executor: fix a bug on generated column cause wrong read result

[tiancaiamao]

What problem does this PR solve?

Issue Number: close #33038

Problem Summary:

What is changed and how it works?

func (us *UnionScanExec) Next(ctx context.Context, req *chunk.Chunk) error {
    ...
    for i, batchSize := 0, req.Capacity(); i < batchSize; i++ {
	    ....
	    matched, _, err := expression.EvalBool(us.ctx, us.conditionsWithVirCol, mutableRow.ToRow())
	    if err != nil {
		    return err
	    }
	    if matched {
		    req.AppendRow(mutableRow.ToRow())
	    }
    }
    return nil
}

When a batch of data (32 rows by default) is processed, but no row match, an empty chunk is returned.
The parent executor treat empty chunk as the reading process has finished.
So the Next() will never be called again!
Only the first batch of chunk is handled, all the others are ignored.

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

Fix a bug that reading from a table with generated column may get wrong result.
It happens when all the following conditions hold:
- A UnionScan executor is used
- The query comes with a filter condition on the generated column
- No row from the first batch of the chunk rows match the filter condition 

[ti-chi-bot]

[REVIEW NOTIFICATION]

This pull request has been approved by:

• lcwangchao
• wjhuang2016

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[sre-bot]

Code Coverage Details: https://codecov.io/github/pingcap/tidb/commit/8d278cc445503e8674c68a756485abed56565466

[tiancaiamao]

/merge

[ti-chi-bot]

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

Commit hash: b5bf9b7

[tiancaiamao]

/merge

[tiancaiamao]

/run-unit-test

[tiancaiamao]

/run-unit-test

[tiancaiamao]

/run-mysql-test

[tiancaiamao]

/run-unit-test

[ti-srebot]

cherry pick to release-4.0 in PR #33099

[ti-srebot]

cherry pick to release-5.0 in PR #33100

[ti-srebot]

cherry pick to release-5.1 in PR #33101

[ti-srebot]

cherry pick to release-5.2 in PR #33102

[ti-srebot]

cherry pick to release-5.3 in PR #33103

[ti-srebot]

cherry pick to release-5.4 in PR #33104

[tiancaiamao]

PTAL @wjhuang2016 @lcwangchao