workflows/get-merge-commit: move to composite action

We don't need a separate workflow anymore, because we don't need to skip
dependent jobs on failures anymore. The biggest failure mode was
"conflict" previously, but we resolved that on the last commit. The
remaining failure modes are so rare, that it's OK to just fail the jobs
in this case instead of marking them as "skipped". Especially, because
the resolve-merge-commit job would have previously failed anyway.

By moving this to an action we avoid running separate jobs each time we
need the merge commit. This also makes the check list in PRs much
cleaner.

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

@@ -0,0 +1,66 @@
+name: Get merge commit
+
+description: 'Checks whether the Pull Request is mergeable and returns two commit hashes: 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.'
+
+outputs:
+  mergedSha:
+    description: "The merge commit SHA"
+    value: ${{ fromJSON(steps.merged.outputs.result).mergedSha }}
+  targetSha:
+    description: "The target commit SHA"
+    value: ${{ fromJSON(steps.merged.outputs.result).targetSha }}
+
+runs:
+  using: composite
+  steps:
+    - id: merged
+      uses: actions/github-script@60a0d83039c74a4aee543508d2ffcb1c3799cdea # v7.0.1
+      with:
+        script: |
+          if (context.eventName == 'push') return { 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 ${retryInterval} seconds before trying again...`)
+              await new Promise(resolve => setTimeout(resolve, retryInterval * 1000))
+              continue
+            }
+
+            if (prInfo.mergeable) {
+              console.log("The PR can be merged.")
+
+              const mergedSha = prInfo.merge_commit_sha
+              const targetSha = (await github.rest.repos.getCommit({
+                owner: context.repo.owner,
+                repo: context.repo.repo,
+                ref: prInfo.merge_commit_sha
+              })).data.parents[0].sha
+
+              console.log(`Checking the commits:\nmerged:${mergedSha}\ntarget:${targetSha}`)
+
+              return { mergedSha, targetSha }
+            } else {
+              console.log("The PR has a merge conflict.")
+
+              const mergedSha = prInfo.head.sha
+              const targetSha = (await github.rest.repos.compareCommitsWithBasehead({
+                owner: context.repo.owner,
+                repo: context.repo.repo,
+                basehead: `${prInfo.base.sha}...${prInfo.head.sha}`
+              })).data.merge_base_commit.sha
+
+              console.log(`Checking the commits:\nmerged:${mergedSha}\ntarget:${targetSha}`)
+
+              return { mergedSha, targetSha }
+            }
+          }
+          throw new Error("Not retrying anymore. It's likely that GitHub is having internal issues: check https://www.githubstatus.com.")

.github/workflows/check-format.yml

@@ -9,18 +9,20 @@ on:
 permissions: {}
 
 jobs:
-  get-merge-commit:
-    uses: ./.github/workflows/get-merge-commit.yml
-
   nixos:
     name: fmt-check
     runs-on: ubuntu-24.04-arm
-    needs: get-merge-commit
-    if: needs.get-merge-commit.outputs.mergedSha
     steps:
       - uses: actions/checkout@11bd71901bbe5b1630ceea73d27597364c9af683 # v4.2.2
         with:
-          ref: ${{ needs.get-merge-commit.outputs.mergedSha }}
+          sparse-checkout: .github/actions
+      - name: Check if the PR can be merged and get the test merge commit
+        uses: ./.github/actions/get-merge-commit
+        id: get-merge-commit
+
+      - uses: actions/checkout@11bd71901bbe5b1630ceea73d27597364c9af683 # v4.2.2
+        with:
+          ref: ${{ steps.get-merge-commit.outputs.mergedSha }}
 
       - uses: cachix/install-nix-action@526118121621777ccd86f79b04685a9319637641 # v31
         with:

.github/workflows/codeowners-v2.yml

@@ -37,17 +37,19 @@ env:
   DRY_MODE: ${{ github.event.pull_request.draft && '1' || '' }}
 
 jobs:
-  get-merge-commit:
-    if: github.repository_owner == 'NixOS'
-    uses: ./.github/workflows/get-merge-commit.yml
-
   # Check that code owners is valid
   check:
     name: Check
     runs-on: ubuntu-24.04-arm
-    needs: get-merge-commit
-    if: github.repository_owner == 'NixOS' && needs.get-merge-commit.outputs.mergedSha
+    if: github.repository_owner == 'NixOS'
     steps:
+      - uses: actions/checkout@11bd71901bbe5b1630ceea73d27597364c9af683 # v4.2.2
+        with:
+          sparse-checkout: .github/actions
+      - name: Check if the PR can be merged and get the test merge commit
+        uses: ./.github/actions/get-merge-commit
+        id: get-merge-commit
+
       - uses: cachix/install-nix-action@526118121621777ccd86f79b04685a9319637641 # v31
 
