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feat: add auto-fix option for automatic clang-format fixing #443

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feat: add auto-fix option to automatically format and commit clang-fo…
shenxianpeng Jun 8, 2026
ff93734
refactor: rename fix-commit-msg to auto-fix-commit-msg for consistency
shenxianpeng Jun 8, 2026
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27 changes: 27 additions & 0 deletions .github/workflows/examples/auto-fix.yml
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name: cpp-linter (auto-fix)
on:
pull_request:
branches: [main, master, develop]
paths: ['**.c', '**.cpp', '**.h', '**.hpp', '**.cxx', '**.hxx', '**.cc', '**.hh', '**CMakeLists.txt', 'meson.build', '**.cmake']

jobs:
cpp-linter:
runs-on: ubuntu-latest
permissions:
contents: write # needed for auto-fix commits
pull-requests: write
steps:
- uses: actions/checkout@v5

- uses: cpp-linter/cpp-linter-action@v2
id: linter
env:
GITHUB_TOKEN: ${{ secrets.GITHUB_TOKEN }}
with:
style: 'file' # Use .clang-format config file
tidy-checks: '-*' # disable clang-tidy
auto-fix: 'true' # auto-apply clang-format fixes

- name: Fail fast?!
if: steps.linter.outputs.clang-format-checks-failed > 0
run: exit 1
31 changes: 31 additions & 0 deletions README.md
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Expand Up @@ -68,6 +68,37 @@ For all explanations of our available input parameters and output variables, see

See also our [example recipes][recipes-doc].

### Auto-fix clang-format issues

You can enable automatic fixing of clang-format issues by setting `auto-fix: 'true'`.
When enabled, the action will:

1. Run clang-format detection as usual
2. Apply `clang-format -i` to fix any files with style issues
3. Commit and push the formatted changes back to the PR branch

```yaml
steps:
- uses: actions/checkout@v5
- uses: cpp-linter/cpp-linter-action@v2
id: linter
env:
GITHUB_TOKEN: ${{ secrets.GITHUB_TOKEN }}
with:
style: 'file'
auto-fix: 'true' # automatically fix format issues
```

> [!TIP]
> To prevent the auto-fix commit from triggering another CI run, include a
> `[skip ci]` (or `[ci skip]`, `[no ci]`, etc.) tag in your custom commit message:
>
> ```yaml
> with:
> auto-fix: 'true'
> auto-fix-commit-msg: 'style: apply styling format fix [skip ci]'
> ```

## Used By

<p align="center">
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46 changes: 46 additions & 0 deletions action.yml
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Expand Up @@ -209,6 +209,24 @@ inputs:
Set this option to `true` to prevent Pull Request reviews from approving or requesting changes.
default: 'false'
required: false
auto-fix:
description: |
Set this option to `true` to automatically apply clang-format fixes
and commit them back to the PR branch.

When enabled, cpp-linter runs with ``--fix``, which applies
``clang-format -i`` on files with style issues. After that,
a new commit is pushed to the PR branch with the formatted changes.

This option has no effect on clang-tidy issues.
default: 'false'
required: false
auto-fix-commit-msg:
description: |
Custom commit message for the auto-fix commit.
Only used when ``auto-fix`` is ``true``.
default: 'style: apply styling format fix'
required: false
jobs:
description: |
The number of jobs to run in parallel.
Expand Down Expand Up @@ -458,6 +476,9 @@ runs:
'--passive-reviews=${{ inputs.passive-reviews }}'
'--jobs=${{ inputs.jobs }}'
]
if '${{ inputs.auto-fix }}' == 'true' {
$args = ($args | append ['--fix'])

@2bndy5 2bndy5 Jun 9, 2026 •
edited
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The self-test CI fails here because all variables are immutable by default in nushell. To declare them mutable, use mut instead of let (above this line):

-       let args = [
+       mut args = [

}
mut uv_args = [run --no-sync --project $action_path --directory (pwd)]

let gh_action_debug = $env | get --optional 'ACTIONS_STEP_DEBUG'
Expand All @@ -483,3 +504,28 @@ runs:

print $"\n(ansi purple)Running cpp-linter(ansi reset)"
^$'($env.UV_INSTALL_DIR)/uv' ...$uv_args cpp-linter ...$args

- name: Auto-commit clang-format fixes
if: inputs.auto-fix == 'true' || inputs.auto-fix == true
shell: nu {0}
run: |
let has_changes = (^git diff --exit-code) | complete | $in.exit_code != 0

@2bndy5 2bndy5 Jun 9, 2026

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If this returns true, then it is because cpp-linter left some cache artifacts behind. Currently, cpp-linter makes sure the scanned source files are unchanged. Any changes are done in memory or in cache. If changes are done to the C/C++ sources, then they are undone before exiting run_clang_format() or run_clang_tidy().

This feature needs special handling in cpp-linter first; see cpp-linter/cpp-linter#82 for a discussion of the complexity that hasn't been addressed yet.

