tso: fix the corner case that may cause TSO fallback (#4885)

[ti-chi-bot]

This is an automated cherry-pick of #4885

Signed-off-by: JmPotato [email protected]

What problem does this PR solve?

Issue Number: Close #4884.

This bug will occur when the following conditions are met:

1. PD-0 resigns the leader.
2. PD-0 updates the TSO right after resetting it concurrently and leaves the non-empty TSO in memory.
3. PD-0 becomes the leader again.
4. PD-0 receives a TSO request after the leadership is available and before the TSO synchronization is done.
5. A smaller TSO will be generated with this request.
6. BOOM!!!

What is changed and how does it work?

tso: fix the corner case that may cause TSO fallback

Do not update the TSO in memory if it's zero.

Check List

Tests

• Unit test
• Integration test

Release note

Fix the corner case that may cause TSO fallback.

[ti-chi-bot]

[REVIEW NOTIFICATION]

This pull request has been approved by:

• JmPotato
• lhy1024

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[JmPotato]

Close since we don't cherry-pick PR for 5.4 anymore.

[nolouch]

why 5.4 no need to cherry-pick?

[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: b519e15

[ti-chi-bot]

@ti-chi-bot: Your PR was out of date, I have automatically updated it for you.

At the same time I will also trigger all tests for you:

/run-all-tests

If the CI test fails, you just re-trigger the test that failed and the bot will merge the PR for you after the CI passes.

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[codecov]

Codecov Report

Merging #4892 (0baaab6) into release-5.4 (273fe3d) will increase coverage by 0.13%.
The diff coverage is 75.00%.

@@               Coverage Diff               @@
##           release-5.4    #4892      +/-   ##
===============================================
+ Coverage        74.69%   74.83%   +0.13%     
===============================================
  Files              265      265              
  Lines            27964    27969       +5     
===============================================
+ Hits             20889    20930      +41     
+ Misses            5204     5184      -20     
+ Partials          1871     1855      -16     

Flag	Coverage Δ
unittests	74.83% <75.00%> (+0.13%)	⬆️

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

Impacted Files	Coverage Δ
server/server.go	71.68% <50.00%> (-0.18%)	⬇️
server/tso/tso.go	71.91% <100.00%> (+7.13%)	⬆️
server/region_syncer/server.go	81.41% <0.00%> (-5.13%)	⬇️
server/schedule/operator_controller.go	83.02% <0.00%> (-1.23%)	⬇️
server/election/leadership.go	77.31% <0.00%> (-1.04%)	⬇️
server/cluster/cluster.go	82.62% <0.00%> (-0.81%)	⬇️
server/core/storage.go	69.31% <0.00%> (-0.76%)	⬇️
client/client.go	72.83% <0.00%> (+0.10%)	⬆️
server/grpc_service.go	52.36% <0.00%> (+0.77%)	⬆️
... and 9 more

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