carenas / action

Cross-platform GitHub action

Cross-Platform GitHub Action

This project provides a GitHub action for running GitHub Action workflows on multiple platforms. This includes platforms that GitHub Actions doesn't currently natively support.

Features

Some of the features that are supported include:

• Multiple operating system with one single action
• Multiple versions of each operating system
• Allows to use default shell or Bash shell
• Low boot overhead
• Fast execution

Usage

Here's a sample workflow file which will setup a matrix resulting in two jobs. One which will run on FreeBSD 12.2 and one which runs on OpenBSD 6.8.

name: CI

on: [push]

jobs:
  test:
    runs-on: macos-10.15
    strategy:
      matrix:
        os:
          - name: freebsd
            version: '12.2'
          - name: openbsd
            version: '6.8'

    steps:
      - uses: actions/checkout@v2

      - name: Test on ${{ matrix.os.name }}
        uses: cross-platform-actions/[email protected]
        env:
          MY_ENV1: MY_ENV1
          MY_ENV2: MY_ENV2
        with:
          environment_variables: MY_ENV1 MY_ENV2
          operating_system: ${{ matrix.os.name }}
          version: ${{ matrix.os.version }}
          shell: bash
          run: |
            uname -a
            echo $SHELL
            pwd
            ls -lah
            whoami
            env | sort

The FreeBSD and OpenBSD jobs need to run on: macos-10.15. Jobs for the other platforms need to run on a Linux runner.

Inputs

This section lists the available inputs for the action.

Input	Required	Default Value	Description
run	✓	✗	Runs command-line programs using the operating system's shell. This will be executed inside the virtual machine.
operating_system	✓	✗	The type of operating system to run the job on. See Supported Platforms.
version	✓	✗	The version of the operating system to use. See Supported Platforms.
shell	✗	default	The shell to use to execute the commands. Defaults to the default shell for the given operating system. Allowed values are: default, sh and bash
environment_variables	✗	""	A list of environment variables to forward to the virtual machine. The list should be separated with spaces.

All inputs are expected to be strings. It's important that especially the version is explicitly specified as a string, using single or double quotes. Otherwise YAML might interpet the value as a numeric value instead of a string. This might lead to some unexpected behavior. If the version is specified as version: 13.0, YAML will interpet 13.0 as a floating point number, drop the fraction part (because 13 and 13.0 are the same) and the GitHub action will only see 13 instead of 13.0. The solution is to explicitly state that a string is required by using quotes: version: '13.0'.

Supported Platforms

This sections lists the currently supported platforms by operating system. Each operating system will list which versions are supported.

OpenBSD (openbsd)

Version	x86-64
6.9	✓
6.8	✓

FreeBSD (freebsd)

Version	x86-64
13.0	✓
12.2	✓

NetBSD (netbsd)

Version	x86-64
9.2	✓

Under the Hood

GitHub Actions currently only support the following platforms: macOS, Linux and Windows. To be able to run other platforms, this GitHub action runs the commands inside a virtual machine (VM). If the host platform is macOS the hypervisor can take advantage of nested virtualization.

The FreeBSD and OpenBSD VMs run on the xhyve hypervisor (on a macOS host), while the other platforms run on the QEMU hypervisor (on a Linux host). xhyve is built on top of Apple's Hypervisor framework. The Hypervisor framework allows to implement hypervisors with support for hardware acceleration without the need for kernel extensions. xhyve is a lightweight hypervisor that boots the guest operating systems quickly and requires no dependencies outside of what's provided by the system. QEMU is a more general purpose hypervisor that runs on most host platforms and supports most guest systems. It's a bit slower than xhyve because it's general purpose and it cannot use nested virtualization on the Linux hosts provided by GitHub.

The VM images running inside the hypervisor are built using Packer. It's a tool for automatically creating VM images, installing the guest operating system and doing any final provisioning.

The GitHub action uses SSH to communicate and execute commands inside the VM. It uses rsync to share files between the guest VM and the host. xhyve does not have any native support for sharing files. To authenticate the SSH connection a unique key pair is used. This pair is generated each time the action is run. The public key is added to the VM image and the private key is stored on the host. Since xhyve does not support file sharing, a secondar hard drive, which is backed by a file, is created. The public key is stored on this hard drive, which is then mounted by the VM. At boot time, the secondary hard drive will be identified and the public key will be copied to the appropriate location.

To reduce the time it takes for the GitHub action to start executing the commands specified by the user, it aims to boot the guest operating systems as fast as possible. This is achieved in a couple of ways:

• By downloading resources, like the hypervisor and a few other tools, instead of installing them through a package manager

• No compression is used for the resources that are downloaded. The size is small enough anyway and it's faster to download the uncompressed data than it is to download compressed data and then uncompress it.

• It leverages async/await to perform tasks asynchronously. Like downloading the VM image and other resources at the same time

• It performs as much as possible of the setup ahead of time when the VM image is provisioned