From: "Ara.T.Howard" <Ara.T.Howard@...>
Date: 2004-08-24T23:50:45+09:00
Subject: POLS - exception comparisons



what is the meaning of this?


   jib:~ > cat a.rb
   def method
     raise 'foobar'
   end

   errors = []

   2.times do
     Thread.new do
       begin
         method
       rescue => e
         errors << e
       end
     end.join
   end

   p errors

   p(errors[-2] == errors[-1])

   p((errors[-2].class == errors[-1].class and
     errors[-2].message == errors[-1].message and
     errors[-2].backtrace == errors[-1].backtrace))


   jib:~ > ruby a.rb
   [#<RuntimeError: foobar>, #<RuntimeError: foobar>]
   false
   true


i'm writing some code which must be fault tolerant: something like


   def error_wrap errors = [], &block
     begin
       block.call errors
     rescue => e
       errors << e
       if errors.size >= @fault_tolerance
         raise
       else
         if errors.size == 1 or (errors.size >= 2 and errors[-2] != errors[-1])
           @logger.warn{ errors.last }
           error_wrap errors, &block
         end
       end
     end
   end

which runs a block of code, handling errors by logging them if it's a new
error (don't busy the log duplicate errors), eventually failing if too many
errors are seen.

the problems is that

   errors[-2] != errors[-1]

is always true.  as the first bit of code shows, the '==' operator seems to be
implemented in a counter intuitive way.  doesn't it make sense that exceptions
with the same class, message, and backtrace should be considered the same?

looking briefly at error.c leads me to believe that it's Object#== used to
compare exceptions - does this make sense?

regards.


-a
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