From: Joel VanderWerf <vjoel@...>
Date: 2003-11-29T07:37:18+09:00
Subject: re-raising an exception with the original backtrace


Is there a way to re-raise an exception without replacing its backtrace 
with the current one?

The reason I want to do this is that I am catching an exception and 
passing it to a handler. The handler needs to make a decision and then, 
possibly, re-raise the same exception, preferrably with the same 
information as it originally contained.

But if I do the following, the exception takes on the backtrace of the 
handler, which has lost some information. I know I can add the original 
backtrace to the exception's message, but that's not the same thing...

$ cat exception.rb

   class MyError < StandardError; end

   def foo
     raise MyError
   end

   def main
     foo
   rescue Exception => e
     handler(e)
   end

   def handler(e)
     ok = false
     if ok
       return true
     else
       puts "backtrace:", e.backtrace, "---"
       raise e
     end
   end

   main

$ ruby exception.rb
backtrace:
exception.rb:4:in `foo'
exception.rb:8:in `main'
exception.rb:23
---
exception.rb:19:in `handler': MyError (MyError)
         from exception.rb:10:in `main'
         from exception.rb:23

Note that the backtrace of e knows about foo, but the re-raised 
exception does not.