Check write permissions

.github/workflows/ci.yml

@@ -25,6 +25,7 @@ jobs:
     name: GitHub CODEOWNERS Validator
     runs-on: ubuntu-latest
     steps:
+    - uses: cachix/install-nix-action@v26
 
     - uses: actions/checkout@v4
       with:
@@ -55,3 +56,12 @@ jobs:
 
         # Specifies whether only teams are allowed as owners of files.
         owner_checker_owners_must_be_teams: "false"
+
+    # The above validator doesn't currently ensure that people have write access: https://github.com/mszostok/codeowners-validator/issues/157
+    # So we're doing it manually instead
+    - name: Check that codeowners have write access
+      # Important that we run the script from the base branch,
+      # because otherwise a PR from a fork could change it to extract the secret
+      run: trusted-base/scripts/unprivileged-owners.sh untrusted-pr ${{ github.repository }}
+      env:
+        GH_TOKEN: "${{ secrets.OWNERS_VALIDATOR_GITHUB_SECRET }}"

scripts/unprivileged-owners.sh

@@ -0,0 +1,50 @@
+#!/usr/bin/env nix-shell
+#!nix-shell -i bash --pure -p codeowners github-cli
+
+set -euo pipefail
+
+tmp=$(mktemp -d)
+trap 'rm -rf "$tmp"' exit
+
+if (( $# != 2 )); then
+  echo "Usage: $0 PATH OWNER/REPO"
+  exit 1
+fi
+
+root=$1
+repo=$2
+
+# Writes all code owners into $tmp/codeowners, one user per line (without @)
+while read -r -a fields; do
+  # The first field is the filename
+  unset 'fields[0]'
+  if [[ "${fields[1]}" != "(unowned)" ]]; then
+    (IFS=$'\n'; echo "${fields[*]##@}")
+  fi
+done < <(codeowners --file "$root/.github/CODEOWNERS") |
+  sort -u > "$tmp/codeowners"
+
+# Get all users with push access
+gh api \
+  -H "Accept: application/vnd.github+json" \
+  -H "X-GitHub-Api-Version: 2022-11-28" \
+  --method GET \
+  -f permission=push \
+  /repos/"$repo"/collaborators \
+  -F per_page=100 \
+  --paginate \
+  --jq '.[].login' |
+  sort > "$tmp/collaborators"
+
+# Figure out all the owners that aren't collaborators
+readarray -t unprivilegedOwners < <(comm -23 "$tmp/codeowners" "$tmp/collaborators")
+
+if (( "${#unprivilegedOwners[@]}" == 0 )); then
+  echo "All code owners have write permission"
+else
+  echo "These code owners don't have write permission:"
+  for handle in "${unprivilegedOwners[@]}"; do
+    echo "- [ ] @$handle"
+  done
+  exit 1
+fi