The runner is open source, but once you read the code you'd wish it wasn't.
https://github.com/actions/runner
For anyone in the audience who didn't know: GitHub Actions is based on Visual Studio Team Foundation Server's CI, and later Azure DevOps. And nobody, including the current maintainers, seem to know exactly how it all works (or how it doesn't).[1] The random exit codes is just the cherry on top!
> GitHub Actions is based on Visual Studio Team Foundation Server's CI, and later Azure DevOps
Yes and no, ADO Agent (https://github.com/microsoft/azure-pipelines-agent) is far more secretive and "black-box" alike. Like stuck in old version of NodeJS, Powershell, API without documentation or even enough tests/samples...