Make ActiveRecord::Base.logger a class_attribute

[casperisfine]

cattr_accessor rely on class variables which has terrible
performance on long ancestor chains. See https://bugs.ruby-lang.org/issues/17763
for a detailed description of the problem.

In comparison class_attribute on ActiveRecord::Base is almost 7x
faster:

Calculating -------------------------------------
              logger      1.700M (± 0.9%) i/s -      8.667M in   5.097595s
             clogger     11.556M (± 0.9%) i/s -     58.806M in   5.089282s

Comparison:
             clogger: 11555754.2 i/s
              logger:  1700280.4 i/s - 6.80x  (± 0.00) slower

This is because ActiveRecord::Base.ancestors.size == 62.

Other cattr usages on ActiveRecord::Base should be considered too, but logger is by far the biggest hostspot, so probably the best one to start the discussion.

@eileencodes @jhawthorn @tenderlove @rafaelfranca

[jhawthorn]

👍 Probably deserves a mention in the changelog. I suspect most will just be using logger as a method and won't ever be setting logger=, but you never know. Also seems plausible some folks will have used @@logger

[casperisfine]

Probably deserves a mention in the changelog.

Good point, I always forget -_-.

Done.

[pixeltrix]

Seems fine to me but I'd need @tenderlove to clarify this from the bug discussion:

I think the logger is one of the places where we depend on class variable semantics and it's unlikely to change.

[casperisfine]

This quote is in opposition to "class instance variables":

class AR::Base
  class << self
    attr_accessor :logger
  end
end

Because indeed we expect MyModel.logger to work.

[eregon]

Nice! I thought that change made sense too when I wrote about it in https://bugs.ruby-lang.org/issues/17763#note-7