region_scatterer: fix the bug that could generate schedule with too few peers (#4570)

[ti-chi-bot]

This is an automated cherry-pick of #4570

What problem does this PR solve?

close #4565

What is changed and how it works?

If an originPeer is already in selectStore, continue directly. Otherwise, it will end up picking a targetPeer which is either itself or some other store that is not in the initial peer list. Since entering the select phase, it is guaranteed that it is not in the selectStore, so it is always guaranteed that the worst-case selection itself is feasible at this time. In the end, the number of peers before and after can be guaranteed to remain unchanged.

Check List

Tests

• Unit test

When not modified, the following operator may be generated:

scatter-region {rm peer: store [3]} (kind:region,leader, region:201(0,0), createAt:2022-01-13 18:24:32.375399 +0800 CST m=+0.014571751, startAt:0001-01-01 00:00:00 +0000 UTC, currentStep:0, steps:[transfer leader from store 3 to store 4, remove peer on store 3])

Code changes

Side effects

Related changes

• Need to cherry-pick to the release branch

Release note

Fix the bug that the region scatterer may generate the schedule with too few peers.

[ti-chi-bot]

[REVIEW NOTIFICATION]

This pull request has been approved by:

• HunDunDM
• nolouch

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[nolouch]

/merge

[ti-chi-bot]

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

/run-all-tests

Details

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[ti-chi-bot]

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

Details

Commit hash: 05c49e0

[nolouch]

/run-all-tests

[codecov]

Codecov Report

Merging #4581 (05c49e0) into release-5.4 (9413840) will decrease coverage by 0.08%.
The diff coverage is 100.00%.

@@               Coverage Diff               @@
##           release-5.4    #4581      +/-   ##
===============================================
- Coverage        74.77%   74.68%   -0.09%     
===============================================
  Files              265      265              
  Lines            27959    27963       +4     
===============================================
- Hits             20905    20885      -20     
- Misses            5193     5213      +20     
- Partials          1861     1865       +4     

Flag	Coverage Δ
unittests	74.68% <100.00%> (-0.09%)	⬇️

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

Impacted Files	Coverage Δ
server/schedule/region_scatterer.go	78.99% <100.00%> (-3.92%)	⬇️
pkg/errs/errs.go	75.00% <0.00%> (-25.00%)	⬇️
server/tso/local_allocator.go	64.86% <0.00%> (-6.76%)	⬇️
server/statistics/hot_cache_task.go	81.81% <0.00%> (-4.55%)	⬇️
server/schedulers/random_merge.go	60.00% <0.00%> (-3.34%)	⬇️
server/region_syncer/client.go	83.59% <0.00%> (-1.57%)	⬇️
server/schedule/operator_controller.go	83.02% <0.00%> (-1.23%)	⬇️
server/tso/allocator_manager.go	64.72% <0.00%> (-1.00%)	⬇️
pkg/btree/btree.go	85.78% <0.00%> (-0.97%)	⬇️
... and 14 more

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