Cannot get exception backtrace after its inspect method is called (in C)

[seanchas116]

I tried to inspect and get backtrace of exceptions in C
and I noticed that backtrace returns an empty array after inspect is called.

#include <stdio.h>
#include <mruby.h>
#include <mruby/compile.h>
#include <mruby/string.h>

mrb_value mrb_get_backtrace(mrb_state *mrb, mrb_value self);

int main(void)
{
    mrb_state *mrb = mrb_open();

    mrbc_context *context = mrbc_context_new(mrb);
    mrbc_filename(mrb, context, "file");

    mrb_load_string_cxt(mrb,
                        "def foo\n"
                        "  xxx(0)\n"
                        "end\n"
                        "foo",
                        context);

    mrb_value exc = mrb_obj_value(mrb->exc);

    // this works //
    mrb_value backtrace = mrb_get_backtrace(mrb, exc);
    puts(mrb_str_to_cstr(mrb, mrb_inspect(mrb, backtrace)));
    // => ["file:2:in Object.foo", "file:4"]

    mrb_value inspect = mrb_inspect(mrb, exc);
    puts(mrb_str_to_cstr(mrb, inspect));

    // this does not works //
    /*
    mrb_value inspect = mrb_inspect(mrb, exc);
    puts(mrb_str_to_cstr(mrb, inspect));

    mrb_value backtrace = mrb_get_backtrace(mrb, exc);
    puts(mrb_str_to_cstr(mrb, mrb_inspect(mrb, backtrace)));
    // => []
    */

    mrbc_context_free(mrb, context);
    mrb_close(mrb);

    return 0;
}

The same thing works fine when I loaded this file in mruby command.

begin
  5.times do
    raise "error"
  end
rescue => exc
  p exc.inspect
  p exc.backtrace
end

[matz]

mrb_get_backtrace() retrieve backtrace information from the stack in the mrb_state.
If you call methods the call stack will be overwritten. So call mrb_get_backtrace() just after you catch the exception. I will add description about this issue.