
If you want to get approval from someone else, you can cc or assign.

/assign

[XiShanYongYe-Chang]

If you want to get approval from someone else, you can cc or assign.

Got it.

[RainbowMango]

modification to newStatus will also be applied to the rebalancer.Status, which will lead to patch status failed.

Won't lead to patch failed, but non-ops, right? That means the patch will be ignored.

[chaosi-zju]

Won't lead to patch failed, but non-ops, right? That means the patch will be ignored.

yes, the patch will be ignored.

[RainbowMango]

/lgtm
/approve

[karmada-bot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: RainbowMango

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[RainbowMango]

What's the next PR blocked by this？

[chaosi-zju]

What's the next PR blocked by this？

#4860

[chaosi-zju]

What's the next PR blocked by this？

or #4876 if you prefer

[RainbowMango]

I can review them but please also ask for review from controller owners.

[chaosi-zju]

I can review them but please also ask for review from controller owners.

ok