ci/github-script/prepare: move more steps from workflow (#435302)

.github/actions/checkout/action.yml

···
+name: Checkout
+
+description: 'Checkout into trusted / untrusted / pinned folders consistently.'
+
+inputs:
+  merged-as-untrusted-at:
+    description: "Whether and which SHA to checkout for the merge commit in the ./untrusted folder."
+  pinned-from:
+    description: "Whether to checkout the pinned nixpkgs for CI and from where (trusted, untrusted)."
+  target-as-trusted-at:
+    description: "Whether and which SHA to checkout for the target commit in the ./trusted folder."
+
+runs:
+  using: composite
+  steps:
+    - if: inputs.merged-as-untrusted-at
+      # 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.merged-as-untrusted-at }}
+        path: untrusted
+
+    - if: inputs.target-as-trusted-at
+      uses: actions/checkout@08c6903cd8c0fde910a37f88322edcfb5dd907a8 # v5.0.0
+      with:
+        ref: ${{ inputs.target-as-trusted-at }}
+        path: trusted
+
+    - if: inputs.pinned-from
+      id: pinned
+      uses: actions/github-script@60a0d83039c74a4aee543508d2ffcb1c3799cdea # v7.0.1
+      env:
+        PINNED_FROM: ${{ inputs.pinned-from }}
+      with:
+        script: |
+          const path = require('node:path')
+          const pinned = require(path.resolve(path.join(process.env.PINNED_FROM, 'ci', 'pinned.json')))
+          core.setOutput('pinned-at', pinned.pins.nixpkgs.revision)
+
+    - if: steps.pinned.outputs.pinned-at
+      uses: actions/checkout@08c6903cd8c0fde910a37f88322edcfb5dd907a8 # v5.0.0
+      with:
+        ref: ${{ steps.pinned.outputs.pinned-at }}
+        path: pinned
+        sparse-checkout: |
+          lib
+          maintainers
+          nixos/lib
+          pkgs

.github/actions/get-merge-commit/action.yml

···
-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: |
-          require('./ci/github-script/prepare.js')({
-            github,
-            context,
-            core,
-          })
-
-    - 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
-

.github/workflows/README.md

···
   This is a temporary commit that GitHub creates automatically as "what would happen, if this PR was merged into the base branch now?".
   The checkout could be done via the virtual branch `refs/pull/<pr-number>/merge`, but doing so would cause failures when this virtual branch doesn't exist (anymore).
   This can happen when the PR has conflicts, in which case the virtual branch is not created, or when the PR is getting merged while workflows are still running, in which case the branch won't exist anymore at the time of checkout.
-  Thus, we use the `get-merge-commit.yml` workflow to check whether the PR is mergeable and the test merge commit exists and only then run the relevant jobs.
+  Thus, we use the `prepare` job to check whether the PR is mergeable and the test merge commit exists and only then run the relevant jobs.
 
 - Various workflows need to make comparisons against the base branch.
   In this case, we checkout the parent of the "test merge commit" for best results.

.github/workflows/build.yml

···
       - uses: actions/checkout@08c6903cd8c0fde910a37f88322edcfb5dd907a8 # v5.0.0
         with:
           sparse-checkout: .github/actions
-      - name: Check if the PR can be merged and checkout the merge commit
-        uses: ./.github/actions/get-merge-commit
+      - name: Checkout the merge commit
+        uses: ./.github/actions/checkout
         with:
-          mergedSha: ${{ inputs.mergedSha }}
-          merged-as-untrusted: true
-          pinnedFrom: untrusted
+          merged-as-untrusted-at: ${{ inputs.mergedSha }}
+          pinned-from: untrusted
 
       - uses: cachix/install-nix-action@fc6e360bedc9ee72d75e701397f0bb30dce77568 # v31
         with:

.github/workflows/check.yml

···
       - uses: actions/checkout@08c6903cd8c0fde910a37f88322edcfb5dd907a8 # v5.0.0
         with:
           sparse-checkout: .github/actions
-      - name: Check if the PR can be merged and checkout the merge and target commits
-        uses: ./.github/actions/get-merge-commit
+      - name: Checkout merge and target commits
+        uses: ./.github/actions/checkout
         with:
-          mergedSha: ${{ inputs.mergedSha }}
-          merged-as-untrusted: true
-          pinnedFrom: trusted
-          targetSha: ${{ inputs.targetSha }}
-          target-as-trusted: true
+          merged-as-untrusted-at: ${{ inputs.mergedSha }}
+          pinned-from: trusted
+          target-as-trusted-at: ${{ inputs.targetSha }}
 
