syncer(dm): prevent checkpoint flush after BeginTx error

[GMHDBJD]

What problem does this PR solve?

Issue Number: close #12626

What is changed and how it works?

This PR fixes a DM correctness issue where checkpoint flush could still advance after a downstream transaction BeginTx failure.

Changes:

• Introduce a shared downstream execution error predicate that includes ErrDBExecuteFailedBegin together with existing ErrDBExecuteFailed and ErrDBUnExpect.
• Use the predicate in sync, async, and checkpoint flush worker guards so checkpoint flush is skipped after downstream begin failures.
• Treat begin failures as downstream execution errors when flushing the safe-mode exit point on task exit.
• Avoid running key-not-found diagnostics after ExecuteSQL already returned an execution error.
• Add a regression test for ErrDBExecuteFailedBegin(sql.ErrConnDone) to ensure checkpoint flush is skipped.

Check List

Tests

• Unit test

go test ./dm/syncer -run TestCheckpointFlushWorkerSkipsCheckpointOnBeginError -count=1
go test ./dm/syncer -run 'Test(CheckpointFlushWorkerSkipsCheckpointOnBeginError|JudgeKeyNotFound|EnableSafeModeInitializationPhase)$' -count=1

Also ran:

make fmt
git diff --check

Questions

Will it cause performance regression or break compatibility?

No. The change only broadens existing checkpoint-blocking error classification to include downstream transaction begin failures and suppresses a misleading diagnostic after failed execution.

Do you need to update user documentation, design documentation or monitoring documentation?

No.

Release note

Fix a DM correctness issue that could flush checkpoints after downstream transaction begin failures.

[ti-chi-bot]

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[gemini-code-assist]

Code Review

This pull request centralizes downstream execution error handling by introducing the isDownstreamExecutionError helper function, which now includes terror.ErrDBExecuteFailedBegin in its checks. This ensures that checkpoint flushes are correctly skipped when a downstream transaction fails to begin, preventing potential data inconsistency. The changes also include a new unit test to verify this behavior and a safety check in dml_worker.go to ensure key-not-found logic only executes when no other errors are present. I have no feedback to provide as there were no review comments to assess.

[ti-chi-bot]

@GMHDBJD: 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
pull-verify	ad6182a	link	true	/test pull-verify
pull-dm-integration-test	ad6182a	link	true	/test pull-dm-integration-test
pull-dm-integration-test-next-gen	ad6182a	link	false	/test pull-dm-integration-test-next-gen

Full PR test history. Your PR dashboard.

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