fix regression reported by @cyfdecyf in #1454 (#1517)

tests/test_virtual_functions.cpp

@@ -10,6 +10,7 @@
 #include "pybind11_tests.h"
 #include "constructor_stats.h"
 #include <pybind11/functional.h>
+#include <thread>
 
 /* This is an example class that we'll want to be able to extend from Python */
 class ExampleVirt  {
@@ -157,6 +158,28 @@ struct DispatchIssue : Base {
     }
 };
 
+static void test_gil() {
+    {
+        py::gil_scoped_acquire lock;
+        py::print("1st lock acquired");
+
+    }
+
+    {
+        py::gil_scoped_acquire lock;
+        py::print("2nd lock acquired");
+    }
+
+}
+
+static void test_gil_from_thread() {
+    py::gil_scoped_release release;
+
+    std::thread t(test_gil);
+    t.join();
+}
+
+
 // Forward declaration (so that we can put the main tests here; the inherited virtual approaches are
 // rather long).
 void initialize_inherited_virtuals(py::module &m);
@@ -414,7 +437,6 @@ public:
 };
 */
 
-
 void initialize_inherited_virtuals(py::module &m) {
     // test_inherited_virtuals
 
@@ -447,4 +469,8 @@ void initialize_inherited_virtuals(py::module &m) {
     py::class_<D_Tpl, C_Tpl, PyB_Tpl<D_Tpl>>(m, "D_Tpl")
         .def(py::init<>());
 
+
+    // Fix issue #1454 (crash when acquiring/releasing GIL on another thread in Python 2.7)
+    m.def("test_gil", &test_gil);
+    m.def("test_gil_from_thread", &test_gil_from_thread);
 };