Forum: Ruby-core Possible mruby bug in mrb_load_string

Hi all,

Originally posted to ruby-talk, in case it was a mistake on my end. It
was suggested I post the message to ruby-core:

I have been having a bit of a play around with mruby (note: mruby, not
MRI Ruby), and I believe I may have found a bug in a recent (20130222)
version.

Basically, if you call mrb_load_string with something that returns a
class, then make a call on the returned object that throws an exception,
and then call mrb_load_string with a string that makes the same kind of
return, the class in question has somehow vanished from the environment,
and the second load will fail.

Test program and Makefile below. The test string is simply "Float",
which returns the Float class. The first time it calls it, it is okay.
The second time, "Float" no longer exists.

Naturally, if you remove the call to 'dummyCall', the second load works
as it should.

I have seen the problem with other classes as well, although the
specifics of what triggers the problem can vary. It is hard to make a
general statement as to the exact fault. As a general rule, if any Class
is returned from mrb_load_string, and a subsequent call on the return
generates an exception, depending on what you do, the whole Class may or
may not (seem to) be removed from the current environment.

I can't guarantee my code below is bug-free- I'm still fairly new to
mruby. It is possible I have made an incorrect assumption in the code
below. If not, it looks like a mruby bug.

I am using mruby as checked out on 20130222 (22/2/13), Linux, Debian,
64-bit. Can supply additional information if needed.

Cheers,
Garth

-----------------
floatnom.c
-----------------

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

static void show_result(mrb_state *mrb, mrb_value v)
{
   switch (mrb_type(v))
   {
     case MRB_TT_CLASS:
       fprintf(stderr, "- Type Class returned.\n");
       break;
     case MRB_TT_FALSE:
       fprintf(stderr, "- Type false/nil returned.\n");
       break;
     default:
       fprintf(stderr, "- Type %d returned.\n", mrb_type(v));
       break;
   }

   if (mrb->exc)
   {
     struct RObject *exc = mrb->exc;
     mrb->exc = 0;
     mrb_value e = mrb_obj_value(exc);
     mrb_value e_to_s = mrb_funcall(mrb, e, "to_s", 0);
     assert(mrb->exc == 0);
     if (mrb_type(e_to_s) == MRB_TT_STRING)
     {
       char buff[512];
       memcpy(buff, RSTRING_PTR(e_to_s), RSTRING_LEN(e_to_s));
       buff[RSTRING_LEN(e_to_s)] = '\0';
       fprintf(stderr, "- Exception was: %s\n", buff);
     }
     else
       fprintf(stderr, "- Exception could not be identified.\n");
     assert(mrb->exc == 0);
   }
   else
     fprintf(stderr, "- No exception.\n");
}

int main(int argc, char *argv[])
{
   mrb_state *mrb = mrb_open();

   fprintf(stderr, "Loading first string:\n");
   mrb_value v1 = mrb_load_string(mrb, "Float");
   show_result(mrb, v1);

   fprintf(stderr, "Making bad call:\n");
   mrb_value v2 = mrb_funcall(mrb, v1, "dummyCall", 0);
   show_result(mrb, v2);

   fprintf(stderr, "Loading same string again. Should work, but
doesn't.\n");
   mrb_value v3 = mrb_load_string(mrb, "Float");
   show_result(mrb, v3);

   mrb_close(mrb);
   return 0;
}

-----------------
Makefile
-----------------

floatnom: floatnom.o
  gcc -o floatnom floatnom.o -L/path/to/mruby/lib -lmruby -lm

floatnom.o: floatnom.c
  gcc -Wall -o floatnom.o -I/path/to/mruby/include -c floatnom.c

clean:
  rm -f floatnom floatnom.o


-----------------
Typical output
-----------------

(May be typos- transcribed by hand)

Loading first string:
- Type Class returned.
- No exception.
Making bad call:
- Type false/nil returned.
- Exception was: undefined method 'dummyCall' for Float
Loading same string again. Should work, but doesn't.
- Type false/nil returned.
- Exception was: uninitialized constant Float

Hi all,

Please disregard my previous email- the (potential) bug is now submitted
as an issue on github for mruby.

Cheers,
Garth