Ruby Forum Ruby-core > "fiber called across trap" - why would this be bad?

Hello,

I have a C program that embeds Ruby 1.9.1. When I call into the Ruby 
interpreter, and Ruby yield()s an object from a fiber back to my C 
program, then later call back the object from C, which causes the fiber 
to be resume()d, it causes a Ruby error with message "fiber called 
across trap".

If I comment out the relevant code in Ruby's cont.c (lines 876-877), the 
above case works fine. My question is, what is the danger of not doing 
the "fiber called across trap" check, or what is the badness that this 
check is protecting from? I know nothing about the Ruby VM, so go easy 
on me. :-)

If you want to know why I'm doing this, please read on. If this is a 
dumb way to be doing things, can anyone tell me what I should be doing 
instead of resuming fibers across C calls?

--8<----

First, my C program calls out to the Ruby interpreter with something 
like

  rb_funcall(obj, rb_intern("call"), argc, args...);

Ruby does some "business logic" processing then returns a Ruby array to 
C (actually yield()ed from a fiber) like this:

  [REQ_DB, query_str, callback_obj, callback_method]

The C program (the "server") then saves the pointers to callback_obj and 
callback_method, turns query_str into a query, and sends it to our 
internal database in a non-blocking fashion.

[There are also other response types, like REQ_HTTP (which performs a 
non-blocking HTTP request to a backend web service).]

On receiving the response from the database, the server then calls Ruby 
back with the result of the query as a Ruby string:

  rb_funcall(callback_obj, SYM2ID(callback_method), 
INT2FIX(db_result_code),
             rb_str_new(db_result_start, 
db_result_end-db_result_start));

This callback results in a call to resume() on the fiber that yielded 
(and was wrapped and docked into a Hash of in-flight requests).

The intention is that in Ruby we can write code that looks like:

  def dostuff
    return something_based_on(req_db(query_string1), 
req_db(query_string2))
  end

rather than

  def dostuff
    req_db(query_string1, self, :handle_query1)
  end

  def handle_query1(resultset1)
    @resultset1 = resultset1 # ugh.
    req_db(query_string2, self, :handle_query2)
  end

  def handle_query2(resultset2)
    return something_based_on(@resultset1, resultset2) # ugh!
  end

Thanks for reading,

James


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