From: Joel VanderWerf <vjoel@...>
Date: 2008-04-06T15:47:33+09:00
Subject: augmented exceptions


I was working with exceptions bubbling up out of some C code, and having 
trouble crafting an exception message that was descriptive enough to 
identify the object(s) involved so I could easily find and inspect them.

The following is some ruby and C code (the ruby code is useful by 
itself) for "augmented exceptions". Maybe there's better terminology, 
but the idea is that you can tell the handler which object is causing 
the problem, or if there is no handler then at least the stack dump 
tells you a little more about the object. It falls back gracefully if 
#inspect (or #to_s) fails for the object in question.

========================================================================

# Include this module in an exception class to pass an object along
# with the exception so that it can be used by the rescuer. To raise
# an exception with an object, use this syntax:
#
#   raise YourExceptionClass, [message_string, object], ...
#
# To raise an exception normally, with just a message string:
#
#   raise YourExceptionClass, message_string, ...
#
# The object is used in two ways.
#
# First, the message you provide when you raise the exception is
# concatenated with a string describing the object so that the message
# string on the receiving end of the exception says something helpful
# about this object. To generate the string, we first use #inspect on
# the object. If #inspect fails, we fall back to #to_s; if that fails,
# we fall back to class name and #object_id.
#
# Second, the object can be accessed from $! (or, equivalently, the
# exception object captured using the "rescue => ex" syntax) using
# the #object method.
#
# The rationale for all of this is that sometimes you want to be able
# to do "post-mortem" analysis on an object involved in an exception,
# either in a rescue clause or by looking at the exception message.
#
module AugmentedException
   attr_reader :object

   def initialize(msg)
     if defined?(msg.first)
       msg, @object = *msg
       s = ((@object.inspect rescue
             @object.to_s) rescue
             "id ##{@object.object_id} of class #{@object.class}")
       msg += " Object is: $!.object == #{s}"
     end
     super msg
   end
end

# Example:

class MyError < StandardError
   include AugmentedException
end

class C
   attr_reader :x
   def initialize x
     @x = x
   end

   def inspect
     #raise # <-- try this!
     super
   end

   def to_s
     #raise # <-- try this!
     "C with x=#{@x}"
   end
end

# If you are rescuing, then you can work with the object itself.
begin
   raise MyError, ["foo bar", C.new(3)]
rescue MyError => ex
   p ex.message # "foo bar Object is: $!.object == #<C:0xb7cfb6d0 @x=3>"
   puts ex      # foo bar Object is: $!.object == #<C:0xb7cfb6d0 @x=3>
   p ex.object  # #<C:0xb7cfb6d0 @x=3>
end

puts
# If all you have is the stack dump, you still get more info
# than before.
raise MyError, ["zap", C.new(4)]
   # c.rb:78: zap Object is: $!.object == #<C:0xb7da65d0 @x=4> (MyError)

__END__

/*
   If you want to generate these exceptions from C code, you will find
   that rb_raise() doesn't quite work (it wants a string, not an object).
   But the following does work nicely. It's almost a drop-in replacement
   for rb_raise (and the implementation is based on MRI). You just add an
   argument referencing the object you wish to propagate to the rescuer.
   You can still use format strings as with rb_raise.
*/

#include <stdarg.h>

void aug_ex_raise(VALUE exc, VALUE obj, const char *fmt, ...)
{
     va_list args;
     char buf[BUFSIZ];
     VALUE ex, ary;

     va_start(args, fmt);
     vsnprintf(buf, BUFSIZ, fmt, args);
     va_end(args);

     ary = rb_ary_new3(2, rb_str_new2(buf), obj);
     ex = rb_funcall(exc, ID_new, 1, ary);
     rb_exc_raise(ex);
}

// Sample use:
aug_ex_raise(exception_class, object, "foo: %s, %d", "bar", 13);

-- 
       vjoel : Joel VanderWerf : path berkeley edu : 510 665 3407