tso: fix an invalid err != nil check (#8523)

[ti-chi-bot]

This is an automated cherry-pick of #8523

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.

[ti-chi-bot]

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This pull-request has been approved by: JmPotato

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[codecov]

Codecov Report

Attention: Patch coverage is 50.00000% with 1 line in your changes missing coverage. Please review.

Project coverage is 73.76%. Comparing base (10ecdbe) to head (689a480).

Additional details and impacted files

@@               Coverage Diff               @@
##           release-8.1    #8525      +/-   ##
===============================================
+ Coverage        73.67%   73.76%   +0.08%     
===============================================
  Files              437      437              
  Lines            48750    48749       -1     
===============================================
+ Hits             35918    35960      +42     
+ Misses            9772     9735      -37     
+ Partials          3060     3054       -6     

Flag	Coverage Δ
unittests	73.76% <50.00%> (+0.08%)	⬆️

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[lhy1024]

Do we need to pick it to release-8.3?