Serialize rule writes and add tests

tests/mocks/persist-mock.sh

@@ -3,9 +3,31 @@ set -euo pipefail
 
 : "${PERSIST_MOCK_LOG:=/tmp/persist-mock.log}"
 : "${PERSIST_MOCK_FAIL:=0}"
+: "${PERSIST_MOCK_DELAY_SECS:=0}"
+: "${PERSIST_MOCK_ACTIVE_DIR:=}"
+: "${PERSIST_MOCK_FAIL_ON_CONCURRENT:=0}"
 
 printf '%s %s\n' "$(basename -- "$0")" "$*" >>"${PERSIST_MOCK_LOG}"
 
+cleanup() {
+  [[ -n ${PERSIST_MOCK_ACTIVE_DIR} ]] || return 0
+  rmdir "${PERSIST_MOCK_ACTIVE_DIR}" 2>/dev/null || true
+}
+
+if [[ -n ${PERSIST_MOCK_ACTIVE_DIR} ]]; then
+  if ! mkdir "${PERSIST_MOCK_ACTIVE_DIR}" 2>/dev/null; then
+    if [[ ${PERSIST_MOCK_FAIL_ON_CONCURRENT} == 1 ]]; then
+      exit 1
+    fi
+  else
+    trap cleanup EXIT
+  fi
+fi
+
+if [[ ${PERSIST_MOCK_DELAY_SECS} != 0 ]]; then
+  sleep "${PERSIST_MOCK_DELAY_SECS}"
+fi
+
 if [[ ${PERSIST_MOCK_FAIL} == 1 ]]; then
   exit 1
 fi