Pre commit file fetcher

[robaiken]

What are you trying to accomplish?

Add the FileFetcher class for the pre-commit ecosystem, enabling Dependabot to fetch pre-commit configuration files from repositories.

Anything you want to highlight for special attention from reviewers?

• The CONFIG_FILE_PATTERN regex (/\.pre-commit(-config)?\.ya?ml$/i) matches all valid pre-commit config file variations
• Pre-commit support is currently in beta and requires ALLOW_BETA_ECOSYSTEMS=true to be enabled
• The implementation follows the established patterns from other ecosystems (bundler, terraform, etc.)

How will you know you've accomplished your goal?

We can fetch pre-commit files

Checklist

• I have run the complete test suite to ensure all tests and linters pass.
• I have thoroughly tested my code changes to ensure they work as expected, including adding additional tests for new functionality.
• I have written clear and descriptive commit messages.
• I have provided a detailed description of the changes in the pull request, including the problem it addresses, how it fixes the problem, and any relevant details about the implementation.
• I have ensured that the code is well-documented and easy to understand.

[Copilot]

Pull request overview

This PR introduces a concrete FileFetcher implementation for the pre_commit ecosystem so Dependabot can discover and download pre-commit configuration files, gated behind the beta-ecosystem experiment flag.

Changes:

• Implement Dependabot::PreCommit::FileFetcher with a regex-based config filename matcher, beta-ecosystem gating, and exclude_paths-aware filtering.
• Add an RSpec suite for the pre-commit file fetcher covering required file detection, error messaging, fetch behaviour (including missing files and beta disabled), and ecosystem versions.
• Add GitHub API and pre-commit config fixtures to support the new tests.

Reviewed changes

Copilot reviewed 5 out of 5 changed files in this pull request and generated 2 comments.

Show a summary per file

File	Description
pre_commit/lib/dependabot/pre_commit/file_fetcher.rb	Implements the pre-commit FileFetcher, including config filename pattern, beta gating, and final exclude_paths filtering.
pre_commit/spec/dependabot/pre_commit/file_fetcher_spec.rb	Adds tests for required files detection, error messaging, fetch behaviour under various conditions, exclude_paths, and ecosystem_versions.
pre_commit/spec/fixtures/pre_commit_configs/basic.yaml	Provides a canonical pre-commit configuration fixture used to validate decoded file content.
pre_commit/spec/fixtures/github/repo_contents.json	Mocks GitHub repository contents API response including .pre-commit-config.yaml and a README for list-directory behaviour tests.
pre_commit/spec/fixtures/github/pre_commit_config.json	Mocks GitHub file contents API response for .pre-commit-config.yaml, including base64-encoded YAML content.

[Copilot]

We exercise all supported pre-commit config filename variations in .required_files_in?, but #fetch_files is only tested with .pre-commit-config.yaml. This means that regressions in config_file_name (e.g., failing to locate .pre-commit.yml or .pre-commit.yaml in repo_contents) would not be caught; consider adding at least one integration-style #fetch_files example that stubs repo_contents with a non--config filename and verifies that the correct file is fetched.

[Copilot]

The expectation in this context appears inconsistent with Dependabot::FileFetchers::Base#files: when fetch_files returns an empty array (as it will when the only manifest is excluded by FileFiltering.should_exclude_path?), #files raises Dependabot::DependencyFileNotFound with "No files found in ..." rather than returning an empty array. To test the exclude_paths behaviour without conflicting with the base class contract, this example should either assert that file_fetcher_instance.files raises DependencyFileNotFound, or call fetch_files directly and assert that it returns an empty array.

Suggested change

	it "excludes the matching file" do
	expect(file_fetcher_instance.files).to be_empty
	it "raises a DependencyFileNotFound error when all files are excluded" do
	expect { file_fetcher_instance.files }
	.to raise_error(Dependabot::DependencyFileNotFound) do |error|
	expect(error.message).to include("No files found in")
	end

[AbhishekBhaskar]

LGTM, but copilot's suggestion to add another integration style spec seemed relevant to me.

[MajorTanya]

Apologies for butting in on an already merged pull request, but I believe pre-commit does not actually support .yml config files immediately (a .yml must be passed manually, according to pre-commit's issue tracker here and the most recent duplicate here).
Has that been considered in this file fetcher? I'm unfamiliar with dependabot development and Ruby, so I thought it'd be important to ask this.

[robaiken]

@MajorTanya No need to apologise, open source is a collaborative effort and input is always welcome! We're supporting both .yaml and .yml extensions. While pre-commit requires the --config flag to use a .yml file, we want to support users who've chosen that configuration