       - uses: cachix/install-nix-action@fc6e360bedc9ee72d75e701397f0bb30dce77568 # v31
 

.github/workflows/eval.yml

···
         with:
           sparse-checkout: .github/actions
       - name: Check out the PR at the test merge commit
-        uses: ./.github/actions/get-merge-commit
+        uses: ./.github/actions/checkout
         with:
-          mergedSha: ${{ inputs.mergedSha }}
-          merged-as-untrusted: true
-          pinnedFrom: untrusted
+          merged-as-untrusted-at: ${{ inputs.mergedSha }}
+          pinned-from: untrusted
 
       - name: Install Nix
         uses: cachix/install-nix-action@fc6e360bedc9ee72d75e701397f0bb30dce77568 # v31
···
         with:
           sparse-checkout: .github/actions
       - name: Check out the PR at the target commit
-        uses: ./.github/actions/get-merge-commit
+        uses: ./.github/actions/checkout
         with:
-          targetSha: ${{ inputs.targetSha }}
-          target-as-trusted: true
-          pinnedFrom: trusted
+          target-as-trusted-at: ${{ inputs.targetSha }}
+          pinned-from: trusted
 
       - name: Download output paths and eval stats for all systems
         uses: actions/download-artifact@634f93cb2916e3fdff6788551b99b062d0335ce0 # v5.0.0
···
       - uses: actions/checkout@08c6903cd8c0fde910a37f88322edcfb5dd907a8 # v5.0.0
         with:
           sparse-checkout: .github/actions
-      - name: Check if the PR can be merged and checkout the merge commit
-        uses: ./.github/actions/get-merge-commit
+      - name: Checkout the merge commit
+        uses: ./.github/actions/checkout
         with:
-          mergedSha: ${{ inputs.mergedSha }}
-          merged-as-untrusted: true
+          merged-as-untrusted-at: ${{ inputs.mergedSha }}
 
       - name: Install Nix
         uses: cachix/install-nix-action@fc6e360bedc9ee72d75e701397f0bb30dce77568 # v31

.github/workflows/lint.yml

···
       - uses: actions/checkout@08c6903cd8c0fde910a37f88322edcfb5dd907a8 # v5.0.0
         with:
           sparse-checkout: .github/actions
-      - name: Check if the PR can be merged and checkout the merge commit
-        uses: ./.github/actions/get-merge-commit
+      - name: Checkout the merge commit
+        uses: ./.github/actions/checkout
         with:
-          mergedSha: ${{ inputs.mergedSha }}
-          merged-as-untrusted: true
-          pinnedFrom: untrusted
+          merged-as-untrusted-at: ${{ inputs.mergedSha }}
+          pinned-from: untrusted
 
       - uses: cachix/install-nix-action@fc6e360bedc9ee72d75e701397f0bb30dce77568 # v31
 
···
       - uses: actions/checkout@08c6903cd8c0fde910a37f88322edcfb5dd907a8 # v5.0.0
         with:
           sparse-checkout: .github/actions
-      - name: Check if the PR can be merged and checkout the merge commit
-        uses: ./.github/actions/get-merge-commit
+      - name: Checkout the merge commit
+        uses: ./.github/actions/checkout
         with:
-          mergedSha: ${{ inputs.mergedSha }}
-          merged-as-untrusted: true
-          pinnedFrom: untrusted
+          merged-as-untrusted-at: ${{ inputs.mergedSha }}
+          pinned-from: untrusted
 
       - uses: cachix/install-nix-action@fc6e360bedc9ee72d75e701397f0bb30dce77568 # v31
 
···
       - uses: actions/checkout@08c6903cd8c0fde910a37f88322edcfb5dd907a8 # v5.0.0
         with:
           sparse-checkout: .github/actions
-      - name: Check if the PR can be merged and checkout merged and target commits
-        uses: ./.github/actions/get-merge-commit
+      - name: Checkout merge and target commits
+        uses: ./.github/actions/checkout
         with:
-          mergedSha: ${{ inputs.mergedSha }}
-          merged-as-untrusted: true
-          pinnedFrom: untrusted
-          targetSha: ${{ inputs.targetSha }}
-          target-as-trusted: true
+          merged-as-untrusted-at: ${{ inputs.mergedSha }}
+          pinned-from: untrusted
+          target-as-trusted-at: ${{ inputs.targetSha }}
 
