schedule: fix race for region scatter

[Yisaer]

Signed-off-by: yisaer disxiaofei@163.com

What problem does this PR solve?

As totalCountByStore calling the function getDistributionByGroupLocked, this function should be procteded by rw mutex otherwise it will cause data race panic

What is changed and how it works?

Add mutex to protect it.

Check List

Tests

• Unit test

Release note

Fix pd may get panic during scattering region

[ti-chi-bot]

[REVIEW NOTIFICATION]

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• HunDunDM
• rleungx

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[Yisaer]

/merge

[ti-chi-bot]

@Yisaer: 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: 2405e70

[codecov]

Codecov Report

Merging #3760 (2405e70) into master (3ba158c) will decrease coverage by 0.03%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master    #3760      +/-   ##
==========================================
- Coverage   75.09%   75.05%   -0.04%     
==========================================
  Files         245      245              
  Lines       24544    24546       +2     
==========================================
- Hits        18432    18424       -8     
- Misses       4496     4502       +6     
- Partials     1616     1620       +4     

Flag	Coverage Δ
unittests	75.05% <100.00%> (-0.04%)	⬇️

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

Impacted Files	Coverage Δ
server/schedule/region_scatterer.go	83.87% <100.00%> (-4.51%)	⬇️
server/tso/local_allocator.go	80.55% <0.00%> (-9.73%)	⬇️
server/region_syncer/server.go	83.33% <0.00%> (-6.07%)	⬇️
server/statistics/hot_cache_task.go	71.92% <0.00%> (-3.51%)	⬇️
server/encryptionkm/key_manager.go	71.78% <0.00%> (-1.66%)	⬇️
server/tso/tso.go	70.78% <0.00%> (-1.13%)	⬇️
server/server.go	71.62% <0.00%> (-0.76%)	⬇️
server/cluster/coordinator.go	72.42% <0.00%> (-0.72%)	⬇️
server/tso/allocator_manager.go	79.63% <0.00%> (-0.67%)	⬇️
... and 10 more

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[ti-chi-bot]

In response to a cherrypick label: new pull request created: #3761.

[ti-chi-bot]

In response to a cherrypick label: new pull request created: #3762.

[ti-chi-bot]

In response to a cherrypick label: new pull request created: #3763.