CODEOWNERS syntax/format validation

Release notes

You can now see in the UI if your CODEOWNERS file has syntax errors.

Problem

Research justification: Help users better understand CODEOWNERS (&4498 - closed)

Code owners offers great flexibility, allowing multiple file locations, sections, and rules to be configured by users. However, code owners does not currently validate the information being entered into the file. This can lead to users experiencing problems such as:

1. Creating rules for files/paths that don't exist
2. Creating rules that create conflict with other existing rules
3. Creating rules that don't apply because of incorrect syntax

The initial version in this MVP would return a true/false if the file is valid to be presented by the frontend. Details about why the file was invalid would be captured in a later version.

Further details

The future introduction of support for multiple code owners files will compound this problem and could lead to a degradation of the experience if validation does not follow soon.

This issue has been split out into a MVP (this issue) and a more advanced version captured in CODEOWNERS editor and validation tool (#389409 - closed)

Proposal

See epic: &4498 (closed)

Lint/verify that the CODEOWNERS file syntax/format is valid.

• Display status of syntax validity of code owners file when viewing in repository

Design

Figma

Scenario	Screenshot
Valid syntax
Contains syntax errors - Default
Contains syntax errors - Expanded
Contains syntax errors - Error expanded
Fallback option: If we are unable to get the line reference, we can start with listing the errors

Requirements

Checks for the following scenarios as referenced in error handling in Code Owners.

1. Entries with spaces
2. Unparsable sections
3. Malformed owners
4. Inaccessible owners
5. Zero owners
6. Less than 1 required approvals

• When the code owners file has valid syntax
  • Display checkmark icon with text Syntax is valid.
• When the code owners file has syntax errors
  • Display error icon with text Contains {X} syntax errors. [> Show errors].
    • Where X is equal to the total number of syntax errors
    • Clicking on "Show errors" expands the list of errors
  • When expanded, display a link to docs for text "How are errors handled?"
    • Links to https://docs.gitlab.com/ee/user/project/codeowners/reference.html#error-handling-in-code-owners
  • Each of the type of errors is displayed as a bullet point.
    • The reason it is in a list rather than a comma separated sentence is for easy parsing of the list. The vertical height of the list is not a concern because we want users to be aware of the errors.
    • Clicking on the error name will expand to reveal the lines as a list where there is a syntax error.
• Validation is done when the changes to codeowners is made.
  • This result should be cached so that we don't need to do a validation every time the codeowners file is opened.

Availability and Testing

• Add feature specs for above 6 error scenarios and happy path scenario.
• Ensure multiple errors can be handled together.
• Test for expanding errors.
• Test count of errors.
• Ensure validation is performed on update also.
• Exploratory testing of feature.