From: senderista@... (Tobin Baker)
Date: 2001-07-15T14:07:29+09:00
Subject: [ruby-talk:17864] Trapping Ruby exceptions from C

I'm trying to solve the problem of how to trap a Ruby exception from a
C call to a Ruby method.  (Specifically, I'm writing server-side CORBA
bindings which unmarshal an incoming request and dynamically dispatch
it to a Ruby servant object registered with the ORB.)  The code for
this is completely generic; it has no idea what methods it's calling
or what exceptions these might raise.  What it needs to do is get a
copy of the exception raised so it can marshal it over the wire back
to the client.  Right now I'm using rb_rescue for the method call,
along with a helper class to hold any exception raised.  The code
looks like this:

    class ErrorHolder
      def initialize(error=nil)
        unless error.nil?
          raise TypeError unless error.kind_of?(StandardError)
        end
        @error = error
      end
    end

// wrapper for rb_apply, necessary since all arguments for a function
passed to rb_rescue must be packed into a single array

static VALUE
rorbit_apply_try(VALUE packed_args)
{
    int argc;
    VALUE *argv;
    VALUE recv;
    VALUE sym;
    VALUE args;

    Check_Type(packed_args, T_ARRAY);
    argc = RARRAY(packed_args)->len;
    argv = RARRAY(packed_args)->ptr;
    rb_scan_args(argc, argv, "3", &recv, &sym, &args);
    Check_Type(args, T_ARRAY);
    return rb_apply(recv, SYM2ID(sym), args);
}

// Use as second function pointer to rb_rescue (with error_holder used
by caller to retrieve error value)
static VALUE
rorbit_trap_exception(VALUE error_holder, VALUE error)
{
    if (!RTEST(rb_obj_is_kind_of(error, rb_eStandardError)))
	rb_iv_set(error_holder, "@error", rb_class_new_instance(0, 0,
rb_eTypeError);
    else
	rb_iv_set(error_holder, "@error", error);
    return Qnil;
}

This is the code that calls an arbitrary method on the Ruby servant
using rb_apply:

// use rb_rescue to catch exception
    VALUE packed_args = rb_ary_new3(3, servant->impl,
ID2SYM(rb_intern(name)), args);
    VALUE error_holder = rb_class_new_instance(0, 0,
rorbit_c_CORBA_ORBit_ErrorHolder);
    results = rb_rescue(rorbit_apply_try, packed_args,
rorbit_trap_exception, error_holder);
    error = rb_iv_get(error_holder, "@error");

Is there a way to retrieve an exception from the method called by
rb_apply without having to use a helper class?  I was frankly
surprised there wasn't a simpler way to do it, since this is something
C extensions would have to do all the time.  Am I missing something
obvious?