I updated from ruby 2.7.1 to 3.1.1, then removed Gemfile.lock and ran bundle update (it's on a dev branch, so I can throw it away if this is a bad idea, I just wanted to see if it would work).

bundle update succeeds, but when I start the server:

rails s
=> Booting Puma
=> Rails 7.0.2.2 application starting in development 
=> Run `bin/rails server --help` for more startup options
Exiting
/Users/st/.rbenv/versions/3.1.1/lib/ruby/3.1.0/psych/visitors/to_ruby.rb:430:in `visit_Psych_Nodes_Alias': Unknown alias: default (Psych::BadAlias)
    from /Users/st/.rbenv/versions/3.1.1/lib/ruby/3.1.0/psych/visitors/visitor.rb:30:in `visit'
    from /Users/st/.rbenv/versions/3.1.1/lib/ruby/3.1.0/psych/visitors/visitor.rb:6:in `accept'
    from /Users/st/.rbenv/versions/3.1.1/lib/ruby/3.1.0/psych/visitors/to_ruby.rb:35:in `accept'
    from /Users/st/.rbenv/versions/3.1.1/lib/ruby/3.1.0/psych/visitors/to_ruby.rb:345:in `block in revive_hash'
    from /Users/st/.rbenv/versions/3.1.1/lib/ruby/3.1.0/psych/visitors/to_ruby.rb:343:in `each'
    from /Users/st/.rbenv/versions/3.1.1/lib/ruby/3.1.0/psych/visitors/to_ruby.rb:343:in `each_slice'
    from /Users/st/.rbenv/versions/3.1.1/lib/ruby/3.1.0/psych/visitors/to_ruby.rb:343:in `revive_hash'
    from /Users/st/.rbenv/versions/3.1.1/lib/ruby/3.1.0/psych/visitors/to_ruby.rb:167:in `visit_Psych_Nodes_Mapping'
    from /Users/st/.rbenv/versions/3.1.1/lib/ruby/3.1.0/psych/visitors/visitor.rb:30:in `visit'
    from /Users/st/.rbenv/versions/3.1.1/lib/ruby/3.1.0/psych/visitors/visitor.rb:6:in `accept'
    from /Users/st/.rbenv/versions/3.1.1/lib/ruby/3.1.0/psych/visitors/to_ruby.rb:35:in `accept'
    from /Users/st/.rbenv/versions/3.1.1/lib/ruby/3.1.0/psych/visitors/to_ruby.rb:345:in `block in revive_hash'
    from /Users/st/.rbenv/versions/3.1.1/lib/ruby/3.1.0/psych/visitors/to_ruby.rb:343:in `each'
    from /Users/st/.rbenv/versions/3.1.1/lib/ruby/3.1.0/psych/visitors/to_ruby.rb:343:in `each_slice'
    from /Users/st/.rbenv/versions/3.1.1/lib/ruby/3.1.0/psych/visitors/to_ruby.rb:343:in `revive_hash'
    from /Users/st/.rbenv/versions/3.1.1/lib/ruby/3.1.0/psych/visitors/to_ruby.rb:167:in `visit_Psych_Nodes_Mapping'
    from /Users/st/.rbenv/versions/3.1.1/lib/ruby/3.1.0/psych/visitors/visitor.rb:30:in `visit'
    from /Users/st/.rbenv/versions/3.1.1/lib/ruby/3.1.0/psych/visitors/visitor.rb:6:in `accept'
    from /Users/st/.rbenv/versions/3.1.1/lib/ruby/3.1.0/psych/visitors/to_ruby.rb:35:in `accept'
    from /Users/st/.rbenv/versions/3.1.1/lib/ruby/3.1.0/psych/visitors/to_ruby.rb:318:in `visit_Psych_Nodes_Document'
    from /Users/st/.rbenv/versions/3.1.1/lib/ruby/3.1.0/psych/visitors/visitor.rb:30:in `visit'
    from /Users/st/.rbenv/versions/3.1.1/lib/ruby/3.1.0/psych/visitors/visitor.rb:6:in `accept'
