ddl: sync the job access in failpoint mockParallelSameDDLJobTwice

[tangenta]

What problem does this PR solve?

Issue Number: close #42987

Problem Summary:

The job is modified in another goroutine at line 1055, and it is read at line 1060.

tidb/ddl/ddl.go

Lines 1054 to 1067 in c1ce67d

	task := &limitJobTask{job, make(chan error)}
	d.limitJobCh <- task
	failpoint.Inject("mockParallelSameDDLJobTwice", func(val failpoint.Value) {
	if val.(bool) {
	// The same job will be put to the DDL queue twice.
	job = job.Clone()
	task1 := &limitJobTask{job, make(chan error)}
	d.limitJobCh <- task1
	<-task.err
	// The second job result is used for test.
	task = task1
	}
	})

What is changed and how it works?

We should wait for limitJobCh's modification before reading the job.

Check List

Tests

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

Side effects

• Performance regression: Consumes more CPU
• Performance regression: Consumes more Memory
• Breaking backward compatibility

Documentation

• Affects user behaviors
• Contains syntax changes
• Contains variable changes
• Contains experimental features
• Changes MySQL compatibility

Release note

Please refer to Release Notes Language Style Guide to write a quality release note.

None

[ti-chi-bot]

[REVIEW NOTIFICATION]

This pull request has been approved by:

• hawkingrei
• wjhuang2016

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[hawkingrei]

/merge

[ti-chi-bot]

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

Commit hash: b87d0ae

[hawkingrei]

/retest

[ti-chi-bot]

@tangenta: 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
idc-jenkins-ci-tidb/unit-test	b87d0ae	link	true	/test unit-test

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