election: fix the data race of the leadership resetting

[JmPotato]

Signed-off-by: JmPotato ghzpotato@gmail.com

What problem does this PR solve?

Fix the data race of the leadership resetting.

ref: https://ci2.pingcap.net/blue/rest/organizations/jenkins/pipelines/pd_test/runs/2203/nodes/50/steps/94/log/?start=0

What is changed and how it works?

Make sure the leadership is not expired before resetting it.

Check List

Tests

• Unit test
• Integration test

Release note

None.

[ti-chi-bot]

[REVIEW NOTIFICATION]

This pull request has been approved by:

• nolouch

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[rleungx]

Do we need to cherry-pick this PR?

[codecov]

Codecov Report

Merging #4102 (4daed02) into master (740fac1) will increase coverage by 0.12%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master    #4102      +/-   ##
==========================================
+ Coverage   74.40%   74.53%   +0.12%     
==========================================
  Files         258      258              
  Lines       26322    26323       +1     
==========================================
+ Hits        19585    19619      +34     
+ Misses       4986     4958      -28     
+ Partials     1751     1746       -5     

Flag	Coverage Δ
unittests	74.53% <100.00%> (+0.12%)	⬆️

Flags with carried forward coverage won't be shown. Click here to find out more.

Impacted Files	Coverage Δ
server/election/lease.go	85.71% <100.00%> (+1.58%)	⬆️
server/tso/allocator_manager.go	68.50% <100.00%> (+1.05%)	⬆️
server/tso/local_allocator.go	64.86% <0.00%> (-6.76%)	⬇️
pkg/dashboard/adapter/manager.go	78.72% <0.00%> (-4.26%)	⬇️
pkg/etcdutil/etcdutil.go	82.35% <0.00%> (-2.36%)	⬇️
server/election/leadership.go	82.41% <0.00%> (-2.20%)	⬇️
server/cluster/cluster.go	82.20% <0.00%> (-0.84%)	⬇️
server/core/storage.go	68.04% <0.00%> (-0.76%)	⬇️
server/grpc_service.go	46.93% <0.00%> (-0.35%)	⬇️
pkg/tempurl/tempurl.go	60.00% <0.00%> (ø)
... and 12 more

---

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[JmPotato]

Do we need to cherry-pick this PR?

I think cherry-pick is needed since this data race may affect the normal leadership campaign, i.e, cancel the context immediately after it's set.

[JmPotato]

/merge

[ti-chi-bot]

@JmPotato: It seems you want to merge this PR, I will help you trigger all the tests:

/run-all-tests

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[ti-chi-bot]

This pull request has been accepted and is ready to merge.

Commit hash: c21137a

[JmPotato]

/run-unit-test

[ti-chi-bot]

In response to a cherrypick label: new pull request created: #4108.