When CircleCI does not work for pull requests from forks... what to do?

[Rob--W]

I created a new pull request at #4573, but CircleCI failed to trigger a build at first. I think that I fixed it, but there may still be an issue with integration. I'm documenting my experience here in case we encounter it again in the future.

• Completely failing build: https://app.circleci.com/pipelines/github/mozilla/addons-linter/4020
• Working build (after following the steps below): https://app.circleci.com/pipelines/github/mozilla/addons-linter/4021

The pull request still shows a failing build because it links to the old build. The new builds is green, so I think that the "failing build" icon on Github is just an integration bug.

The log message of the initial failed build is:

Could not find a usable config.yml, you need to reauthorize and approve the SAML-protected org. Please sign out of CircleCI and log back in with your VCS before triggering a new pipeline.

What have I tried:

• I already tried to sign out and sign into CircleCI as suggested by the comment, but after doing that, the "Rerun Workflow from Start" option is still disabled. The tooltip says "You need write permissions to trigger builds". I already have write access to this project on Github (e.g. I can merge pull requests), so I don't know what this means.
• I tried revoking access to the CircleCI OAuth app as suggested here: https://support.circleci.com/hc/en-us/articles/360043002793-Troubleshooting-CircleCI-access-after-enabling-Github-SSO. Didn't work.
• I visited https://app.circleci.com/settings/project/github/mozilla/addons-linter/advanced and confirmed that the "Build forked pull requests" option is checked ("Run builds for pull requests from forks. CircleCI will automatically update the commit status shown on GitHub's pull request page.").
• After the above steps failed, I closed and re-opened the pull request. That worked!

┆Issue is synchronized with this Jira Task

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