Clean module state at m_free time

We were not doing this previously, meaning that we always left some
references (and therefore memory) leaks behind after the module was
unloaded.

While here, we're also now freeing a reference to the modules where we
import our own exceptions, and the Decimal object, from.

Signed-off-by: Rodrigo Tobar <rtobar@icrar.org>

src/ijson/backends/ext/_yajl2/module.c

@@ -32,6 +32,7 @@ static PyMethodDef yajl2_methods[] = {
 };
 
 static int _yajl2_mod_exec(PyObject *m);
+static void _yajl2_mod_free(void *m);
 
 static PyModuleDef_Slot yajl2_slots[] = {
 	{Py_mod_exec, _yajl2_mod_exec},
@@ -97,6 +98,7 @@ static struct PyModuleDef moduledef = {
 	.m_size = sizeof(yajl2_state),
 	.m_methods = yajl2_methods,
 	.m_slots = yajl2_slots,
+	.m_free = _yajl2_mod_free,
 };
 
 static yajl2_state *get_state(PyObject *module)
@@ -174,16 +176,46 @@ static int _yajl2_mod_exec(PyObject *m)
 
 	// Import globally-used names
 	PyObject *ijson_common = PyImport_ImportModule("ijson.common");
-	PyObject *decimal_module = PyImport_ImportModule("decimal");
 	M1_N(ijson_common);
-	M1_N(decimal_module);
-
 	state->JSONError = PyObject_GetAttrString(ijson_common, "JSONError");
 	state->IncompleteJSONError = PyObject_GetAttrString(ijson_common, "IncompleteJSONError");
-	state->Decimal = PyObject_GetAttrString(decimal_module, "Decimal");
+	Py_DECREF(ijson_common);
 	M1_N(state->JSONError);
 	M1_N(state->IncompleteJSONError);
+
+	PyObject *decimal_module = PyImport_ImportModule("decimal");
+	M1_N(decimal_module);
+	state->Decimal = PyObject_GetAttrString(decimal_module, "Decimal");
+	Py_DECREF(decimal_module);
 	M1_N(state->Decimal);
 
 	return 0;
+}
+
+void _yajl2_mod_free(void *self)
+{
+	yajl2_state *state = PyModule_GetState((PyObject *)self);
+	if (!state) {
+		// module could have been initialised but not executed.
+		// We saw this in 3.8 when importing with importlib.module_from_spec
+		// and *not* executing the resulting module with spec.loader.exec_module.
+		return;
+	}
+	Py_XDECREF(state->Decimal);
+	Py_XDECREF(state->IncompleteJSONError);
+	Py_XDECREF(state->JSONError);
+	Py_XDECREF(state->dotitem);
+	Py_XDECREF(state->item);
+	Py_XDECREF(state->dot);
+	Py_XDECREF(state->enames.end_array_ename);
+	Py_XDECREF(state->enames.start_array_ename);
+	Py_XDECREF(state->enames.end_map_ename);
+	Py_XDECREF(state->enames.map_key_ename);
+	Py_XDECREF(state->enames.start_map_ename);
+	Py_XDECREF(state->enames.string_ename);
+	Py_XDECREF(state->enames.number_ename);
+	Py_XDECREF(state->enames.double_ename);
+	Py_XDECREF(state->enames.integer_ename);
+	Py_XDECREF(state->enames.boolean_ename);
+	Py_XDECREF(state->enames.null_ename);
 }