[LRUG] To debug the impossible bug

Wed Aug 3 03:57:49 PDT 2011

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