ext: Update pybind11 to v2.8.1

Change-Id: Ia1c7081377f53fd470addf35526f8b28a949a7b0
Signed-off-by: Jason Lowe-Power <jason@lowepower.com>
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/52523
Maintainer: Bobby R. Bruce <bbruce@ucdavis.edu>
Tested-by: kokoro <noreply+kokoro@google.com>
Reviewed-by: Gabe Black <gabe.black@gmail.com>

ext/pybind11/tests/test_thread.cpp

@@ -0,0 +1,66 @@
+/*
+    tests/test_thread.cpp -- call pybind11 bound methods in threads
+
+    Copyright (c) 2021 Laramie Leavitt (Google LLC) <lar@google.com>
+
+    All rights reserved. Use of this source code is governed by a
+    BSD-style license that can be found in the LICENSE file.
+*/
+
+#include <pybind11/cast.h>
+#include <pybind11/pybind11.h>
+
+#include <chrono>
+#include <thread>
+
+#include "pybind11_tests.h"
+
+namespace py = pybind11;
+
+namespace {
+
+struct IntStruct {
+    explicit IntStruct(int v) : value(v) {};
+    ~IntStruct() { value = -value; }
+    IntStruct(const IntStruct&) = default;
+    IntStruct& operator=(const IntStruct&) = default;
+
+    int value;
+};
+
+} // namespace
+
+TEST_SUBMODULE(thread, m) {
+
+    py::class_<IntStruct>(m, "IntStruct").def(py::init([](const int i) { return IntStruct(i); }));
+
+    // implicitly_convertible uses loader_life_support when an implicit
+    // conversion is required in order to lifetime extend the reference.
+    //
+    // This test should be run with ASAN for better effectiveness.
+    py::implicitly_convertible<int, IntStruct>();
+
+    m.def("test", [](int expected, const IntStruct &in) {
+        {
+            py::gil_scoped_release release;
+            std::this_thread::sleep_for(std::chrono::milliseconds(5));
+        }
+
+        if (in.value != expected) {
+            throw std::runtime_error("Value changed!!");
+        }
+    });
+
+    m.def(
+        "test_no_gil",
+        [](int expected, const IntStruct &in) {
+            std::this_thread::sleep_for(std::chrono::milliseconds(5));
+            if (in.value != expected) {
+                throw std::runtime_error("Value changed!!");
+            }
+        },
+        py::call_guard<py::gil_scoped_release>());
+
+    // NOTE: std::string_view also uses loader_life_support to ensure that
+    // the string contents remain alive, but that's a C++ 17 feature.
+}