Store global objects into thread-local variables

[ildarkayumov]

Context

Bullet stores global objects into fiber-local variables before each request, uses these global objects to collect all N+1 queries and later shows all collected problems based on these global objects result. This approach works well until someone don't try to run his code inside Fiber like this

class PostsController < ApplicationController
  def index
    fiber = ::Fiber.new do
      Post.find_each do |post|
        post.comments.each do |comment|
          comment.name
        end
      end
    end

    fiber.resume
  end
end

In that case bullet warning is not shown because all global objects are stored as fiber-local variables for current Fiber and not passed to all new Fibers

Decision

The easiest way is to store bullet objects into thread-local variables instead: https://ruby-doc.org/core-2.6/Thread.html#class-Thread-label-Fiber-local+vs.+Thread-local

Changes

1. move Bullet objects to thread-local variables
2. combine variables into single thread local variable

Notes

this problem can easily occur even when rails app does not use fibers explicitly but instead uses some 3rd party libraries like dry-rb/dry-effects#82

[flyerhzm]

@ildarkayumov I know it's easier to use thread_local_data to replace Thread.current, but I don't want to define the data structure { bullet: { bullet_start: true } }, would you mind using the following code instead

Thread.current.thread_variable_set(:bullet_start, true)

[flyerhzm]

@ildarkayumov thank you for fixing it.