tso: Fix keyspace-group split causing keyspace to fallback to group 0

[ystaticy]

What problem does this PR solve?

The keyspace tso group does not fall back to group 0 and is updated directly to the target group

Issue Number: Close #9822

What is changed and how does it work?

Save the split target keyspace group first, then save the split source keyspace group. The order matters: if we save splitSourceKg first (which removes some keyspaces from it), there will be a brief moment where those keyspaces don't belong to any group, causing them to fallback to group 0. By saving splitTargetKg first (which contains the split keyspaces), we ensure the keyspaces always belong to a valid group during the transition.

1.During the split process, save the target group first, then the source group. This avoids deleting the keyspace-to-group mapping from memory (caused by saving the source group first) and thus prevents falling back to group 0.
2. The watch process ensures order based on the revision, and the processing of putFn is also serial.
3. Updating the target group first does not trigger the issue of "premature" updates to the keyspace-group ID mapping. This is because during the split process, the client will keep retrying until the split is finally completed, after which the client can send requests normally.
4. The delayBeforeCheckingElectionMember failpoint is used to lengthen the split process, making it easier to reproduce the scenario where TSO requests are received between the two keyspace group save operations during the split.

Check List

Tests

• Unit test
• Integration test
• Manual test (add detailed scripts or steps below)
• No code

Code changes

• Has the configuration change
• Has HTTP API interfaces changed (Don't forget to add the declarative for the new API)
• Has persistent data change

Side effects

• Possible performance regression
• Increased code complexity
• Breaking backward compatibility

Related changes

• PR to update pingcap/docs/pingcap/docs-cn:
• PR to update pingcap/tiup:
• Need to cherry-pick to the release branch

Release note

tso: Fix keyspace-group split operation fallback to group 0

[ti-chi-bot]

Hi @ystaticy. Thanks for your PR.

I'm waiting for a tikv member to verify that this patch is reasonable to test. If it is, they should reply with /ok-to-test on its own line. Until that is done, I will not automatically test new commits in this PR, but the usual testing commands by org members will still work. Regular contributors should join the org to skip this step.

Once the patch is verified, the new status will be reflected by the ok-to-test label.

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[JmPotato]

/ok-to-test

[ystaticy]

/test pull-unit-test-next-gen

[rleungx]

/retest

[codecov]

Codecov Report

❌ Patch coverage is 83.33333% with 3 lines in your changes missing coverage. Please review.
✅ Project coverage is 76.97%. Comparing base (33d00c5) to head (9f088f1).
⚠️ Report is 1 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #9823      +/-   ##
==========================================
+ Coverage   76.82%   76.97%   +0.14%     
==========================================
  Files         491      491              
  Lines       78440    78454      +14     
==========================================
+ Hits        60261    60389     +128     
+ Misses      14478    14388      -90     
+ Partials     3701     3677      -24     

Flag	Coverage Δ
unittests	76.97% <83.33%> (+0.14%)	⬆️

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[ystaticy]

/test pull-unit-test-next-gen

[ti-chi-bot]

@ystaticy: The following test failed, say /retest to rerun all failed tests or /retest-required to rerun all mandatory failed tests:

Test name	Commit	Details	Required	Rerun command
non-block/pull-unit-test-next-gen	9f088f1	link	false	/test pull-unit-test-next-gen

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[ti-chi-bot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: JmPotato, rleungx

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[ti-chi-bot]

[LGTM Timeline notifier]

Timeline:

• 2025-10-13 10:25:04.740009694 +0000 UTC m=+89810.817262255: ☑️ agreed by rleungx.
• 2025-10-14 03:01:16.03044973 +0000 UTC m=+149582.107702290: ☑️ agreed by JmPotato.