extmod/modussl: Add finaliser support for ussl objects.

Per the comment found here
#209 (comment),
this patch adds finaliser code to prevent memory leaks from ussl objects,
which is especially useful when memory for a ussl context is allocated
outside the uPy heap.  This patch is in-line with the finaliser code found
in many modsocket implementations for various ports.

This feature is configured via MICROPY_PY_USSL_FINALISER and is disabled by
default because there may be issues using it when the ussl state *is*
allocated on the uPy heap, rather than externally.

Author: MrSurly

extmod/modussl_axtls.c

@@ -51,7 +51,11 @@ struct ssl_args {
 STATIC const mp_obj_type_t ussl_socket_type;
 
 STATIC mp_obj_ssl_socket_t *socket_new(mp_obj_t sock, struct ssl_args *args) {
+#if MICROPY_PY_USSL_FINALISER
+    mp_obj_ssl_socket_t *o = m_new_obj_with_finaliser(mp_obj_ssl_socket_t);
+#else
     mp_obj_ssl_socket_t *o = m_new_obj(mp_obj_ssl_socket_t);
+#endif
     o->base.type = &ussl_socket_type;
     o->buf = NULL;
     o->bytes_left = 0;
@@ -178,6 +182,9 @@ STATIC const mp_rom_map_elem_t ussl_socket_locals_dict_table[] = {
     { MP_ROM_QSTR(MP_QSTR_write), MP_ROM_PTR(&mp_stream_write_obj) },
     { MP_ROM_QSTR(MP_QSTR_setblocking), MP_ROM_PTR(&socket_setblocking_obj) },
     { MP_ROM_QSTR(MP_QSTR_close), MP_ROM_PTR(&socket_close_obj) },
+#if MICROPY_PY_USSL_FINALISER
+    { MP_ROM_QSTR(MP_QSTR___del__), MP_ROM_PTR(&socket_close_obj) },
+#endif
 };
 
 STATIC MP_DEFINE_CONST_DICT(ussl_socket_locals_dict, ussl_socket_locals_dict_table);

extmod/modussl_mbedtls.c

@@ -111,7 +111,11 @@ int _mbedtls_ssl_recv(void *ctx, byte *buf, size_t len) {
 
 
 STATIC mp_obj_ssl_socket_t *socket_new(mp_obj_t sock, struct ssl_args *args) {
+#if MICROPY_PY_USSL_FINALISER
+    mp_obj_ssl_socket_t *o = m_new_obj_with_finaliser(mp_obj_ssl_socket_t);
+#else
     mp_obj_ssl_socket_t *o = m_new_obj(mp_obj_ssl_socket_t);
+#endif
     o->base.type = &ussl_socket_type;
 
     int ret;
@@ -272,6 +276,9 @@ STATIC const mp_rom_map_elem_t ussl_socket_locals_dict_table[] = {
     { MP_ROM_QSTR(MP_QSTR_write), MP_ROM_PTR(&mp_stream_write_obj) },
     { MP_ROM_QSTR(MP_QSTR_setblocking), MP_ROM_PTR(&socket_setblocking_obj) },
     { MP_ROM_QSTR(MP_QSTR_close), MP_ROM_PTR(&socket_close_obj) },
+#if MICROPY_PY_USSL_FINALISER
+    { MP_ROM_QSTR(MP_QSTR___del__), MP_ROM_PTR(&socket_close_obj) },
+#endif
     { MP_ROM_QSTR(MP_QSTR_getpeercert), MP_ROM_PTR(&mod_ssl_getpeercert_obj) },
 };
 

py/mpconfig.h

@@ -1109,6 +1109,8 @@ typedef double mp_float_t;
 
 #ifndef MICROPY_PY_USSL
 #define MICROPY_PY_USSL (0)
+// Whether to add finaliser code to ussl objects
+#define MICROPY_PY_USSL_FINALISER (0)
 #endif
 
 #ifndef MICROPY_PY_WEBSOCKET