planner: activate stale-read txn when autocommit=0

[wfxr]

Issue Number: close #64198

Problem Summary:

When autocommit=0 and tidb_read_staleness is set, each SELECT statement recalculates its own read_ts instead of reusing the same snapshot. This causes inconsistent reads across multiple statements within what users expect to be a single transaction.

What changed and how does it work?

Changed EnterNewTxnWithReplaceProvider to EnterNewTxnWithBeginStmt when !IsAutocommit() in updateStateFromStaleReadProcessor(). This triggers activateStaleTxn() in StalenessTxnContextProvider.This ensures:

• InTxn() becomes true after the first SELECT
• Subsequent reads reuse the same snapshot (StartTS)
• Write statements are rejected (existing behavior)
• COMMIT/ROLLBACK ends the transaction normally

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.

Fix an issue that stale read with `tidb_read_staleness` does not provide consistent snapshot when `autocommit=0`, causing each SELECT to use a different `read_ts` instead of reusing the same snapshot within the transaction.

[ti-chi-bot]

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If you want CI signal for your change, please convert it to an actual PR.
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[wfxr]

/test all

[wfxr]

/retest

[wfxr]

/test all

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 77.6133%. Comparing base (060097f) to head (a487a74).
⚠️ Report is 1 commits behind head on master.

Additional details and impacted files

@@               Coverage Diff                @@
##             master     #65908        +/-   ##
================================================
- Coverage   77.7756%   77.6133%   -0.1623%     
================================================
  Files          2001       1923        -78     
  Lines        545524     533113     -12411     
================================================
- Hits         424285     413767     -10518     
+ Misses       119577     119340       -237     
+ Partials       1662          6      -1656     

Flag	Coverage Δ
integration	41.5259% <100.0000%> (-6.6389%)	⬇️
unit	76.7337% <100.0000%> (+0.3211%)	⬆️

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

Components	Coverage Δ
dumpling	56.7974% <ø> (ø)
parser	∅ <ø> (∅)
br	48.8491% <ø> (-12.1366%)	⬇️

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[wfxr]

/test all

[wfxr]

/retest

[ti-chi-bot]

@wfxr: 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
pull-unit-test-next-gen	a487a74	link	true	/test pull-unit-test-next-gen

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