fix PD peer svc update repeatly

[csuzhangxc]

What problem does this PR solve?

in #4973, we refactored the logic of PD service update, after that the PD peer service is updated repeatly.

I0519 09:53:29.198719       1 service_control.go:91] update Service: [default/tc-1-pd-peer] successfully, kind: TidbCluster, name: tc-1
I0519 09:54:29.199537       1 service_control.go:91] update Service: [default/tc-1-pd-peer] successfully, kind: TidbCluster, name: tc-1
I0519 09:55:29.220976       1 service_control.go:91] update Service: [default/tc-1-pd-peer] successfully, kind: TidbCluster, name: tc-1

This is because the service get from the cache may lack the pingcap.com/last-applied-configuration annotation as it is deleted by a previous reconcile. also see #462

tidb-operator/pkg/controller/service_control.go

Line 157 in e97423e

delete(oldSvc.Annotations, LastAppliedConfigAnnotation)

tidb-operator/pkg/controller/equality.go

Line 77 in e97423e

if lastAppliedConfig, ok := oldSvc.Annotations[LastAppliedConfigAnnotation]; ok {

What is changed and how does it work?

Code changes

• Has Go code change
• Has CI related scripts change

Tests

• Unit test
• E2E test
• Manual test
• No code

check not pd-peer service updated log after this fix.

Side effects

• Breaking backward compatibility
• Other side effects:

Related changes

• Need to cherry-pick to the release branch
• Need to update the documentation

Release Notes

Please refer to Release Notes Language Style Guide before writing the release note.

[ti-chi-bot]

[REVIEW NOTIFICATION]

This pull request has been approved by:

• hanlins
• liubog2008

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[codecov-commenter]

Codecov Report

Merging #5015 (7225b76) into master (e97423e) will increase coverage by 3.79%.
The diff coverage is 100.00%.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #5015      +/-   ##
==========================================
+ Coverage   56.95%   60.75%   +3.79%     
==========================================
  Files         243      247       +4     
  Lines       27490    30765    +3275     
==========================================
+ Hits        15658    18691    +3033     
- Misses      10302    10430     +128     
- Partials     1530     1644     +114     

Flag	Coverage Δ
e2e	30.79% <100.00%> (?)
unittest	56.96% <100.00%> (+<0.01%)	⬆️

[hanlins]

LGTM

[csuzhangxc]

/cherry-pick release-1.5

[ti-chi-bot]

@csuzhangxc: The label(s) /release-1.5 cannot be applied, because the repository doesn't have them.

In response to this:

/cherry-pick release-1.5

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[ti-chi-bot]

@csuzhangxc: new pull request created to branch release-1.5: #5122.

In response to this:

/cherry-pick release-1.5

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