       - uses: cachix/cachix-action@0fc020193b5a1fa3ac4575aa3a7d3aa6a35435ad # v16
@@ -77,7 +79,7 @@ jobs:
 
       - uses: actions/checkout@11bd71901bbe5b1630ceea73d27597364c9af683 # v4.2.2
         with:
-          ref: ${{ needs.get-merge-commit.outputs.mergedSha }}
+          ref: ${{ steps.get-merge-commit.outputs.mergedSha }}
           path: pr
 
       - name: Validate codeowners

.github/workflows/eval-aliases.yml

@@ -9,18 +9,21 @@ on:
 permissions: {}
 
 jobs:
-  get-merge-commit:
-    uses: ./.github/workflows/get-merge-commit.yml
-
   eval-aliases:
     name: Eval nixpkgs with aliases enabled
     runs-on: ubuntu-24.04-arm
-    needs: [ get-merge-commit ]
     steps:
+      - uses: actions/checkout@11bd71901bbe5b1630ceea73d27597364c9af683 # v4.2.2
+        with:
+          sparse-checkout: .github/actions
+      - 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: Check out the PR at the test merge commit
         uses: actions/checkout@11bd71901bbe5b1630ceea73d27597364c9af683 # v4.2.2
         with:
-          ref: ${{ needs.get-merge-commit.outputs.mergedSha }}
+          ref: ${{ steps.get-merge-commit.outputs.mergedSha }}
           path: nixpkgs
 
       - name: Install Nix

.github/workflows/eval.yml

@@ -19,17 +19,36 @@ on:
 permissions: {}
 
 jobs:
-  get-merge-commit:
-    uses: ./.github/workflows/get-merge-commit.yml
+  prepare:
+    name: Prepare
+    runs-on: ubuntu-24.04-arm
+    outputs:
+      mergedSha: ${{ steps.get-merge-commit.outputs.mergedSha }}
+      targetSha: ${{ steps.get-merge-commit.outputs.targetSha }}
+      systems: ${{ steps.systems.outputs.systems }}
+    steps:
+      - uses: actions/checkout@11bd71901bbe5b1630ceea73d27597364c9af683 # v4.2.2
+        with:
+          sparse-checkout: |
+            .github/actions
+            ci/supportedSystems.json
+      - name: Check if the PR can be merged and get the test merge commit
+        uses: ./.github/actions/get-merge-commit
+        id: get-merge-commit
+
+      - name: Load supported systems
+        id: systems
+        run: |
+          echo "systems=$(jq -c <ci/supportedSystems.json)" >> "$GITHUB_OUTPUT"
 
   outpaths:
     name: Outpaths
     runs-on: ubuntu-24.04-arm
-    needs: [ get-merge-commit ]
+    needs: [ prepare ]
     strategy:
       fail-fast: false
       matrix:
-        system: ${{ fromJSON(needs.get-merge-commit.outputs.systems) }}
+        system: ${{ fromJSON(needs.prepare.outputs.systems) }}
     steps:
       - name: Enable swap
         run: |
@@ -41,7 +60,7 @@ jobs:
       - name: Check out the PR at the test merge commit
         uses: actions/checkout@11bd71901bbe5b1630ceea73d27597364c9af683 # v4.2.2
         with:
-          ref: ${{ needs.get-merge-commit.outputs.mergedSha }}
+          ref: ${{ needs.prepare.outputs.mergedSha }}
           path: nixpkgs
 
       - name: Install Nix
@@ -67,7 +86,7 @@ jobs:
   process:
     name: Process
     runs-on: ubuntu-24.04-arm
-    needs: [ outpaths, get-merge-commit ]
+    needs: [ prepare, outpaths ]
     outputs:
       targetRunId: ${{ steps.targetRunId.outputs.targetRunId }}
     steps:
@@ -80,7 +99,7 @@ jobs:
       - name: Check out the PR at the test merge commit
         uses: actions/checkout@11bd71901bbe5b1630ceea73d27597364c9af683 # v4.2.2
         with:
-          ref: ${{ needs.get-merge-commit.outputs.mergedSha }}
+          ref: ${{ needs.prepare.outputs.mergedSha }}
           fetch-depth: 2
           path: nixpkgs
 
