tso: fix an invalid err != nil check

[JmPotato]

What problem does this PR solve?

Issue Number: close #8524.

pd/pkg/tso/keyspace_group_manager.go

Lines 1365 to 1384 in 1c1cd99

	var mergedTS time.Time
	for _, id := range mergeList {
	ts, err := kgm.tsoSvcStorage.LoadTimestamp(endpoint.KeyspaceGroupGlobalTSPath(id))
	if err != nil {
	log.Error("failed to load the keyspace group TSO",
	zap.String("member", kgm.tsoServiceID.ServiceAddr),
	zap.Uint32("merge-target-id", mergeTargetID),
	zap.Any("merge-list", mergeList),
	zap.Uint32("merge-id", id),
	zap.Time("ts", ts),
	zap.Error(err))
	break
	}
	if ts.After(mergedTS) {
	mergedTS = ts
	}
	}
	if err != nil {
	continue
	}

Since the err in the loop is a local variable, so the check at line 1382 will always be false before this fix.

What is changed and how does it work?

Use the continue label to retry the whole merge loop correctly.

Check List

Tests

• Unit test
• Integration test

Release note

None.

[lhy1024]

We also need to pick it to other branches and pd-cse.

[okJiang]

It is better to use continue xxxxLable in Line 1376 instead of break

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 77.45%. Comparing base (1c1cd99) to head (5b02529).
Report is 1 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #8523      +/-   ##
==========================================
+ Coverage   77.40%   77.45%   +0.04%     
==========================================
  Files         473      473              
  Lines       61934    61934              
==========================================
+ Hits        47941    47968      +27     
+ Misses      10409    10379      -30     
- Partials     3584     3587       +3     

Flag	Coverage Δ
unittests	77.45% <100.00%> (+0.04%)	⬆️

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

[ti-chi-bot]

[LGTM Timeline notifier]

Timeline:

• 2024-08-14 03:47:05.750903953 +0000 UTC m=+326110.454373594: ☑️ agreed by lhy1024.
• 2024-08-14 03:55:38.032198005 +0000 UTC m=+326622.735667645: ☑️ agreed by rleungx.

[ti-chi-bot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: lhy1024, okJiang, rleungx

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• OWNERS [lhy1024,rleungx]

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[ti-chi-bot]

In response to a cherrypick label: new pull request created to branch release-8.1: #8525.

[ti-chi-bot]

In response to a cherrypick label: new pull request created to branch release-7.5: #8526.

[JmPotato]

/cherry-pick release-8.3

[ti-chi-bot]

@JmPotato: new pull request created to branch release-8.3: #8527.

In response to this:

/cherry-pick release-8.3

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