ddl: remove reorg ctx when current instance is not owner

[tangenta]

Cherry-pick #55049 to v7.5.

What problem does this PR solve?

Issue Number: close #54897

Problem Summary:

Timeline:

1. tidb-0 run ingest backfill workers as the DDL owner.
2. Inject network latency to tidb-0.
3. tidb-1 get DDL owner, and switch to merge temp index step.
4. Inject network latency to tidb-1. tidb-1's merge temp index backfill workers failed with "DDL is not owner"
5. tidb-0 get DDL owner AGAIN, but reorgCtx is the one added in step(1).
6. Merge temp index step is skipped because the reorgCtx is done.

What changed and how does it work?

Remove reorgCtx before quiting because of DDLNotOwner error.

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]

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[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

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Additional details and impacted files

@@               Coverage Diff                @@
##             release-7.5     #55050   +/-   ##
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  Lines                  ?     411664           
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  Hits                   ?     296423           
  Misses                 ?      95343           
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Flag	Coverage Δ
unit	72.0060% <100.0000%> (?)

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Components	Coverage Δ
dumpling	52.9400% <0.0000%> (?)
parser	∅ <0.0000%> (?)
br	53.5383% <0.0000%> (?)

[ti-chi-bot]

[LGTM Timeline notifier]

Timeline:

• 2024-07-30 10:22:31.468435838 +0000 UTC m=+264867.748483902: ☑️ agreed by lance6716.
• 2024-07-30 10:47:53.523516803 +0000 UTC m=+266389.803564873: ☑️ agreed by D3Hunter.

[tangenta]

/approve

[ti-chi-bot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: D3Hunter, lance6716, tangenta

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