@@ -102,7 +121,7 @@ jobs:
           path: prResult/*
 
       - name: Get target run id
-        if: needs.get-merge-commit.outputs.targetSha
+        if: needs.prepare.outputs.targetSha
         id: targetRunId
         run: |
           # Get the latest eval.yml workflow run for the PR's target commit
@@ -131,7 +150,7 @@ jobs:
           echo "targetRunId=$runId" >> "$GITHUB_OUTPUT"
         env:
           REPOSITORY: ${{ github.repository }}
-          TARGET_SHA: ${{ needs.get-merge-commit.outputs.targetSha }}
+          TARGET_SHA: ${{ needs.prepare.outputs.targetSha }}
           GH_TOKEN: ${{ github.token }}
 
       - uses: actions/download-artifact@v4
@@ -145,8 +164,8 @@ jobs:
       - name: Compare against the target branch
         if: steps.targetRunId.outputs.targetRunId
         run: |
-          git -C nixpkgs worktree add ../target ${{ needs.get-merge-commit.outputs.targetSha }}
-          git -C nixpkgs diff --name-only ${{ needs.get-merge-commit.outputs.targetSha }} \
+          git -C nixpkgs worktree add ../target ${{ needs.prepare.outputs.targetSha }}
+          git -C nixpkgs diff --name-only ${{ needs.prepare.outputs.targetSha }} \
             | jq --raw-input --slurp 'split("\n")[:-1]' > touched-files.json
 
           # Use the target branch to get accurate maintainer info
@@ -172,7 +191,7 @@ jobs:
   tag:
     name: Tag
     runs-on: ubuntu-24.04-arm
-    needs: [ get-merge-commit, process ]
+    needs: [ prepare, process ]
     if: needs.process.outputs.targetRunId
     permissions:
       pull-requests: write
@@ -204,7 +223,7 @@ jobs:
       - name: Check out Nixpkgs at the base commit
         uses: actions/checkout@11bd71901bbe5b1630ceea73d27597364c9af683 # v4.2.2
         with:
-          ref: ${{ needs.get-merge-commit.outputs.targetSha }}
+          ref: ${{ needs.prepare.outputs.targetSha }}
           path: base
           sparse-checkout: ci
 

.github/workflows/get-merge-commit.yml

@@ -1,90 +0,0 @@
-name: Get merge commit
-
-on:
-  pull_request:
-    paths:
-      - .github/workflows/get-merge-commit.yml
-  workflow_call:
-    outputs:
-      mergedSha:
-        description: "The merge commit SHA"
-        value: ${{ jobs.resolve-merge-commit.outputs.mergedSha }}
-      targetSha:
-        description: "The target commit SHA"
-        value: ${{ jobs.resolve-merge-commit.outputs.targetSha }}
-      systems:
-        description: "The supported systems"
-        value: ${{ jobs.resolve-merge-commit.outputs.systems }}
-
-permissions: {}
-
-jobs:
-  resolve-merge-commit:
-    runs-on: ubuntu-24.04-arm
-    outputs:
-      mergedSha: ${{ fromJSON(steps.merged.outputs.result).mergedSha }}
-      targetSha: ${{ fromJSON(steps.merged.outputs.result).targetSha }}
-      systems: ${{ steps.systems.outputs.systems }}
-    steps:
-      - uses: actions/checkout@11bd71901bbe5b1630ceea73d27597364c9af683 # v4.2.2
-        with:
-          path: base
-          sparse-checkout: ci
-
-      - name: Check if the PR can be merged and get the test merge commit
-        id: merged
-        uses: actions/github-script@60a0d83039c74a4aee543508d2ffcb1c3799cdea # v7.0.1
-        with:
-          script: |
-            if (context.eventName == 'push') return { 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 ${retryInterval} seconds before trying again...`)
-                await new Promise(resolve => setTimeout(resolve, retryInterval * 1000))
-                continue
-              }
-
-              if (prInfo.mergeable) {
-                console.log("The PR can be merged.")
-
-                const mergedSha = prInfo.merge_commit_sha
-                const targetSha = (await github.rest.repos.getCommit({
-                  owner: context.repo.owner,
-                  repo: context.repo.repo,
-                  ref: prInfo.merge_commit_sha
-                })).data.parents[0].sha
-
-                console.log(`Checking the commits:\nmerged:${mergedSha}\ntarget:${targetSha}`)
-
-                return { mergedSha, targetSha }
-              } else {
-                console.log("The PR has a merge conflict.")
-
-                const mergedSha = prInfo.head.sha
-                const targetSha = (await github.rest.repos.compareCommitsWithBasehead({
-                  owner: context.repo.owner,
-                  repo: context.repo.repo,
-                  basehead: `${prInfo.base.sha}...${prInfo.head.sha}`
-                })).data.merge_base_commit.sha
-
-                console.log(`Checking the commits:\nmerged:${mergedSha}\ntarget:${targetSha}`)
-
-                return { mergedSha, targetSha }
-              }
-            }
-            throw new Error("Not retrying anymore. It's likely that GitHub is having internal issues: check https://www.githubstatus.com.")
-
-      - name: Load supported systems
-        id: systems
-        run: |
-          echo "systems=$(jq -c <base/ci/supportedSystems.json)" >> "$GITHUB_OUTPUT"

