br: fix lightning split large csv file error and adjust s3 seek result (#27769)

[ti-srebot]

cherry-pick #27769 to release-5.2
You can switch your code base to this Pull Request by using git-extras:

# In tidb repo:
git pr https://github.com/pingcap/tidb/pull/33883

After apply modifications, you can push your change to this PR via:

git push git@github.com:ti-srebot/tidb.git pr/33883:release-5.2-9146cba1ddcc

---

What problem does this PR solve?

Fix the bug that lightning split large csv file may failed if the file size if slightly bigger the region-split-size and the csv has header.

close #27763

What is changed and how it works?

• If the backend if s3, when the seek position >= file size, the seek will succeed and the reader will be replaced with a reader that always returns io.EOF
• Add a check in lightning makeTableRegions, after preprocess the header line, if the remain file is smaller than region-split-size, directly return the result with 1 region.

Check List

Tests

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

Side effects

Documentation

Release note

Fix the bug that import from s3 may fail if the csv file size is similar to region-split-size(default is 256MiB)

[ti-chi-bot]

[REVIEW NOTIFICATION]

This pull request has been approved by:

• glorv
• gozssky

To complete the pull request process, please ask the reviewers in the list to review by filling /cc @reviewer in the comment.
After your PR has acquired the required number of LGTMs, you can assign this pull request to the committer in the list by filling /assign @committer in the comment to help you merge this pull request.

The full list of commands accepted by this bot can be found here.

Reviewer can indicate their review by submitting an approval review.
Reviewer can cancel approval by submitting a request changes review.

[ti-srebot]

/run-all-tests

[ti-srebot]

@glorv you're already a collaborator in bot's repo.

[sleepymole]

/run-integration-br-test

[sleepymole]

/hold

[ti-chi-bot]

@ti-srebot: This cherry pick PR is for a release branch and has not yet been approved by release team.
Adding the do-not-merge/cherry-pick-not-approved label.

To merge this cherry pick, it must first be approved by the collaborators.

AFTER it has been approved by collaborators, please ping the release team in a comment to request a cherry pick review.

Instructions for interacting with me using PR comments are available here. If you have questions or suggestions related to my behavior, please file an issue against the kubernetes/test-infra repository.

[VelocityLight]

This pull request is closed because it's related version has closed automatic cherry-picking.
If it's still needed, you can reopen it or just regenerate it using bot,
see:

• https://prow.tidb.io/command-help#cherrypick
• https://book.prow.tidb.net/#/plugins/cherrypicker

You can find more details at:

• https://internals.tidb.io/t/topic/785