operator: avoid unnecessary reconcile

[zhzhuang-zju]

What type of PR is this? /kind cleanup

What this PR does / why we need it: Currently, default values for Karmada CRs are set during the operator’s reconcile loop. This introduces two issues:

1. Unnecessary reconciliation: When a Karmada CR is updated but becomes identical to its previous state after applying defaults, the controller still performs a full reconcile twice—once for the original change and again due to the update caused by setting defaults.

2. Unstable CR status: The operator sets the Ready condition of the Karmada CR to False at the start of each reconcile and updates it to True upon successful completion. For example, during initial deployment, the first reconcile succeeds and sets Ready=True. However, because the reconcile process itself updates the CR (by filling in default values), it triggers another reconcile, which resets Ready=False, causing flickering in the status. This also causes occasional flakiness in the operator's e2e tests.

To address this, I propose filtering out such "no-op" updates in the UpdateEvent handler—skipping reconciliation if the change is solely due to defaulting and results in no semantic difference.

Note that #6819 would also resolve this problem by moving defaulting logic into an admission controller. Therefore, I’ve left a TODO: once admission-based defaulting is implemented, this workaround can be removed.

Which issue(s) this PR fixes:

Part of #6841

Special notes for your reviewer:

Does this PR introduce a user-facing change?:

[gemini-code-assist[bot]]

Summary of Changes

Hello @zhzhuang-zju, I'm Gemini Code Assist[^1]! I'm currently reviewing this pull request and will post my feedback shortly. In the meantime, here's a summary to help you and other reviewers quickly get up to speed!

This pull request optimizes the Karmada operator's reconciliation process by preventing redundant reconciliations triggered solely by the application of default values to Custom Resources. By applying defaults before comparing the old and new states of a CR, the operator can now accurately identify and skip updates that do not introduce semantic changes, thereby improving performance and eliminating the flickering Ready status observed in Karmada CRs.

Highlights

• Prevent Unnecessary Reconciliations: The operator currently performs redundant reconciliations when default values are applied to Karmada Custom Resources (CRs), even if the CR's semantic state remains unchanged. This PR addresses this by filtering out such "no-op" updates.
• Stabilize CR Status: The current defaulting mechanism causes the Ready condition of Karmada CRs to flicker between True and False due to repeated reconciliations. This change aims to stabilize the CR status by avoiding these unnecessary updates.
• Update Event Filtering Logic: The onKarmadaUpdate handler now applies default values to both the old and new objects before performing a deep comparison of their Spec fields. If the specs are identical after defaulting, the reconciliation is skipped.

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[zhzhuang-zju]

cc @jabellard @RainbowMango

[codecov-commenter]

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Codecov Report

:x: Patch coverage is 0% with 9 lines in your changes missing coverage. Please review. :white_check_mark: Project coverage is 46.39%. Comparing base (f26d76a) to head (603e4c9). :warning: Report is 2 commits behind head on master.

Files with missing lines	Patch %	Lines
operator/pkg/controller/karmada/controller.go	0.00%	9 Missing :warning:
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Additional details and impacted files

@@            Coverage Diff             @@
##           master    #6908      +/-   ##
==========================================
+ Coverage   46.37%   46.39%   +0.01%     
==========================================
  Files         698      698              
  Lines       47763    47767       +4     
==========================================
+ Hits        22152    22160       +8     
+ Misses      23940    23938       -2     
+ Partials     1671     1669       -2     

Flag	Coverage Δ
unittests	46.39% <0.00%> (+0.01%)	:arrow_up:

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