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RFC 175 - Restricted Roles for Github Pull Requests

Summary

Assume different roles for Github Pull Requests based on the PR author's association to the project.

Motivation

Currently Taskcluster's pullRequest policy is very basic. It can either generate graphs for everyone, or only for collaborators. If a project has a task that can access secrets or perform actions that shouldn't be triggered publicly, then the only recourse is to not generate any graph so long as the author is not at least a collaborator.

However, projects commonly have non-sensitive tasks that could otherwise be run publicly. Having distinct roles based on author association would allow projects to generate graphs for public pull requests, while simultaneously blocking these graphs from running sensitive tasks via scopes.

The motivations for this RFC overlap with RFC 168.

Details

A new pullRequest policy called public_restricted will be invented. When used, Taskcluster Github will inspect the author association of the pull request to determine whether or not they are a collaborator. If they are determined to be a collaborator, the current pull-request role will be assumed. So far this behavior is identical to the collaborators policy.

However if the author is not a collaborator, a new role called repo:github.com/${ payload.organization }/${ payload.repository }:pull-request-untrusted will be assumed instead.

To allow projects to tell whether a pull request was created by a collaborator or not, a new tasks_for value of github-pull-request-untrusted will be used to evaluate the .taskcluster.yml file.

Using the public_restricted policy

Using the public_restricted policy will require that both the Taskcluster instance and the project are configured properly. While the details around this configuration are out of scope for this RFC, it's worth providing a brief example to help illustrate how the feature might be used.

Instance Configuration

Taskcluster administrators will need to ensure the pull-request-untrusted roles exist for any projects that use the public_restricted policy. They'll also need to ensure that scopes are properly assigned to block running any sensitive tasks on untrusted pull requests.

Task Configuration

Projects that use the public_restricted policy will need to make sure they don't try to run trusted tasks on untrusted PRs, otherwise they'll get a scope expression error. They'll be able to accomplish this by inspecting the tasks_for field that Taskcluster Github now passes down to the JSON-e context and checking for a value of github-pull-request-untrusted.

In the case of a plain .taskcluster.yml file, this value could be used in a JSON-e conditional statement. In the case of Taskgraph, this could be passed in via a new parameter.