github-ci: close PRs updated after being opened

Enforce our policy of requiring a new pull request whenever changes are
made to an existing one. On any push to an open, non-draft pull request
the workflow comments with the policy and closes the pull request.

Author: jasonish

.github/workflows/close-updated-prs.yml

@@ -0,0 +1,70 @@
+name: Close updated PRs
+
+on:
+  pull_request_target:
+    types:
+      - synchronize
+
+permissions:
+  actions: write
+  contents: read
+  issues: write
+  pull-requests: write
+
+concurrency:
+  group: ${{ github.workflow }}-${{ github.event.pull_request.number }}
+  cancel-in-progress: false
+
+jobs:
+  close-updated-pr:
+    name: Close updated PR
+    if: ${{ github.event.pull_request.draft == false }}
+    runs-on: ubuntu-latest
+    env:
+      GH_TOKEN: ${{ github.token }}
+      REPOSITORY: ${{ github.repository }}
+      PR_NUMBER: ${{ github.event.pull_request.number }}
+      AFTER: ${{ github.event.after }}
+    steps:
+      - name: Close updated PR
+        run: |
+          set -euo pipefail
+
+          body=$(cat <<'EOF'
+          Closing this pull request: its branch was updated after the pull
+          request was opened.
+
+          Per our policy, a new pull request is required when changes are made
+          to an existing one. Please open a new pull request with the updated
+          changes.
+          EOF
+          )
+
+          gh api \
+              --method POST \
+              "/repos/${REPOSITORY}/issues/${PR_NUMBER}/comments" \
+              -f body="${body}" \
+              --silent
+
+          gh api \
+              --method PATCH \
+              "/repos/${REPOSITORY}/pulls/${PR_NUMBER}" \
+              -f state=closed \
+              --silent
+
+      # Run last: cancelling the runs for this push also cancels this run, so
+      # this must come after the PR has already been closed.
+      - name: Cancel running workflows for this push
+        run: |
+          set -euo pipefail
+
+          for status in queued in_progress; do
+              gh api --paginate \
+                  "/repos/${REPOSITORY}/actions/runs?head_sha=${AFTER}&status=${status}" \
+                  --jq '.workflow_runs[].id' | while read -r run_id; do
+                  gh api \
+                      --method POST \
+                      "/repos/${REPOSITORY}/actions/runs/${run_id}/cancel" \
+                      --silent || true
+              done
+          done