@2bndy5 2bndy5 Jun 9, 2026

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Oh sorry, I didn't see you submitted cpp-linter/cpp-linter#202

if $has_changes {
^git config user.name 'github-actions[bot]'
^git config user.email '41898282+github-actions[bot]@users.noreply.github.com'
Comment on lines +514 to +515

@2bndy5 2bndy5 Jun 9, 2026

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Better to use the env vars present: GITHUB_ACTOR and GITHUB_ACTOR_ID. This way, the commits will be associated with the user that triggered the event for the workflow's run.

Suggested change
^git config user.name 'github-actions[bot]'
^git config user.email '41898282+github-actions[bot]@users.noreply.github.com'
^git config user.name $"($env.GITHUB_ACTOR)"
^git config user.email $"($env.GITHUB_ACTOR_ID)+($env.GITHUB_ACTOR)@users.noreply.github.com"

Also, this can be done for a single git commit command instead of all git commands in the CI workflow:

let git_user = $"user.name=($env.GITHUB_ACTOR)"
let git_email = $"user.email=($env.GITHUB_ACTOR_ID)+($env.GITHUB_ACTOR)@users.noreply.github.com"

^git -c $git_user -c $git_email commit -m $commit_msg

@shenxianpeng shenxianpeng Jun 9, 2026

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I am not sure if the outside contributor can commit with their own user/email in GitHub actions.
If it's not a problem. It seems good. othersie use a bot user like github-actions[bot] dependabot[bot], pre-commit-ci[bot] are also not bad.

@2bndy5 2bndy5 Jun 9, 2026

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github-actions[bot] would be good enough if the GITHUB_TOKEN env var is never supplied a PAT (personal access token). If using a PAT, then the owner of the PAT will not be the github-actions[bot].

By using the GITHUB_ACTOR(_ID), we can at least point to the committer that caused violations to be auto-fixed with a new commit.

@2bndy5 2bndy5 Jun 9, 2026

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I am not sure if the outside contributor can commit with their own user/email in GitHub actions

I do this all the time with my automated publishing scripts. See the nu script I wrote for publishing packages from cpp-linter-rs

^git add -A
let commit_msg = if ('${{ inputs.auto-fix-commit-msg }}' | is-empty) {
'style: apply styling format fix'
} else {
'${{ inputs.auto-fix-commit-msg }}'
}
^git commit -m $"($commit_msg)"

@2bndy5 2bndy5 Jun 9, 2026

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See my previous comment about setting the user name and email of a single git command.

let push_result = (^git push) | complete
if $push_result.exit_code != 0 {
print $"(ansi yellow)::warning title=Auto-fix push failed::($push_result.stderr)(ansi reset)"
} else {
print $"(ansi green)Auto-fix commit pushed successfully(ansi reset)"
}
} else {
print $"(ansi green)No formatting changes to commit(ansi reset)"
}
5 changes: 5 additions & 0 deletions docs/action.yml
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Expand Up @@ -47,6 +47,11 @@ inputs:
passive-reviews:
minimum-version: '2.12.0'
required-permission: 'pull-requests: write #pull-request-reviews'
auto-fix:
minimum-version: '2.19.0'
required-permission: 'contents: write #auto-fix'
auto-fix-commit-msg:
minimum-version: '2.19.0'
jobs:
minimum-version: '2.11.0'
cache-enable:
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11 changes: 11 additions & 0 deletions docs/examples/index.md
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Expand Up @@ -2,11 +2,22 @@
[style]: ../inputs-outputs.md#style
[tidy-checks]: ../inputs-outputs.md#tidy-checks
[thread-comments]: ../inputs-outputs.md#thread-comments
[auto-fix]: ../inputs-outputs.md#auto-fix

# Recipes

Here are some example workflows to get started quickly.

=== "auto-fix clang-format"

``` yaml
--8<-- ".github/workflows/examples/auto-fix.yml"
```

1. See also [`auto-fix`][auto-fix]
2. See also [`style`][style]
3. See also [`tidy-checks`][tidy-checks]

=== "only clang-tidy"

``` yaml
Expand Down
12 changes: 12 additions & 0 deletions docs/permissions.md
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Expand Up @@ -78,3 +78,15 @@ The [`tidy-review`](inputs-outputs.md#tidy-review), [`format-review`](inputs-out
permissions:
pull-requests: write
```

## Auto-fix

The [`auto-fix`](inputs-outputs.md#auto-fix) feature requires the following permission
in addition to any other permissions needed for other features:

```yaml
permissions:
contents: write # (1)!
```

1. Needed to commit and push the formatted changes back to the PR branch.

@2bndy5 2bndy5 Jun 9, 2026

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We should also note that this permission is only needed by the token used in actions/checkout (which defaults to github.token).

However, the github.token is NOT able to trigger new CI runs when used to push a commit. If users want the auto-fix commit to trigger a new CI run, then they need to provide it with a PAT that has contents: write permission.

- uses: actions/checkout@v6
  with:
    # MY_TOKEN must have `contents: write` permission.
    # using default token does not trigger CI runs on `git push`
    token: ${{ secrets.MY_TOKEN }}

@2bndy5 2bndy5 Jun 11, 2026

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Another problem that might arise here is when auto-fixing PR changes from a third-party fork. My previous comment was made with experience about pushing changes to the same remote. If the changes need to be pushed to a different remote, then that is a different set of permissions.

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