shared/runtime/pyexec: Make a raised SystemExit always do a forced exit.

The current situation with SystemExit and soft reset is the following:
- `sys.exit()` follows CPython and just raises `SystemExit`.
- On the unix port, raising `SystemExit` quits the application/MicroPython,
  whether at the REPL or in code (this follows CPython behaviour).
- On bare-metal ports, raising `SystemExit` at the REPL does nothing,
  raising it in code will stop the code and drop into the REPL.
- `machine.soft_reset()` raises `SystemExit` but with a special flag set,
  and bare-metal targets check this flag when it propagates to the
  top-level and do a soft reset when they receive it.

The original idea here was that a bare-metal target can't "quit" like the
unix port can, and so dropping to the REPL was considered the same as
"quit".  But this bare-metal behaviour is arguably inconsistent with unix,
and "quit" should mean terminate everything, including REPL access.

This commit changes the behaviour to the following, which is more
consistent:
- Raising `SystemExit` on a bare-metal port will do a soft reset (unless
  the exception is caught by the application).
- `machine.soft_reset()` is now equivalent to `sys.exit()`.
- unix port behaviour remains unchanged.

Tested running the test suite on an stm32 board and everything still
passes, in particular tests that skip by raising `SystemExit` still
correctly skip.

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

shared/runtime/pyexec.c

@@ -44,7 +44,6 @@
 #include "genhdr/mpversion.h"
 
 pyexec_mode_kind_t pyexec_mode_kind = PYEXEC_MODE_FRIENDLY_REPL;
-int pyexec_system_exit = 0;
 
 #if MICROPY_REPL_INFO
 static bool repl_display_debugging_info = 0;
@@ -74,9 +73,6 @@ static int parse_compile_execute(const void *source, mp_parse_input_kind_t input
     MICROPY_BOARD_BEFORE_PYTHON_EXEC(input_kind, exec_flags);
     #endif
 
-    // by default a SystemExit exception returns 0
-    pyexec_system_exit = 0;
-
     nlr_buf_t nlr;
     nlr.ret_val = NULL;
     if (nlr_push(&nlr) == 0) {
@@ -146,7 +142,7 @@ static int parse_compile_execute(const void *source, mp_parse_input_kind_t input
         // check for SystemExit
         if (mp_obj_is_subclass_fast(MP_OBJ_FROM_PTR(((mp_obj_base_t *)nlr.ret_val)->type), MP_OBJ_FROM_PTR(&mp_type_SystemExit))) {
             // at the moment, the value of SystemExit is unused
-            ret = pyexec_system_exit;
+            ret = PYEXEC_FORCED_EXIT;
         } else {
             mp_obj_print_exception(&mp_plat_print, MP_OBJ_FROM_PTR(nlr.ret_val));
             ret = 0;

shared/runtime/pyexec.h

@@ -35,11 +35,6 @@ typedef enum {
 
 extern pyexec_mode_kind_t pyexec_mode_kind;
 
-// Set this to the value (eg PYEXEC_FORCED_EXIT) that will be propagated through
-// the pyexec functions if a SystemExit exception is raised by the running code.
-// It will reset to 0 at the start of each execution (eg each REPL entry).
-extern int pyexec_system_exit;
-
 #define PYEXEC_FORCED_EXIT (0x100)
 
 int pyexec_raw_repl(void);