tidb lint: add a linter to avoid using `==nil` to assert empty slice or map

It's such a huge change!!! I'm not sure whether merging this PR is good. Need discussion.

They are fixed automatically by the linter.

What problem does this PR solve?

Issue Number: close #52074, ref #51807

Problem Summary:

It's easy to make mistake to use arr == nil to tell whether a slice is empty, but if the slice is []T{}, this condition will not meet. #51807 is one of the bugs made by it. Therefore I tried to add a linter to avoid this situation.

This PR also fixed similar issues for map.

What changed and how does it work?

Don't allow using arr == nil.
If you really want to use arr == nil to check whether is not initialized (or empty return), you can use emptyslice.IsNil(arr), which is ignored by the linter.

It gives a lot of error, and I didn't fix them all yet. I'm not sure whether it's good to add this linter.

Check List

Tests

[ ] Unit test
[x] Integration test
[ ] Manual test (add detailed scripts or steps below)
[ ] No need to test
- [ ] I checked and no code files have been changed.

Release note

Please refer to Release Notes Language Style Guide to write a quality release note.

None

Mar 25 '24 09:03 YangKeao

[APPROVALNOTIFIER] This PR is NOT APPROVED

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Mar 25 '24 15:03 ti-chi-bot[bot]

Codecov Report

:x: Patch coverage is 79.91968% with 100 lines in your changes missing coverage. Please review. :white_check_mark: Project coverage is 54.7550%. Comparing base (3475795) to head (8619e3e). :warning: Report is 3608 commits behind head on master.

Additional details and impacted files

@@                Coverage Diff                @@
##             master     #52073         +/-   ##
=================================================
- Coverage   70.7242%   54.7550%   -15.9692%     
=================================================
  Files          1486       1598        +112     
  Lines        439452     612754     +173302     
=================================================
+ Hits         310799     335514      +24715     
- Misses       109205     253918     +144713     
- Partials      19448      23322       +3874

Flag	Coverage Δ
integration	`49.1448% <55.9006%> (?)`
unit	`71.6143% <75.7085%> (-0.1098%)`	:arrow_down:

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

Components	Coverage Δ
dumpling	`∅ <87.5000%> (∅)`
parser	`∅ <ø> (∅)`
br	`58.2345% <63.6363%> (+7.1946%)`	:arrow_up:

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Mar 26 '24 08:03 codecov[bot]

LGTM, but...

How to trigger this lint manually? I found a lot of checks related to lint like make lint, make static-check, make bazel_lint... :(

I think make bazel_lint can trigger this lint.

This PR also provides a main package in /build/linter/emptynil/cmd. Using -fix argument will fix the found issues automatically.

Mar 27 '24 05:03 YangKeao

i think it's a little over-reacting to the bug in description. misusing between len(xx) == 0 and xx == nil are human errors, that cannot be avoided by replacing xx == nil with isNil(xx), we still might make the same mistake by misusing len(xx) == 0 and isNil(xx), and it sacrifices readability.

Mar 27 '24 05:03 D3Hunter

PR needs rebase.

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Mar 29 '24 08:03 ti-chi-bot[bot]

@YangKeao: 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-br-integration-test	8619e3ea7c2f7be75c84bbefe2fa04db60c6d26a	link	true	`/test pull-br-integration-test`
pull-lightning-integration-test	8619e3ea7c2f7be75c84bbefe2fa04db60c6d26a	link	true	`/test pull-lightning-integration-test`
pull-unit-test-ddlv1	8619e3ea7c2f7be75c84bbefe2fa04db60c6d26a	link	true	`/test pull-unit-test-ddlv1`
pull-integration-e2e-test	8619e3ea7c2f7be75c84bbefe2fa04db60c6d26a	link	true	`/test pull-integration-e2e-test`
pull-integration-realcluster-test-next-gen	8619e3ea7c2f7be75c84bbefe2fa04db60c6d26a	link	true	`/test pull-integration-realcluster-test-next-gen`
pull-unit-test-next-gen	8619e3ea7c2f7be75c84bbefe2fa04db60c6d26a	link	true	`/test pull-unit-test-next-gen`

Full PR test history. Your PR dashboard.

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Sep 26 '25 16:09 ti-chi-bot[bot]