objexcept: No more magic messages in exceptions, only exception arguments.

One of the reason for separate "message" (besides still unfulfilled desire to
optimize memory usage) was apparent special handling of exception with
messages by CPython. Well, the message is still just an exception argument,
it just printed specially. Implement that with PRINT_EXC printing format.

py/obj.c

@@ -62,7 +62,7 @@ void mp_obj_print_exception(mp_obj_t exc) {
             }
         }
     }
-    mp_obj_print(exc, PRINT_REPR);
+    mp_obj_print(exc, PRINT_EXC);
     printf("\n");
 }
 

py/obj.h

@@ -141,7 +141,9 @@ typedef mp_obj_t (*mp_fun_var_t)(uint n, const mp_obj_t *);
 typedef mp_obj_t (*mp_fun_kw_t)(uint n, const mp_obj_t *, mp_map_t *);
 
 typedef enum {
-    PRINT_STR, PRINT_REPR
+    PRINT_STR,
+    PRINT_REPR,
+    PRINT_EXC, // Special format for printing exception in unhandled exception message
 } mp_print_kind_t;
 
 typedef void (*mp_print_fun_t)(void (*print)(void *env, const char *fmt, ...), void *env, mp_obj_t o, mp_print_kind_t kind);

py/objexcept.c

@@ -11,32 +11,25 @@
 #include "runtime.h"
 #include "runtime0.h"
 
-// This is unified class for C-level and Python-level exceptions
-// Python-level exceptions have empty ->msg and all arguments are in
-// args tuple. C-level exceptions likely have ->msg set, and args is empty.
 typedef struct _mp_obj_exception_t {
     mp_obj_base_t base;
     mp_obj_t traceback; // a list object, holding (file,line,block) as numbers (not Python objects); a hack for now
-    vstr_t *msg;
     mp_obj_tuple_t args;
 } mp_obj_exception_t;
 
 // Instance of GeneratorExit exception - needed by generator.close()
 // This would belong to objgenerator.c, but to keep mp_obj_exception_t
 // definition module-private so far, have it here.
-const mp_obj_exception_t mp_const_GeneratorExit_obj = {{&mp_type_GeneratorExit}, MP_OBJ_NULL, NULL, {{&mp_type_tuple}, 0}};
+const mp_obj_exception_t mp_const_GeneratorExit_obj = {{&mp_type_GeneratorExit}, MP_OBJ_NULL, {{&mp_type_tuple}, 0}};
 
 STATIC void mp_obj_exception_print(void (*print)(void *env, const char *fmt, ...), void *env, mp_obj_t o_in, mp_print_kind_t kind) {
     mp_obj_exception_t *o = o_in;
-    if (o->msg != NULL) {
-        print(env, "%s: %s", qstr_str(o->base.type->name), vstr_str(o->msg));
-    } else {
-        // Yes, that's how CPython has it
-        // TODO now that exceptions are classes and instances, I think this needs to be changed to match CPython
     if (kind == PRINT_REPR) {
         print(env, "%s", qstr_str(o->base.type->name));
+    } else if (kind == PRINT_EXC) {
+        print(env, "%s: ", qstr_str(o->base.type->name));
     }
-        if (kind == PRINT_STR) {
+    if (kind == PRINT_STR || kind == PRINT_EXC) {
         if (o->args.len == 0) {
             print(env, "");
             return;
@@ -47,7 +40,6 @@ STATIC void mp_obj_exception_print(void (*print)(void *env, const char *fmt, ...
     }
     tuple_print(print, env, &o->args, kind);
 }
-}
 
 STATIC mp_obj_t mp_obj_exception_make_new(mp_obj_t type_in, uint n_args, uint n_kw, const mp_obj_t *args) {
     mp_obj_type_t *type = type_in;
@@ -59,7 +51,6 @@ STATIC mp_obj_t mp_obj_exception_make_new(mp_obj_t type_in, uint n_args, uint n_
     mp_obj_exception_t *o = m_new_obj_var(mp_obj_exception_t, mp_obj_t, n_args);
     o->base.type = type;
     o->traceback = MP_OBJ_NULL;
-    o->msg = NULL;
     o->args.base.type = &mp_type_tuple;
     o->args.len = n_args;
     memcpy(o->args.items, args, n_args * sizeof(mp_obj_t));
@@ -185,7 +176,7 @@ MP_DEFINE_EXCEPTION(Exception, BaseException)
     */
 
 mp_obj_t mp_obj_new_exception(const mp_obj_type_t *exc_type) {
-    return mp_obj_new_exception_msg_varg(exc_type, NULL);
+    return mp_obj_new_exception_args(exc_type, 0, NULL);
 }
 
 mp_obj_t mp_obj_new_exception_args(const mp_obj_type_t *exc_type, uint n_args, const mp_obj_t *args) {
@@ -202,22 +193,25 @@ mp_obj_t mp_obj_new_exception_msg_varg(const mp_obj_type_t *exc_type, const char
     assert(exc_type->make_new == mp_obj_exception_make_new);
 
     // make exception object
-    mp_obj_exception_t *o = m_new_obj_var(mp_obj_exception_t, mp_obj_t, 0);
+    mp_obj_exception_t *o = m_new_obj_var(mp_obj_exception_t, mp_obj_t, 1);
     o->base.type = exc_type;
     o->traceback = MP_OBJ_NULL;
     o->args.base.type = &mp_type_tuple;
-    o->args.len = 0;
+    o->args.len = 1;
 
     if (fmt == NULL) {
         // no message
-        o->msg = NULL;
+        assert(0);
     } else {
-        // render exception message
-        o->msg = vstr_new();
+        // render exception message and store as .args[0]
+        // TODO: optimize bufferbloat
+        vstr_t *vstr = vstr_new();
         va_list ap;
         va_start(ap, fmt);
-        vstr_vprintf(o->msg, fmt, ap);
+        vstr_vprintf(vstr, fmt, ap);
         va_end(ap);
+        o->args.items[0] = mp_obj_new_str((byte*)vstr->buf, vstr->len, false);
+        vstr_free(vstr);
     }
 
     return o;