static allocation for PyModuleDef, to avoid leak check errors. (#2413)

* Initializing PyModuleDef object with PyModuleDef_HEAD_INIT.

Python 3.8 documentation: m_base - Always initialize this member to PyModuleDef_HEAD_INIT.

Long-standing (since first github commit in 2015), inconsequential bug.

Also removing inconsequential Py_INCREF(def): PyModule_Create() resets the reference count to 1.

* git rebase master

* moving static PyModuleDef declaration to global scope, as requested by @wjakob

* renaming the two new macros, to start with PYBIND11_DETAIL_MODULE

include/pybind11/detail/common.h

@@ -307,13 +307,26 @@ extern "C" {
             });
         }
 \endrst */
+#if PY_MAJOR_VERSION >= 3
+#define PYBIND11_DETAIL_MODULE_STATIC_DEF(name)                                \
+    static PyModuleDef PYBIND11_CONCAT(pybind11_module_def_, name);
+#define PYBIND11_DETAIL_MODULE_CREATE(name)                                    \
+        auto m = pybind11::module(                                             \
+            PYBIND11_TOSTRING(name), nullptr,                                  \
+            &PYBIND11_CONCAT(pybind11_module_def_, name));
+#else
+#define PYBIND11_DETAIL_MODULE_STATIC_DEF(name)
+#define PYBIND11_DETAIL_MODULE_CREATE(name)                                    \
+        auto m = pybind11::module(PYBIND11_TOSTRING(name));
+#endif
 #define PYBIND11_MODULE(name, variable)                                        \
+    PYBIND11_DETAIL_MODULE_STATIC_DEF(name)                                    \
     PYBIND11_MAYBE_UNUSED                                                      \
     static void PYBIND11_CONCAT(pybind11_init_, name)(pybind11::module &);     \
     PYBIND11_PLUGIN_IMPL(name) {                                               \
         PYBIND11_CHECK_PYTHON_VERSION                                          \
         PYBIND11_ENSURE_INTERNALS_READY                                        \
-        auto m = pybind11::module(PYBIND11_TOSTRING(name));                    \
+        PYBIND11_DETAIL_MODULE_CREATE(name)                                    \
         try {                                                                  \
             PYBIND11_CONCAT(pybind11_init_, name)(m);                          \
             return m.ptr();                                                    \

include/pybind11/pybind11.h

@@ -862,18 +862,24 @@ public:
     PYBIND11_OBJECT_DEFAULT(module_, object, PyModule_Check)
 
     /// Create a new top-level Python module with the given name and docstring
-    explicit module_(const char *name, const char *doc = nullptr) {
-        if (!options::show_user_defined_docstrings()) doc = nullptr;
 #if PY_MAJOR_VERSION >= 3
-        auto *def = new PyModuleDef();
-        std::memset(def, 0, sizeof(PyModuleDef));
-        def->m_name = name;
-        def->m_doc = doc;
-        def->m_size = -1;
-        Py_INCREF(def);
+    explicit module_(const char *name, const char *doc = nullptr, PyModuleDef *def = nullptr) {
+        if (!def) def = new PyModuleDef();
+        def = new (def) PyModuleDef {  // Placement new (not an allocation).
+            /* m_base */     PyModuleDef_HEAD_INIT,
+            /* m_name */     name,
+            /* m_doc */      options::show_user_defined_docstrings() ? doc : nullptr,
+            /* m_size */     -1,
+            /* m_methods */  nullptr,
+            /* m_slots */    nullptr,
+            /* m_traverse */ nullptr,
+            /* m_clear */    nullptr,
+            /* m_free */     nullptr
+        };
         m_ptr = PyModule_Create(def);
 #else
-        m_ptr = Py_InitModule3(name, nullptr, doc);
+    explicit module_(const char *name, const char *doc = nullptr) {
+        m_ptr = Py_InitModule3(name, nullptr, options::show_user_defined_docstrings() ? doc : nullptr);
 #endif
         if (m_ptr == nullptr)
             pybind11_fail("Internal error in module_::module_()");