Add rebase CI

.github/scripts/push.sh

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+#!/bin/bash
+
+set -eux
+
+upstream_repo=$1
+base_repo=$2
+output_branches=$3
+
+git remote add upstream "${upstream_repo}" || true
+git fetch upstream master
+
+# push each branch
+while IFS= read -r branch || [[ -n $branch ]]; do
+	git checkout "${branch}"
+	echo "git push -f ${base_repo} ${branch}"
+	# git push -f "${base_repo}" "${branch}"
+done < <(echo "${output_branches}")
+

.github/scripts/rebase.sh

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+#!/bin/bash
+
+set -eux
+
+upstream_repo=$1
+
+{
+
+git remote add upstream "${upstream_repo}" || true
+git fetch upstream
+
+output_branches=()
+
+# rebase each branch
+mkdir -p rebase
+while IFS= read -r branch || [[ -n $branch ]]; do
+	(
+	git checkout "${branch}"
+    common_ancestor=$(git merge-base "${branch}" "origin/master")
+	[ -e rebase/"${branch}" ] && exit 1
+    mkdir -p rebase/"${branch}"
+	cd rebase/"${branch}"
+	echo "${common_ancestor}" > BASE_COMMIT
+    git format-patch "${common_ancestor}".."${branch}"
+	if compgen -G "*.patch" > /dev/null; then
+      git reset --hard "upstream/master"
+      git am --3way *.patch
+	fi
+    )
+	output_branches+=( "${branch}" )
+done < <(gh pr list --label rebase --state open --json headRefName --template '{{range .}}{{tablerow .headRefName}}{{end}}')
+
+# sync master with upstream
+git checkout master
+git reset --hard upstream/master
+output_branches+=( "master" )
+
+} >&2
+
+(
+  IFS=$'\n'
+  echo "${output_branches[*]}"
+)
+

.github/workflows/rebase.yaml

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+name: Rebase against upstream
+
+on:
+  schedule:
+    - cron: '0 0 * * *'
+  workflow_dispatch:
+
+permissions:
+  contents: write
+  issues: write
+  pull-requests: read
+
+jobs:
+  rebase:
+    name: Rebase now!
+    runs-on: ubuntu-latest
+    env:
+      UPSTREAM_REPO: https://github.com/haskell/cabal.git
+    steps:
+      - name: Checkout code
+        uses: actions/checkout@v4
+        with:
+          fetch-depth: 0
+          persist-credentials: false
+
+      - run: |
+          set -eux
+
+          gh repo set-default stable-haskell/cabal
+          git config checkout.defaultRemote origin
+
+          # required to apply patches
+          git config user.email "[email protected]"
+          git config user.name "GitHub CI"
+
+          # this does not push
+          branches=$(bash .github/scripts/rebase.sh ${{ env.UPSTREAM_REPO }})
+
+          # now push
+          git checkout ${{ github.ref_name }}
+          bash .github/scripts/push.sh ${{ env.UPSTREAM_REPO }} https://${{ secrets.REBASE_PAT }}@github.com/${{ github.repository }}.git "${branches}"
+        shell: bash
+        env:
+          GH_TOKEN: ${{ github.token }}
+
+
+      # create an issue with a link to the workflow run on failure
+      - if: failure()
+        run: |
+          set -eux
+          gh repo set-default stable-haskell/cabal
+          for issue in $(gh issue list --label rebase-failure --json url -q '.[] | .url') ; do
+            gh issue close "${issue}"
+          done
+          gh issue create --title "Rebase failed on $(date -u +"%Y-%m-%d")" --label rebase-failure --body "${{ github.server_url }}/${{ github.repository }}/actions/runs/${{ github.run_id }}"
+        env:
+          GH_TOKEN: ${{ github.token }}
+
+      # we always want the original repo and the patches that were applied
+      # for debugging and backup reasons
+      - if: always()
+        run: |
+          git checkout -f master || true
+          git archive master > backup.tar
+          tar -rf backup.tar .git rebase
+      - if: always()
+        name: Upload backup
+        uses: actions/upload-artifact@v4
+        with:
+          if-no-files-found: error
+          retention-days: 7
+          name: backup
+          path: |
+            ./backup.tar