[LRUG] To debug the impossible bug

Wed Aug 3 04:09:31 PDT 2011

On 3 Aug 2011, at 12:05, Matthew Rudy Jacobs wrote:

> I didn't think "case()" was defined
> but my guess is that it means "case nil"
> 
> so effectively you're saying;
> 
> "when nil is @question"?
> and in the case it fails
> its because @question is exactly nil.
> 

This isn't right - irb exploration shows that case with no comparison value is *not* equivalent to case nil.

ruby-1.9.2-p290 :001 > class SomeClass
ruby-1.9.2-p290 :002?>   def page_description
ruby-1.9.2-p290 :003?>     description = case
ruby-1.9.2-p290 :004 >         when @question then @question.background
ruby-1.9.2-p290 :005?>       else "Something"
ruby-1.9.2-p290 :006?>       end
ruby-1.9.2-p290 :007?>     description
ruby-1.9.2-p290 :008?>     end
ruby-1.9.2-p290 :009?>   end
 => nil 
ruby-1.9.2-p290 :010 > class Question
ruby-1.9.2-p290 :011?>   attr_accessor :background
ruby-1.9.2-p290 :012?>   end
 => nil 
ruby-1.9.2-p290 :013 > class SomeClass
ruby-1.9.2-p290 :014?>   attr_writer :question
ruby-1.9.2-p290 :015?>   end
 => nil 
ruby-1.9.2-p290 :016 > some_obj = SomeClass.new
 => #<SomeClass:0x000001010ced90> 
ruby-1.9.2-p290 :017 > some_obj.page_description
 => "Something" 
ruby-1.9.2-p290 :018 > some_obj.question = nil
 => nil 
ruby-1.9.2-p290 :019 > some_obj.page_description
 => "Something" 
ruby-1.9.2-p290 :020 > class SomeClass
ruby-1.9.2-p290 :021?>   def page_description
ruby-1.9.2-p290 :022?>     description = case nil
ruby-1.9.2-p290 :023?>       when @question then @question.background
ruby-1.9.2-p290 :024?>       else "Something"
ruby-1.9.2-p290 :025?>       end
ruby-1.9.2-p290 :026?>     description
ruby-1.9.2-p290 :027?>     end
ruby-1.9.2-p290 :028?>   end
 => nil 
ruby-1.9.2-p290 :029 > some_obj = SomeClass.new
 => #<SomeClass:0x00000100940240> 
ruby-1.9.2-p290 :030 > some_obj.page_description
NoMethodError: undefined method `background' for nil:NilClass
        from (irb):23:in `page_description'
        from (irb):30
        from /Users/Tom/.rvm/rubies/ruby-1.9.2-p290/bin/irb:16:in `<main>'

See page 141 of the pickaxe for this use of case.

Cheers,

Tom

> 
> On 3 August 2011 11:57, Simon Coffey <simon at tribesports.com> wrote:
> Hi LRUG,
> 
> Apologies for posting a bug to the list, but it's weird enough that I
> hope it might be interesting (indeed it appears to be logically
> impossible). It's got me completely stumped, so any thoughts would be
> gratefully received.
> 
> We're getting very occasional (~1 per week) exceptions out of one of
> our helper methods in our app (Rails 3.0.3, ruby 1.9.2-p180). For
> example, from the following line of code...
> 
>  def page_description
>    case
>    when @question then @question.background            # exception raised here
>    <snip other cases>
>    end
>  end
> 
> ... we saw "NoMethodError: undefined method `background' for
> nil:NilClass". (The full method can be seen here:
> https://gist.github.com/1120170 )
> 
> That's logically impossible, right? @question is clearly nil, as shown
> by the exception. But @question is also clearly not nil, otherwise how
> on earth would that case fire?
> 
> Other rules in this case statement are also causing exceptions, but
> with inexplicable things apparently stored in the instance variables.
> For example, we've seen "NoMethodError: undefined method `bio' for
> #<Lead:0xdbc31a0>" from line #6 in the gist, despite the @user
> instance variable only ever being assigned an instance of User, not
> Lead. Another time @user appeared to contain a Hash. Other instance
> variables have contained controller instances.
> 
> This only seems to be happening in the #page_description method, which
> is used at the top of the <head> section of our layouts. As such, for
> any given render pass it will always be the first call to access the
> controller instance variables.
> 
> It looks to me like our instance variables are somehow getting
> corrupted, but I can't for the life of me see how this would happen.
> Unfortunately it happens only on production, and has so far resisted
> being reproduced, so we have no ability to get further info out of the
> debugger.
> 
> Has anyone seen anything like this before? Undying gratitude / a beer
> to anyone who has any bright ideas...
> 
> Cheers,
> Simon
> 
> --
> Simon Coffey
> Developer, Tribesports
> 
> e: simon at tribesports.com
> m: 07960 004 857
> p: Unit 104, Cremer Business Centre, 37 Cremer Street, London, E2 8HD
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