Support requiring owners from both base and head refs

[BakerNet]

Related Issue(s)

closes: #83

Summary / Background

Refer to the Issue for a well-written write up

Adds config require_both_branch_reviewers config option to merge base branch and head branch .codeowners rules such that if there is a change in ownership both teams would be required to review.

[pooyaw]

✅ Snyk checks have passed. No issues have been found so far.

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[gemini-code-assist]

Summary of Changes

Hello @BakerNet, I'm Gemini Code Assist¹! I'm currently reviewing this pull request and will post my feedback shortly. In the meantime, here's a summary to help you and other reviewers quickly get up to speed!

This pull request introduces a significant enhancement to the code ownership system by implementing a "sum owners" feature. This new functionality allows for a more secure and explicit process for transferring file ownership by requiring approvals from both the current owners (from the base branch) and the prospective new owners (from the PR branch). This prevents unilateral ownership changes and ensures mutual agreement during handoffs, making the ownership transfer process more robust and transparent.

Highlights

• Introduced 'sum_owners' configuration: A new boolean option 'sum_owners' (default 'false') has been added to 'codeowners.toml' to enable a new merging behavior for code ownership.
• Enabled dual-branch codeowner evaluation: When 'sum_owners' is true, the system now evaluates codeowner rules from both the base branch and the pull request's head branch, requiring approval from both sets of owners for affected files.
• Implemented codeowner merging logic: A new 'MergeCodeOwners' function and supporting utilities were added to combine reviewer groups from base and head branches using an AND logic, ensuring comprehensive approval requirements.
• Enhanced documentation for ownership handoffs: The 'README.md' has been updated to clearly explain the purpose and usage of 'sum_owners', specifically highlighting its utility for managing ownership transfers between teams.

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[gemini-code-assist]

Code Review

This pull request introduces the sum_owners feature, which requires approvals from codeowners in both the base and head branches, useful for ownership handoffs. The implementation involves adding a new merger.go to combine codeowner rules, updating the application logic to use this merger when the feature is enabled, and adding configuration and tests. The documentation in README.md is also updated to explain the new feature.

The changes are well-implemented and tested. I've found a minor issue related to an unused parameter (reviewerGroupManager) in the new merger logic, which makes the code slightly misleading. I've left comments with suggestions to clean this up. Overall, this is a great addition.

[github-actions]

Codeowners approval required for this PR:

• ✅ @zbedforrest

[zbedforrest]

could extract repeated logic into a local closure

Changes in owned files since last approval