Debugging Rails Itself

Tracking down bugs can be hard. Tracking down bugs in a codebase touched by 5000 people is even harder. Making heads or tails of an inheritance-happy codebase like Rails can be a nightmare. How do you find where the bug in save is when save is overridden by 15 different modules?!

In this talk, we'll look at the process that goes into fixing a bug in Rails itself. You'll learn about every step from the initial report to when the fix is eventually committed. You'll learn how to navigate Rails' internals, and how to find the source of the problem -- even if you've never committed to Rails.

Transcript

1. Please sit as close to the front and center as possible
2. Story Time
3. Story Time
4. Debugging Rails Itself
5. TL;DR: Make Aaron fix it
6. Thank you
7. Debugging Rails Itself
8. Who am I? • Sean Griffin • 10x Hacker Ninja Guru at Shopify • @sgrif on Twitter (please tweet at me during this talk) • Rails Committer • Maintainer of Active Record • Creator of Diesel • Bikeshed co-host
10. Anyone can work on Rails
12. Do we have a bug in Rails?
13. What even is a bug?
14. Expectations vs reality
15. Features aren't always used as designed
16. Check the docs
17. We think it's a bug. What's next?
20. We need a reproducible test case
21. gemfile(true) do source "https://rubygems.org" git_source(:github) { |repo| "https://github.com/#{repo}.git" } gem "rails", github: "rails/rails" gem "sqlite3" end
22. ActiveRecord::Base.establish_connection(adapter: "sqlite3", database: ":memory:") ActiveRecord::Base.logger = Logger.new(STDOUT) ActiveRecord::Schema.define do create_table :posts, force: true do |t| end create_table :comments, force: true do |t| t.integer :post_id end end
23. class BugTest < Minitest::Test def test_association_stuff post = Post.create! post.comments << Comment.create! assert_equal 1, post.comments.count assert_equal 1, Comment.count assert_equal post.id, Comment.first.post.id end end
24. The bug needs to be demonstrated with no gems other than Rails
25. Now what?
27. It's time to go bug hunting
28. We need the commit that introduced the bug
29. git bisect (the greatest tool ever)
30. Changing our test script ENV["BUNDLE_GEMFILE"] = "/path/to/rails/Gemfile" require "bundler" Bundler.require
31. Bisecting basics git checkout master git bisect start git bisect bad git checkout 4-2-stable git bisect good
32. Even if you don't fix the bug, bisecting is incredibly useful
34. What if there's no version without the bug?
36. I'm a dinosaur debugger
37. Methods you should know • Kernel#method
38. (Some objects override the method method) Kernel .instance_method(:method) .bind(self) .call
39. Other methods besides the method method • Kernel#method • Method#source_location • Method#super_method • Method#method
40. Methods unrelated to the method method • Kernel#caller • Kernel#raise • Kernel#p
41. git blame
42. git blame context
43. How to not blame people # ~/.gitconfig [alias] context = blame
44. GitHub's UI is helpful for recontexting
45. gif of how to use it
46. Why do we care so much about the commit?
47. Time to fix it!
49. I'm Stuck
51. Take a Break
54. Find a Pair
56. When you finally fix it
59. Anyone can work on Rails
60. Want more Ruby?
62. Room 315 2:30 PM
66. Please ask me questions now
67. Contact • twitter: @sgrif • github: @sgrif • podcast: bikeshed.fm