py/modsys: Add optional attribute delegation.

To be enabled when needed by specific sys attributes.

Signed-off-by: Damien George <damien@micropython.org>

py/modsys.c

@@ -27,6 +27,7 @@
 
 #include "py/builtin.h"
 #include "py/objlist.h"
+#include "py/objmodule.h"
 #include "py/objtuple.h"
 #include "py/objstr.h"
 #include "py/objint.h"
@@ -182,6 +183,18 @@ STATIC mp_obj_t mp_sys_settrace(mp_obj_t obj) {
 MP_DEFINE_CONST_FUN_OBJ_1(mp_sys_settrace_obj, mp_sys_settrace);
 #endif // MICROPY_PY_SYS_SETTRACE
 
+#if MICROPY_PY_SYS_ATTR_DELEGATION
+STATIC const uint16_t sys_mutable_keys[] = {
+    MP_QSTRnull,
+};
+
+STATIC void mp_module_sys_attr(mp_obj_t self_in, qstr attr, mp_obj_t *dest) {
+    MP_STATIC_ASSERT(MP_ARRAY_SIZE(sys_mutable_keys) == MP_SYS_MUTABLE_NUM + 1);
+    MP_STATIC_ASSERT(MP_ARRAY_SIZE(MP_STATE_VM(sys_mutable)) == MP_SYS_MUTABLE_NUM);
+    mp_module_generic_attr(attr, dest, sys_mutable_keys, MP_STATE_VM(sys_mutable));
+}
+#endif
+
 STATIC const mp_rom_map_elem_t mp_module_sys_globals_table[] = {
     { MP_ROM_QSTR(MP_QSTR___name__), MP_ROM_QSTR(MP_QSTR_sys) },
 
@@ -244,6 +257,11 @@ STATIC const mp_rom_map_elem_t mp_module_sys_globals_table[] = {
     #if MICROPY_PY_SYS_ATEXIT
     { MP_ROM_QSTR(MP_QSTR_atexit), MP_ROM_PTR(&mp_sys_atexit_obj) },
     #endif
+
+    #if MICROPY_PY_SYS_ATTR_DELEGATION
+    // Delegation of attr lookup.
+    MP_MODULE_ATTR_DELEGATION_ENTRY(&mp_module_sys_attr),
+    #endif
 };
 
 STATIC MP_DEFINE_CONST_DICT(mp_module_sys_globals, mp_module_sys_globals_table);

py/mpconfig.h

@@ -1377,6 +1377,12 @@ typedef double mp_float_t;
 #define MICROPY_PY_SYS_STDIO_BUFFER (MICROPY_CONFIG_ROM_LEVEL_AT_LEAST_EXTRA_FEATURES)
 #endif
 
+// Whether the sys module supports attribute delegation
+// This is enabled automatically when needed by other features
+#ifndef MICROPY_PY_SYS_ATTR_DELEGATION
+#define MICROPY_PY_SYS_ATTR_DELEGATION (0)
+#endif
+
 // Whether to provide "uerrno" module
 #ifndef MICROPY_PY_UERRNO
 #define MICROPY_PY_UERRNO (MICROPY_CONFIG_ROM_LEVEL_AT_LEAST_EXTRA_FEATURES)

py/mpstate.h

@@ -40,6 +40,10 @@
 // memory system, runtime and virtual machine.  The state is a global
 // variable, but in the future it is hoped that the state can become local.
 
+enum {
+    MP_SYS_MUTABLE_NUM,
+};
+
 // This structure contains dynamic configuration for the compiler.
 #if MICROPY_DYNAMIC_COMPILER
 typedef struct mp_dynamic_compiler_t {
@@ -158,6 +162,11 @@ typedef struct _mp_state_vm_t {
     // must be initialised after the call to mp_init.
     mp_obj_list_t mp_sys_path_obj;
     mp_obj_list_t mp_sys_argv_obj;
+
+    #if MICROPY_PY_SYS_ATTR_DELEGATION
+    // Contains mutable sys attributes.
+    mp_obj_t sys_mutable[MP_SYS_MUTABLE_NUM];
+    #endif
     #endif
 
     // dictionary for overridden builtins