Six Year Old Optional / Keyword Arguments bug

I recently conducted a workshop about Contributing to Open-Source at first-ever Rubyconf Philippines. In its introductory talk, I spoke about how Aaron Patterson, fixed a 6 year old bug about Optional Arguments, that existed in Rails.

Bug in ruby

Let's try a small program.

1
2class Lab
3
4def day
5puts 'invoked'
6'sunday'
7end
8
9def run
10day = day
11end
12
13end
14
15puts Lab.new.run
16
Copy

What do you think would be printed on your terminal when you run the above program.

If you are using ruby 2.1 or below then you will see nothing. Why is that ? That's because of a bug in ruby.

This is bug number 9593 in ruby issue tracker.

In the statement day = day the left hand side variable assignment is stopping the call to method day. So the method day is never invoked.

Another variation of the same bug

1
2class Lab
3
4def day
5puts 'invoked'
6'sunday'
7end
8
9def run( day: day)
10end
11
12end
13
14puts Lab.new.run
15
Copy

In the above case we are using the keyword argument feature added in Ruby 2.0 . If you are unfamiliar with keyword arguments feature of ruby then checkout this excellent video by Peter Cooper.

In this case again the same behavior is exhibited. The method day is never invoked.

How this bug affects Rails community

You might be thinking that I would never write code like that. Why would you have a variable name same as method name.

Well Rails had this bug because rails has code like this.

1
2def has_cached_counter?(reflection = reflection)
3end
4
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In this case method reflection never got called and the variable reflection was always assigned nil.

Fixing the bug

Nobu fixed this bug in ruby 2.2.0. By the way Nobu is also known as "ruby patch-monster" because of amount of patches he applies to ruby.

So this bug is fixed in ruby 2.2.0. What about the people who are not using ruby 2.2.0.

The simple solution is not to omit the parameter. If we change the above code to

1
2def has_cached_counter?(reflection = reflection())
3end
4
Copy

then we are explicitly invoking the method reflection and the variable reflection will be assigned the output of method reflection.

And this is how Aaron Patterson fixed six years old bug.