Define projects in atlantis.yaml and allow undefined projects to auto plan

[nitrocode]

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• I'd be willing to implement this feature (contributing guide)

Describe the user story

If we have 100 projects in a single repo and only need to customize 1, we'll have to define every project or we can no longer auto plan projects.

This is stated in the docs with this warning

Once an atlantis.yaml file exists in a repo, Atlantis won't try to determine where to run plan automatically. Instead it will just follow the project configuration. This means that you'll need to define each project in your repo.

If you have many directories with Terraform configuration, each directory will need to be defined.

According to the code, the warning in the documentation has been there since 2019. I'm not sure if this was an intentional limitation or if it is an issue that never got prioritized. I couldn't find an existing ticket for this either (happy to close if there is one).

https://github.com/runatlantis/atlantis/blame/ada4e53931953ada1afc75faac3ba9ebda126472/runatlantis.io/docs/repo-level-atlantis-yaml.md#L36

Describe the solution you'd like

Describe the drawbacks of your solution

I'd like all projects to have a default, as if there isn't an atlantis.yaml file, and if there is an atlantis.yaml file with defined projects, only override the configurations of specific directories/projects and continue to autoplan the rest.

Describe alternatives you've considered

I currently run the script in the docs to auto generate the projects in the atlantis.yaml file.

There are other projects that also auto generate the atlantis.yaml file such as terragrunt-atlantis-config and atmos

[jskrill]

Hi, I'd like to tackle this issue. I imagine this can be solved rather simply with a new top level config option (to avoid breaking existing behavior), for example:

version: 3
discover_projects_with_missing_config: true
projects:
- dir: some-dir
  autoplan:
    when_modified: ["*.tf", "custom_file"]
    enabled: true

In this case "discover_projects_with_missing_config" would default to false for the existing behavior. When set to true, we'd run through the DetermineProjects (https://github.com/runatlantis/atlantis/blob/main/server/events/project_finder.go#L140) logic unconditionally and merge the collection with the results of DetermineProjectsViaConfig (https://github.com/runatlantis/atlantis/blob/main/server/events/project_finder.go#L178). To merge, a project returned from DetermineProjectsViaConfig would override any project from DetermineProjects which has the same dir.

Let me know if this sounds reasonable and I can work on a PR.

[nitrocode]

Yes this would be fantastic

cc @GenPage @jamengual thoughts?

[jamengual]

it never make to much sense to me that we did not have a customizable catch all, it would be nice to have.

[jskrill]

Opened a PR, let me know if you have any comments

[nitrocode]

I forgot that i gave this some thought in a previous ticket

Please see this comment #859 (comment)