servo/.github/workflows/self-hosted-runner-select.yml

name: Select Self-hosted Runner
on:
  workflow_call:
    inputs:
      github-hosted-runner-label:
        required: true
        type: string
      self-hosted-image-name:
        required: true
        type: string
      self-hosted-runner-scope:
        required: false
        type: string
        default: /orgs/${{ github.repository_owner }}/actions/runners
      monitor-api-base-url:
        required: false
        type: string
        default: https://ci0.servo.org
      force-github-hosted-runner:
        required: false
        type: boolean
        default: false
    outputs:
      unique-id:
        value: ${{ jobs.runner-select.outputs.unique-id }}
      selected-runner-label:
        value: ${{ jobs.runner-select.outputs.selected-runner-label }}
      is-self-hosted:
        value: ${{ jobs.runner-select.outputs.is-self-hosted }}

jobs:
  # Selects a self-hosted runner if available, or else a GitHub-hosted runner.
  # We generate a unique id for the workload, then ask our monitor API to
  # reserve a self-hosted runner for us.
  runner-select:
    name: Select Runner
    runs-on: ubuntu-latest
    outputs:
      unique-id: ${{ steps.select.outputs.unique_id }}
      selected-runner-label: ${{ steps.select.outputs.selected_runner_label }}
      is-self-hosted: ${{ steps.select.outputs.is_self_hosted }}
    steps:
      - name: Select and reserve best available runner
        id: select
        run: |
          github_hosted_runner_label='${{ inputs.github-hosted-runner-label }}'
          self_hosted_image_name='${{ inputs.self-hosted-image-name }}'
          self_hosted_runner_scope='${{ inputs.self-hosted-runner-scope }}'
          monitor_api_base_url='${{ inputs.monitor-api-base-url }}'

          set -euo pipefail

          fall_back_to_github_hosted() {
            echo 'Falling back to GitHub-hosted runner'
            echo "selected_runner_label=$github_hosted_runner_label" | tee -a $GITHUB_OUTPUT
            echo 'is_self_hosted=false' | tee -a $GITHUB_OUTPUT
            exit 0
          }

          # Generate a unique id that allows the workload job to find the runner
          # we are reserving for it (via runner labels), and allows the timeout
          # job to find the workload job run (via the job’s friendly name), even
          # if there are multiple instances in the workflow call tree.
          unique_id=$(uuidgen)
          echo "unique_id=$unique_id" | tee -a $GITHUB_OUTPUT

          # Disable self-hosted runners by creating a repository variable named
          # NO_SELF_HOSTED_RUNNERS with any non-empty value.
          # <https://github.com/servo/servo/settings/variables/actions>
          if [ -n '${{ vars.NO_SELF_HOSTED_RUNNERS }}' ]; then
            echo 'NO_SELF_HOSTED_RUNNERS is set!'
            fall_back_to_github_hosted
          fi

          if [ '${{ inputs.force-github-hosted-runner }}' = true ]; then
            echo 'inputs.force-github-hosted-runner is set!'
            fall_back_to_github_hosted
          fi

          # Use the monitor API to reserve a runner. If we get an object with
          # runner details, we succeeded. If we get null, we failed.
          take_runner_url=$monitor_api_base_url/$self_hosted_image_name/$unique_id/${{ github.repository }}/${{ github.run_id }}
          result=$(mktemp)
          echo
          echo POST "$take_runner_url"
          if ! curl -sS --fail-with-body --connect-timeout 5 --max-time 30 -X POST "$take_runner_url" \
              -H 'Authorization: Bearer ${{ secrets.SERVO_CI_MONITOR_API_TOKEN }}' > $result \
              || ! jq -e . $result > /dev/null; then
            cat $result
            echo
            echo
            echo 'No self-hosted runners available!'
            fall_back_to_github_hosted
          fi

          echo
          echo "selected_runner_label=reserved-for:$unique_id" | tee -a $GITHUB_OUTPUT
          echo 'is_self_hosted=true' | tee -a $GITHUB_OUTPUT