workflows/periodic-merge: merge merge-base into haskell-updates

Since haskell-updates is based on master, but merges into staging, we
need to base it on a merge-base of staging and master. See #361143.

I'm a bit worried that the information GitHub uses for displaying
Pull-Requests becomes stale and this will “add” commits to the PR
compared to the base anyways. We'll find out, I suppose.

.github/workflows/periodic-merge-24h.yml

@@ -33,8 +33,6 @@ jobs:
       max-parallel: 1
       matrix:
         pairs:
-          - from: master
-            into: haskell-updates
           - from: release-24.11
             into: staging-next-24.11
           - from: staging-next-24.11

.github/workflows/periodic-merge-haskell-updates.yml

@@ -0,0 +1,59 @@
+# This action periodically merges a merge base of master and staging into haskell-updates.
+#
+# haskell-updates is based on master (so there are little unrelated failures and the cache
+# is already prepopulated), but needs to target staging due to the high amount of rebuilds
+# it typically causes. To prevent unrelated commits clattering the GitHub UI, we need to
+# take care to only merge the merge-base of master and staging into haskell-updates.
+#
+# See also https://github.com/NixOS/nixpkgs/issues/361143.
+
+name: "Periodic Merges (haskell-updates)"
+
+
+on:
+  schedule:
+    # * is a special character in YAML so you have to quote this string
+    # Merge every 24 hours
+    - cron:  '0 0 * * *'
+  workflow_dispatch:
+
+permissions:
+  contents: read
+
+jobs:
+  periodic-merge:
+    permissions:
+      contents: write  # for devmasx/merge-branch to merge branches
+      pull-requests: write  # for peter-evans/create-or-update-comment to create or update comment
+    if: github.repository_owner == 'NixOS'
+    runs-on: ubuntu-latest
+    name: git merge-base master staging → haskell-updates
+    steps:
+      - uses: actions/checkout@11bd71901bbe5b1630ceea73d27597364c9af683 # v4.2.2
+        with:
+          fetch-depth: 0
+
+      # Note: If we want to do something similar for more branches, we can move this into a
+      # separate job, so we can use the matrix strategy again.
+      - name: Find merge base of master and staging
+        id: find_merge_base_step
+        run: |
+          merge_base="$(git merge-base refs/remotes/origin/master refs/remotes/origin/staging)"
+          echo "Found merge base: $merge_base" >&2
+          echo "merge_base=$merge_base" >> "$GITHUB_OUTPUT"
+
+      - name: git merge-base master staging → haskell-updates
+        uses: devmasx/merge-branch@854d3ac71ed1e9deb668e0074781b81fdd6e771f # 1.4.0
+        with:
+          type: now
+          head_to_merge: ${{ steps.find_merge_base_step.outputs.merge_base }}
+          target_branch: haskell-updates
+          github_token: ${{ secrets.GITHUB_TOKEN }}
+
+      - name: Comment on failure
+        uses: peter-evans/create-or-update-comment@71345be0265236311c031f5c7866368bd1eff043 # v4.0.0
+        if: ${{ failure() }}
+        with:
+          issue-number: 367709
+          body: |
+            Periodic merge from `${{ steps.find_merge_base_step.outputs.merge_base }}` into `haskell-updates` has [failed](https://github.com/NixOS/nixpkgs/actions/runs/${{ github.run_id }}).