       - uses: cachix/install-nix-action@fc6e360bedc9ee72d75e701397f0bb30dce77568 # v31
 

.github/workflows/pr.yml

···
 on:
   pull_request:
     paths:
-      - .github/actions/get-merge-commit/action.yml
+      - .github/actions/checkout/action.yml
       - .github/workflows/build.yml
       - .github/workflows/check.yml
       - .github/workflows/eval.yml
···
   prepare:
     runs-on: ubuntu-24.04-arm
     outputs:
-      baseBranch: ${{ steps.branches.outputs.base }}
-      headBranch: ${{ steps.branches.outputs.head }}
-      mergedSha: ${{ steps.get-merge-commit.outputs.mergedSha }}
-      targetSha: ${{ steps.get-merge-commit.outputs.targetSha }}
-      systems: ${{ steps.systems.outputs.systems }}
-      touched: ${{ steps.files.outputs.touched }}
+      baseBranch: ${{ steps.prepare.outputs.base }}
+      headBranch: ${{ steps.prepare.outputs.head }}
+      mergedSha: ${{ steps.prepare.outputs.mergedSha }}
+      targetSha: ${{ steps.prepare.outputs.targetSha }}
+      systems: ${{ steps.prepare.outputs.systems }}
+      touched: ${{ steps.prepare.outputs.touched }}
     steps:
       - uses: actions/checkout@08c6903cd8c0fde910a37f88322edcfb5dd907a8 # v5.0.0
         with:
+          sparse-checkout-cone-mode: true # default, for clarity
           sparse-checkout: |
-            .github/actions
             ci/github-script
-            ci/supportedBranches.js
-            ci/supportedSystems.json
-      - name: Check if the PR can be merged and get the test merge commit
-        uses: ./.github/actions/get-merge-commit
-        id: get-merge-commit
-
-      - name: Load supported systems
-        id: systems
-        run: |
-          echo "systems=$(jq -c <ci/supportedSystems.json)" >> "$GITHUB_OUTPUT"
-
-      - name: Determine branch type
-        id: branches
+      - id: prepare
         uses: actions/github-script@60a0d83039c74a4aee543508d2ffcb1c3799cdea # v7.0.1
         with:
           script: |
-            const { classify } = require('./ci/supportedBranches.js')
-            const { base, head } = context.payload.pull_request
-
-            const baseClassification = classify(base.ref)
-            core.setOutput('base', baseClassification)
-            core.info('base classification:', baseClassification)
-
-            const headClassification =
-              (base.repo.full_name == head.repo.full_name) ?
-              classify(head.ref) :
-              // PRs from forks are always considered WIP.
-              { type: ['wip'] }
-            core.setOutput('head', headClassification)
-            core.info('head classification:', headClassification)
-
-      - name: Determine changed files
-        id: files
-        uses: actions/github-script@60a0d83039c74a4aee543508d2ffcb1c3799cdea # v7.0.1
-        with:
-          script: |
-            const files = (await github.paginate(github.rest.pulls.listFiles, {
-              ...context.repo,
-              pull_number: context.payload.pull_request.number,
-              per_page: 100,
-            })).map(file => file.filename)
-
-            if (files.includes('ci/pinned.json')) core.setOutput('touched', ['pinned'])
-            else core.setOutput('touched', [])
+            require('./ci/github-script/prepare.js')({
+              github,
+              context,
+              core,
+            })
 
   check:
     name: Check

ci/github-script/prepare.js

···
+const { classify } = require('../supportedBranches.js')
+
 module.exports = async ({ github, context, core }) => {
   const pull_number = context.payload.pull_request.number
 
···
       await new Promise((resolve) => setTimeout(resolve, retryInterval * 1000))
       continue
     }
+
+    const { base, head } = prInfo
 
     let mergedSha, targetSha
 
···
       targetSha = (
         await github.rest.repos.compareCommitsWithBasehead({
           ...context.repo,
-          basehead: `${prInfo.base.sha}...${prInfo.head.sha}`,
+          basehead: `${base.sha}...${head.sha}`,
         })
       ).data.merge_base_commit.sha
     }
···
     )
     core.setOutput('mergedSha', mergedSha)
     core.setOutput('targetSha', targetSha)
+
+    core.setOutput('systems', require('../supportedSystems.json'))
+
+    const baseClassification = classify(base.ref)
+    core.setOutput('base', baseClassification)
+    console.log('base classification:', baseClassification)
+
+    const headClassification =
+      base.repo.full_name === head.repo.full_name
+        ? classify(head.ref)
+        : // PRs from forks are always considered WIP.
+          { type: ['wip'] }
+    core.setOutput('head', headClassification)
+    console.log('head classification:', headClassification)
+
+    const files = (
+      await github.paginate(github.rest.pulls.listFiles, {
+        ...context.repo,
+        pull_number: context.payload.pull_request.number,
+        per_page: 100,
+      })
+    ).map((file) => file.filename)
+
+    if (files.includes('ci/pinned.json')) core.setOutput('touched', ['pinned'])
+    else core.setOutput('touched', [])
+
     return
   }
   throw new Error(