nixpkgs/ci/github-script/prepare.js

module.exports = async function ({ github, context, core }) {
  const pull_number = context.payload.pull_request.number

  for (const retryInterval of [5, 10, 20, 40, 80]) {
    core.info('Checking whether the pull request can be merged...')
    const prInfo = (
      await github.rest.pulls.get({
        ...context.repo,
        pull_number,
      })
    ).data

    if (prInfo.state !== 'open') throw new Error('PR is not open anymore.')

    if (prInfo.mergeable == null) {
      core.info(
        `GitHub is still computing whether this PR can be merged, waiting ${retryInterval} seconds before trying again...`,
      )
      await new Promise((resolve) => setTimeout(resolve, retryInterval * 1000))
      continue
    }

    let mergedSha, targetSha

    if (prInfo.mergeable) {
      core.info('The PR can be merged.')

      mergedSha = prInfo.merge_commit_sha
      targetSha = (
        await github.rest.repos.getCommit({
          ...context.repo,
          ref: prInfo.merge_commit_sha,
        })
      ).data.parents[0].sha
    } else {
      core.warning('The PR has a merge conflict.')

      mergedSha = prInfo.head.sha
      targetSha = (
        await github.rest.repos.compareCommitsWithBasehead({
          ...context.repo,
          basehead: `${prInfo.base.sha}...${prInfo.head.sha}`,
        })
      ).data.merge_base_commit.sha
    }

    core.info(
      `Checking the commits:\nmerged: ${mergedSha}\ntarget: ${targetSha}`,
    )
    core.setOutput('mergedSha', mergedSha)
    core.setOutput('targetSha', targetSha)
    return
  }
  throw new Error(
    "Not retrying anymore. It's likely that GitHub is having internal issues: check https://www.githubstatus.com.",
  )
}