[mhenrixon]

Rails main has an experimental Rails.application.ractorize! that serves requests from non-main Ractors on Ruby 4.0. Every jbuilder view fails there: Jbuilder#initialize reads the class-level defaults from class variables, and Ruby 4.0 refuses every class variable read from a non-main Ractor.

require "jbuilder"
Ractor.new { Jbuilder.new { |json| json.name "David" }.attributes! }.value
# main, Ruby 4.0.7:
# lib/jbuilder.rb:18:in 'Jbuilder#initialize': can not access class variables from non-main Ractors (@@key_formatter from Jbuilder) (Ractor::IsolationError)
# this branch:
# => {"name" => "David"}

Ruby master (4.1) already lets non-main Ractors read a class variable that holds a shareable value (see rails/rails#58974). That covers the two booleans but not @@key_formatter: a KeyFormatter holds a Mutex and a mutable cache, so a Ractor can't read it on any Ruby.

Changes

• The defaults are now class instance variables of Jbuilder. They are read and written through ::Jbuilder, so JbuilderTemplate and other subclasses still share them, as they did with the class variables. Jbuilder.key_format, ignore_nil and deep_format_keys behave the same.
• KeyFormatter can be frozen. A frozen formatter still returns its cached keys and formats the others without storing them. freeze drops the Mutex, because a Mutex can't be shared between Ractors.
• The Railtie makes the default formatter and JbuilderTemplate.template_lookup_options shareable on Rails' before_ractorize load hook. Rails' own railties use the same hook. Older Rails never runs it, so nothing changes without ractorize!.

The process-wide cache from #597/#607 still works as before on the main Ractor. I measured no difference: 200k builds with 5 camelized keys took 0.216s/0.217s on main and 0.199s/0.212s on this branch (Ruby 4.0.7, best of 5). Under ractorize! the default formatter is frozen, so it formats keys on each call, as it did before #597. A formatter built from a lambda makes the hook raise at ractorize!, unless the lambda was made shareable (Ractor.shareable_lambda), so the failure shows up at boot and not on a request.

Tests

• Defaults set on Jbuilder apply to subclasses, and defaults set on a subclass apply to Jbuilder. A template test checks the same for JbuilderTemplate.
• A frozen KeyFormatter still formats keys, both cached and new.
• The before_ractorize hook makes the default formatter and the lookup options shareable.
• A Jbuilder built in a non-main Ractor picks up the default key format and ignore_nil. This test needs Ractor#value, so it is skipped below Ruby 4.0.
• The existing tests that called class_variable_set/class_variable_get now use the setters and the new readers.

CI=1 bundle exec rake test: Ruby 4.0.7 + Rails 8.1 gives 143 runs, 0 failures, 0 skips. Ruby 3.1.6 + 7.0, 3.2.8 + 7.1 and 3.4.11 + 8.0 give 0 failures, with the one Ruby 4.0-only skip. On rails_head (allowed to fail), 6 collection-partial tests in jbuilder_template_test.rb fail on main too, against today's Rails main. This branch fails the same 6 and nothing else (checked over 5 seeds).

I also checked it in a Rails main app under ractorize!, with GET /posts.json running in a non-main Ractor. Before: Ractor::IsolationError … (@@key_formatter from Jbuilder) @ lib/jbuilder.rb:18. After: 200 both without a default format and with Jbuilder.key_format camelize: :lower plus Jbuilder.ignore_nil in an initializer.

Conformance case

test/jbuilder_test.rb:840: "defaults can be read from a non-main Ractor once shareable"

BUNDLE_GEMFILE=gemfiles/rails_8_1.gemfile bundle exec ruby -Itest test/jbuilder_test.rb -n /non-main_Ractor_once_shareable/

• Without the fix (f141f5e), Ruby 4.0.7: the test itself can't run there (it calls the new Jbuilder.default_key_formatter); its Ractor block alone fails with Ractor::IsolationError: can not access class variables from non-main Ractors (@@key_formatter from Jbuilder)
• With this PR: passes on Ruby 4.0.7.

It builds a Jbuilder inside the Ractor, so it also fails if any default Jbuilder.new reads goes back into a class variable or isn't shareable. It skips below Ruby 4.0.