Exempt specific files from Codeowners rules

Problem to solve

In some cases an organization wants the strict control provided by codeowners on most files, but want to exclude a small number of files from this. For example, updating a pom file as part of a Maven CI pipeline.

Intended users

• Sasha (Software Developer)

Proposal

Introduce a new syntax !#{path} for CODEOWNERS to exclude files from the current section. Any lines starting with ! will exclude matching paths form other rules within the current section.

* @group-x
!*.rb

[Ruby]
*.rb @ruby-devs
!/config/*

The default group would match all files excluding ruby files The Ruby group would match all ruby files except any inside the /config directory

Research

We performed a spike [SPIKE] Refine `Exempt specific files from Code... (#497801 - closed) to research how we would implement this.

1. Add regex to identify exclusion entries within CODEOWNERS file within Gitlab::CodeOwners::File#parse_entry
2. Refactor MergeRequests::SyncCodeOwnerApprovalRules#execute to apply the exclusions

Out of scope

.gitignore allows excluding a file and then reincluding it further down. We will not support this as it introduces a lot of complexity. This may be implemented in future.

Adding errors when the exclusions are not well formatted e.g. !/path/*.rb @user or !*.rb !*.js