ddl: reuse backend for DXF subtasks of same step

[D3Hunter]

What problem does this PR solve?

Issue Number: ref #57229, ref #57497

Problem Summary:

What changed and how does it work?

• the local backend can be created once and shared by many subtasks at the same time,
• solve some data race issues

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

[tiprow]

Hi @D3Hunter. Thanks for your PR.

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[codecov]

Codecov Report

Attention: Patch coverage is 33.03571% with 75 lines in your changes missing coverage. Please review.

Project coverage is 74.8050%. Comparing base (cf4d252) to head (12935e8).
Report is 12 commits behind head on master.

Additional details and impacted files

@@               Coverage Diff                @@
##             master     #59165        +/-   ##
================================================
+ Coverage   73.0909%   74.8050%   +1.7140%     
================================================
  Files          1686       1735        +49     
  Lines        466442     476782     +10340     
================================================
+ Hits         340927     356657     +15730     
+ Misses       104561      97659      -6902     
- Partials      20954      22466      +1512     

Flag	Coverage Δ
integration	48.7408% <8.0357%> (?)
unit	72.2523% <33.0357%> (-0.0322%)	⬇️

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

Components	Coverage Δ
dumpling	52.6910% <ø> (ø)
parser	∅ <ø> (∅)
br	60.3823% <ø> (+15.0706%)	⬆️

[D3Hunter]

fixed in #57249

[D3Hunter]

/retest

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/retest

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[lance6716]

rest lgtm

[ti-chi-bot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: lance6716, tangenta

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• OWNERS [lance6716,tangenta]
• pkg/ddl/OWNERS [lance6716,tangenta]
• pkg/lightning/OWNERS [lance6716]
• pkg/meta/OWNERS [lance6716,tangenta]

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[ti-chi-bot]

[LGTM Timeline notifier]

Timeline:

• 2025-01-24 07:00:31.857161931 +0000 UTC m=+423359.188081335: ☑️ agreed by tangenta.
• 2025-01-24 07:37:23.705167259 +0000 UTC m=+425571.036086663: ☑️ agreed by lance6716.