session: make TxnInfo() return even if process info is empty (#57044)

[ti-chi-bot]

This is an automated cherry-pick of #57044

What problem does this PR solve?

Issue Number: close #57043

Problem Summary:

tidb/pkg/session/session.go

Lines 4026 to 4030 in c240fdf

	txnInfo := tmp.TxnInfo()
	if txnInfo != nil {
	startTS = txnInfo.StartTS
	processInfoID = txnInfo.ConnectionID
	}

TxnInfo() could be nil if process info is not set(see the implementation of TxnInfo()). Such sessions are commonly used in DDL/DXF:

tidb/pkg/ddl/backfilling_operators.go

Lines 533 to 539 in c240fdf

	sessCtx, err := w.sessPool.Get()
	if err != nil {
	logutil.Logger(w.ctx).Error("tableScanWorker get session from pool failed", zap.Error(err))
	w.ctx.onError(err)
	return
	}
	w.se = session.NewSession(sessCtx)

And they only invoke session.Begin() before using:

tidb/pkg/ddl/index_cop.go

Lines 43 to 48 in c240fdf

	func wrapInBeginRollback(se *sess.Session, f func(startTS uint64) error) error {
	err := se.Begin(context.Background())
	if err != nil {
	return errors.Trace(err)
	}
	defer se.Rollback()

As a result, the startTS from internal DDL sessions will not be reported to etcd, and GC safepoint will be advanced unexpectedly.

What changed and how does it work?

Make TxnInfo() return even if process info is empty.

Check List

Tests

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

  • I checked and no code files have been changed.

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]

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[ti-chi-bot]

@ti-chi-bot: The following tests 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	bb692bc	link	true	/test unit-test
idc-jenkins-ci-tidb/check_dev_2	bb692bc	link	true	/test check-dev2
idc-jenkins-ci-tidb/mysql-test	bb692bc	link	true	/test mysql-test
idc-jenkins-ci-tidb/build	bb692bc	link	true	/test build
idc-jenkins-ci-tidb/check_dev	bb692bc	link	true	/test check-dev

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