.github/workflows/lib-tests.yml

@@ -12,18 +12,20 @@ on:
 permissions: {}
 
 jobs:
-  get-merge-commit:
-    uses: ./.github/workflows/get-merge-commit.yml
-
   nixpkgs-lib-tests:
     name: nixpkgs-lib-tests
     runs-on: ubuntu-24.04
-    needs: get-merge-commit
-    if: needs.get-merge-commit.outputs.mergedSha
     steps:
       - uses: actions/checkout@11bd71901bbe5b1630ceea73d27597364c9af683 # v4.2.2
         with:
-          ref: ${{ needs.get-merge-commit.outputs.mergedSha }}
+          sparse-checkout: .github/actions
+      - name: Check if the PR can be merged and get the test merge commit
+        uses: ./.github/actions/get-merge-commit
+        id: get-merge-commit
+
+      - uses: actions/checkout@11bd71901bbe5b1630ceea73d27597364c9af683 # v4.2.2
+        with:
+          ref: ${{ steps.get-merge-commit.outputs.mergedSha }}
 
       - uses: cachix/install-nix-action@526118121621777ccd86f79b04685a9319637641 # v31
         with:

.github/workflows/nix-parse-v2.yml

@@ -9,18 +9,21 @@ on:
 permissions: {}
 
 jobs:
-  get-merge-commit:
-    uses: ./.github/workflows/get-merge-commit.yml
-
   tests:
     name: nix-files-parseable-check
     runs-on: ubuntu-24.04-arm
-    needs: get-merge-commit
-    if: "needs.get-merge-commit.outputs.mergedSha && !contains(github.event.pull_request.title, '[skip treewide]')"
+    if: "!contains(github.event.pull_request.title, '[skip treewide]')"
     steps:
       - uses: actions/checkout@11bd71901bbe5b1630ceea73d27597364c9af683 # v4.2.2
         with:
-          ref: ${{ needs.get-merge-commit.outputs.mergedSha }}
+          sparse-checkout: .github/actions
+      - name: Check if the PR can be merged and get the test merge commit
+        uses: ./.github/actions/get-merge-commit
+        id: get-merge-commit
+
+      - uses: actions/checkout@11bd71901bbe5b1630ceea73d27597364c9af683 # v4.2.2
+        with:
+          ref: ${{ steps.get-merge-commit.outputs.mergedSha }}
 
       - uses: cachix/install-nix-action@526118121621777ccd86f79b04685a9319637641 # v31
         with:

.github/workflows/nixpkgs-vet.yml

@@ -17,21 +17,23 @@ permissions: {}
 # There is a feature request for suppressing notifications on concurrency-canceled runs: https://github.com/orgs/community/discussions/13015
 
 jobs:
-  get-merge-commit:
-    uses: ./.github/workflows/get-merge-commit.yml
-
   check:
     name: nixpkgs-vet
     # This needs to be x86_64-linux, because we depend on the tooling being pre-built in the GitHub releases.
     runs-on: ubuntu-24.04
     # This should take 1 minute at most, but let's be generous. The default of 6 hours is definitely too long.
     timeout-minutes: 10
-    needs: get-merge-commit
-    if: needs.get-merge-commit.outputs.mergedSha
     steps:
       - uses: actions/checkout@11bd71901bbe5b1630ceea73d27597364c9af683 # v4.2.2
         with:
-          ref: ${{ needs.get-merge-commit.outputs.mergedSha }}
+          sparse-checkout: .github/actions
+      - name: Check if the PR can be merged and get the test merge commit
+        uses: ./.github/actions/get-merge-commit
+        id: get-merge-commit
+
+      - uses: actions/checkout@11bd71901bbe5b1630ceea73d27597364c9af683 # v4.2.2
+        with:
+          ref: ${{ steps.get-merge-commit.outputs.mergedSha }}
           # Fetches the merge commit and its parents
           fetch-depth: 2