ci: add hotfix cherry-pick provenance check (#36340)

.github/scripts/check-hotfix-cherry-picks.sh

@@ -0,0 +1,73 @@
+#!/usr/bin/env bash
+set -euo pipefail
+
+BASE_SHA=${BASE_SHA:-}
+HEAD_SHA=${HEAD_SHA:-}
+MAIN_REF=${MAIN_REF:-origin/main}
+REMEDIATION_HINT="Changes should be made from the main branch using git cherry-pick -x."
+
+error() {
+  printf 'ERROR: %s\n' "$1" >&2
+}
+
+if [[ -z "$BASE_SHA" || -z "$HEAD_SHA" ]]; then
+  error "BASE_SHA and HEAD_SHA are required. $REMEDIATION_HINT"
+  exit 2
+fi
+
+if ! git rev-parse --verify "$BASE_SHA^{commit}" > /dev/null 2>&1; then
+  error "Base commit '$BASE_SHA' is not available in the local git checkout."
+  exit 2
+fi
+
+if ! git rev-parse --verify "$HEAD_SHA^{commit}" > /dev/null 2>&1; then
+  error "Head commit '$HEAD_SHA' is not available in the local git checkout."
+  exit 2
+fi
+
+if ! git rev-parse --verify "$MAIN_REF^{commit}" > /dev/null 2>&1; then
+  error "Main ref '$MAIN_REF' is not available in the local git checkout. $REMEDIATION_HINT"
+  exit 2
+fi
+
+failed=0
+checked=0
+
+while IFS= read -r commit_sha; do
+  [[ -n "$commit_sha" ]] || continue
+
+  checked=$((checked + 1))
+  subject=$(git log -1 --format=%s "$commit_sha")
+  source_sha=$(
+    git log -1 --format=%B "$commit_sha" \
+      | sed -nE 's/^\(cherry picked from commit ([0-9a-fA-F]{7,64})\)$/\1/p' \
+      | tail -n 1
+  )
+
+  if [[ -z "$source_sha" ]]; then
+    error "Commit $commit_sha ($subject) is missing cherry-pick provenance. $REMEDIATION_HINT"
+    failed=1
+    continue
+  fi
+
+  if ! git cat-file -e "$source_sha^{commit}" 2> /dev/null; then
+    error "Commit $commit_sha ($subject) references source $source_sha, but that commit is not available locally. $REMEDIATION_HINT"
+    failed=1
+    continue
+  fi
+
+  if ! git merge-base --is-ancestor "$source_sha" "$MAIN_REF"; then
+    error "Commit $commit_sha ($subject) references source $source_sha, but that source is not reachable from main ($MAIN_REF). $REMEDIATION_HINT"
+    failed=1
+  fi
+done < <(git rev-list --reverse "$BASE_SHA..$HEAD_SHA")
+
+if [[ "$failed" -ne 0 ]]; then
+  exit 1
+fi
+
+if [[ "$checked" -eq 0 ]]; then
+  echo "No PR commits to check."
+else
+  echo "Verified $checked PR commit(s) include cherry-pick provenance from main."
+fi