Safeguard default argument against mutation

src/pb_stub.cc

@@ -76,6 +76,27 @@ SignalHandler(int signum)
   // Skip the SIGINT and SIGTERM
 }
 
+template <typename PYTYPE>
+PYTYPE
+PyDefaultArgumentToMutableType(const py::object& argument)
+{
+  // The default argument on Python functions always reference the same copy,
+  // meaning if the default argument is changed by the function, then it is
+  // changed for all subsequent calls to the function. Thus, default arguments
+  // should be limited to basic types (i.e. None). This helper function returns
+  // an empty expected type, if the argument is None (i.e. default initialized).
+  // If the argument is neither None nor expected type, an exception is thrown.
+  if (py::isinstance<py::none>(argument)) {
+    return PYTYPE();
+  }
+  if (py::isinstance<PYTYPE>(argument)) {
+    return argument;
+  }
+  throw PythonBackendException(
+      std::string("Expect ") + typeid(PYTYPE).name() + ", got " +
+      std::string(py::str(argument.get_type())));
+}
+
 void
 Stub::Instantiate(
     int64_t shm_growth_size, int64_t shm_default_size,
@@ -1464,7 +1485,10 @@ PYBIND11_EMBEDDED_MODULE(c_python_backend_utils, module)
                       const int64_t model_version, const uint32_t flags,
                       const int32_t timeout,
                       const PreferredMemory& preferred_memory,
-                      const InferenceTrace& trace, const py::dict& parameters) {
+                      const InferenceTrace& trace,
+                      const py::object& parameters_) {
+            py::dict parameters =
+                PyDefaultArgumentToMutableType<py::dict>(parameters_);
             std::set<std::string> requested_outputs;
             for (auto& requested_output_name : requested_output_names) {
               requested_outputs.emplace(requested_output_name);
@@ -1503,7 +1527,7 @@ PYBIND11_EMBEDDED_MODULE(c_python_backend_utils, module)
           py::arg("preferred_memory").none(false) =
               PreferredMemory(PreferredMemory::DEFAULT, 0),
           py::arg("trace").none(false) = InferenceTrace(),
-          py::arg("parameters").none(false) = py::dict())
+          py::arg("parameters").none(true) = py::none())
       .def(
           "inputs", &InferRequest::Inputs,
           py::return_value_policy::reference_internal)