Question

In my Rails 3.0.11 app, we have very simple code in a controller:

def index
  @record = Record.valid # scope around 80,000 records
  asdfasdfsa # consider this is a typo to raise NameError Exception
end

The interesting thing is that when it came to the typo, the app seems to query/execute the @record instance variable first before raising an exception. The query costs almost 1 min to get records. So in browser, the page hanges for a long while before coming into an exception template.

If I replace @record with a local variable “record”, the querying doesn’t happen at all. Anyone knows what it is going on?

Answer 1

See my blog post Ruby’s Inspect Considered Harmful for details about this very issue. In short, though:

1. NameError calls inspect when formatting its error message
2. The default implementation of inspect calls inspect on all instance variables recursively
3. NameError throws away the result of inspect if it is longer than 65 characters
4. For us, this meant that a typo in a View caused Rails to hang for 20 minutes while Ruby built a huge, 20MB string and then proceeded to throw it away
5. It took us 7 months to get a trivial fix for this into Rails core

In short, I consider the behaviour of NameError to be a heinous bug within the Ruby interpreter. I can think of no sane reason for this implementation.