common: Prevent module re-import duplication in Python3

Author: EricCousineau-TRI

include/pybind11/detail/common.h

@@ -171,7 +171,18 @@
 #define PYBIND11_PLUGIN_IMPL(name) \
     extern "C" PYBIND11_EXPORT PyObject *PyInit_##name()
 
-#else
+#define PYBIND11_CHECK_REIMPORT(m)                                \
+    {                                                             \
+        static py::handle m##_original;                           \
+        if (m##_original) {                                       \
+            m = py::reinterpret_borrow<py::module>(m##_original); \
+            return m.ptr();                                       \
+        } else {                                                  \
+            m##_original = m;                                     \
+        }                                                         \
+    }
+
+#else // PY_MAJOR_VERSION >= 3
 #define PYBIND11_INSTANCE_METHOD_NEW(ptr, class_) PyMethod_New(ptr, nullptr, class_)
 #define PYBIND11_INSTANCE_METHOD_CHECK PyMethod_Check
 #define PYBIND11_INSTANCE_METHOD_GET_FUNCTION PyMethod_GET_FUNCTION
@@ -198,6 +209,7 @@
         (void)pybind11_init_wrapper();                      \
     }                                                       \
     PyObject *pybind11_init_wrapper()
+#define PYBIND11_CHECK_REIMPORT(variable)
 #endif
 
 #if PY_VERSION_HEX >= 0x03050000 && PY_VERSION_HEX < 0x03050200
@@ -285,6 +297,7 @@ extern "C" {
     PYBIND11_PLUGIN_IMPL(name) {                                               \
         PYBIND11_CHECK_PYTHON_VERSION                                          \
         auto m = pybind11::module(PYBIND11_TOSTRING(name));                    \
+        PYBIND11_CHECK_REIMPORT(m)                                             \
         try {                                                                  \
             PYBIND11_CONCAT(pybind11_init_, name)(m);                          \
             return m.ptr();                                                    \