From: Paul Brannan <pbrannan@...>
Date: 2002-04-16T00:56:31+09:00
Subject: C++ Exceptions and Ruby's catch/throw

Because Ruby exceptions are implemented using longjmp, it is not safe to
allow a Ruby exception to propogate over C++ code.

To deal with this problem, I use rb_rescue2 around all my calls from C++
into Ruby.  This is unsafe in two cases:
  1) The Ruby code I am calling uses throw()
  2) The Ruby code I am calling calls a continuation

I don't there's much I can do about #2 except document it as having
undefined behavior.  However, I'd like to do something about #1.

When I catch an exception, I:
  1) re-throw it as a C++ exception (so it can propogate over C++ code
     and allow cleanup to occur), then
  2) catch it as a C++ exception, and
  3) re-raise it as a Ruby exception before returning to Ruby code.

I can use rb_protect instead of rb_rescue2, and this will catch both
exceptions and Ruby throws.  But how do I then re-throw what was thrown?

I.e.:

  extern VALUE ruby_errinfo;

  VALUE myfuncall(VALUE) {
    rb_eval_string("throw :foo");
    return Qnil;
  }

  VALUE foo(VALUE self) {
    int state = 0;
    VALUE retval = rb_protect(
        RUBY_METHOD_FUNC(myfuncall),
        Qnil,
        &state);
    if(state != 0) {
      // ruby_errinfo is nil here
    }
    return retval;
  }

How do I retrieve :foo from inside the foo() function?

Paul