name: Get merge commit

description: 'Checks whether the Pull Request is mergeable and checks out the repo at up to two commits: The result of a temporary merge of the head branch into the target branch ("merged"), and the parent of that commit on the target branch ("target"). Handles push events and merge conflicts gracefully.'

inputs:
  mergedSha:
    description: "The merge commit SHA, previously collected."
    type: string
  merged-as-untrusted:
    description: "Whether to checkout the merge commit in the ./untrusted folder."
    type: boolean
  pinnedFrom:
    description: "Whether to checkout the pinned nixpkgs for CI and from where (trusted, untrusted)."
    type: string
  targetSha:
    description: "The target commit SHA, previously collected."
    type: string
  target-as-trusted:
    description: "Whether to checkout the target commit in the ./trusted folder."
    type: boolean

outputs:
  mergedSha:
    description: "The merge commit SHA"
    value: ${{ steps.commits.outputs.mergedSha }}
  targetSha:
    description: "The target commit SHA"
    value: ${{ steps.commits.outputs.targetSha }}

runs:
  using: composite
  steps:
    - id: commits
      if: ${{ !inputs.mergedSha && !inputs.targetSha }}
      uses: actions/github-script@60a0d83039c74a4aee543508d2ffcb1c3799cdea # v7.0.1
      with:
        script: |
          if (context.eventName == 'push') return core.setOutput('mergedSha', context.sha)

          for (const retryInterval of [5, 10, 20, 40, 80]) {
            console.log("Checking whether the pull request can be merged...")
            const prInfo = (await github.rest.pulls.get({
              owner: context.repo.owner,
              repo: context.repo.repo,
              pull_number: context.payload.pull_request.number
            })).data

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

            if (prInfo.mergeable == null) {
              console.log(`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) {
              console.log("The PR can be merged.")

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

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

            console.log(`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.")

    - if: inputs.merged-as-untrusted && (inputs.mergedSha || steps.commits.outputs.mergedSha)
      # Would be great to do the checkouts in git worktrees of the existing spare checkout instead,
      # but Nix is broken with them:
      # https://github.com/NixOS/nix/issues/6073
      uses: actions/checkout@08c6903cd8c0fde910a37f88322edcfb5dd907a8 # v5.0.0
      with:
        ref: ${{ inputs.mergedSha || steps.commits.outputs.mergedSha }}
        path: untrusted

    - if: inputs.target-as-trusted && (inputs.targetSha || steps.commits.outputs.targetSha)
      uses: actions/checkout@08c6903cd8c0fde910a37f88322edcfb5dd907a8 # v5.0.0
      with:
        ref: ${{ inputs.targetSha || steps.commits.outputs.targetSha }}
        path: trusted

    - if: inputs.pinnedFrom
      id: pinned
      uses: actions/github-script@60a0d83039c74a4aee543508d2ffcb1c3799cdea # v7.0.1
      env:
        PINNED_FROM: ${{ inputs.pinnedFrom }}
      with:
        script: |
          const path = require('node:path')
          const pinned = require(path.resolve(path.join(process.env.PINNED_FROM, 'ci', 'pinned.json')))
          core.setOutput('pinnedSha', pinned.pins.nixpkgs.revision)

    - if: steps.pinned.outputs.pinnedSha
      uses: actions/checkout@08c6903cd8c0fde910a37f88322edcfb5dd907a8 # v5.0.0
      with:
        ref: ${{ steps.pinned.outputs.pinnedSha }}
        path: pinned
        sparse-checkout: |
          lib
          maintainers
          nixos/lib
          pkgs