    from /Users/st/.rbenv/versions/3.1.1/lib/ruby/3.1.0/psych/visitors/to_ruby.rb:35:in `accept'
    from /Users/st/.rbenv/versions/3.1.1/lib/ruby/3.1.0/psych.rb:335:in `safe_load'
    from /Users/st/.rbenv/versions/3.1.1/lib/ruby/3.1.0/psych.rb:370:in `load'
    from /Users/st/.rbenv/versions/3.1.1/lib/ruby/gems/3.1.0/gems/webpacker-4.3.0/lib/webpacker/env.rb:30:in `available_environments'
    from /Users/st/.rbenv/versions/3.1.1/lib/ruby/gems/3.1.0/gems/webpacker-4.3.0/lib/webpacker/env.rb:21:in `current'
    from /Users/st/.rbenv/versions/3.1.1/lib/ruby/gems/3.1.0/gems/webpacker-4.3.0/lib/webpacker/env.rb:15:in `inquire'
    from /Users/st/.rbenv/versions/3.1.1/lib/ruby/gems/3.1.0/gems/webpacker-4.3.0/lib/webpacker/env.rb:7:in `inquire'
    from /Users/st/.rbenv/versions/3.1.1/lib/ruby/gems/3.1.0/gems/webpacker-4.3.0/lib/webpacker/instance.rb:11:in `env'
    from /Users/st/.rbenv/versions/3.1.1/lib/ruby/gems/3.1.0/gems/webpacker-4.3.0/lib/webpacker/instance.rb:18:in `config'
    from /Users/st/.rbenv/versions/3.1.1/lib/ruby/gems/3.1.0/gems/webpacker-4.3.0/lib/webpacker.rb:34:in `config'
    from /Users/st/.rbenv/versions/3.1.1/lib/ruby/gems/3.1.0/gems/webpacker-4.3.0/lib/webpacker/railtie.rb:32:in `block in <class:Engine>'
    from /Users/st/.rbenv/versions/3.1.1/lib/ruby/gems/3.1.0/gems/railties-7.0.2.2/lib/rails/initializable.rb:32:in `instance_exec'
    from /Users/st/.rbenv/versions/3.1.1/lib/ruby/gems/3.1.0/gems/railties-7.0.2.2/lib/rails/initializable.rb:32:in `run'
    from /Users/st/.rbenv/versions/3.1.1/lib/ruby/gems/3.1.0/gems/railties-7.0.2.2/lib/rails/initializable.rb:61:in `block in run_initializers'
    from /Users/st/.rbenv/versions/3.1.1/lib/ruby/3.1.0/tsort.rb:228:in `block in tsort_each'
    from /Users/st/.rbenv/versions/3.1.1/lib/ruby/3.1.0/tsort.rb:350:in `block (2 levels) in each_strongly_connected_component'
    from /Users/st/.rbenv/versions/3.1.1/lib/ruby/3.1.0/tsort.rb:431:in `each_strongly_connected_component_from'
    from /Users/st/.rbenv/versions/3.1.1/lib/ruby/3.1.0/tsort.rb:349:in `block in each_strongly_connected_component'
    from /Users/st/.rbenv/versions/3.1.1/lib/ruby/3.1.0/tsort.rb:347:in `each'
    from /Users/st/.rbenv/versions/3.1.1/lib/ruby/3.1.0/tsort.rb:347:in `call'
    from /Users/st/.rbenv/versions/3.1.1/lib/ruby/3.1.0/tsort.rb:347:in `each_strongly_connected_component'
    from /Users/st/.rbenv/versions/3.1.1/lib/ruby/3.1.0/tsort.rb:226:in `tsort_each'
    from /Users/st/.rbenv/versions/3.1.1/lib/ruby/3.1.0/tsort.rb:205:in `tsort_each'
    from /Users/st/.rbenv/versions/3.1.1/lib/ruby/gems/3.1.0/gems/railties-7.0.2.2/lib/rails/initializable.rb:60:in `run_initializers'
    from /Users/st/.rbenv/versions/3.1.1/lib/ruby/gems/3.1.0/gems/railties-7.0.2.2/lib/rails/application.rb:372:in `initialize!'
    from /Users/st/rails/hangswith/config/environment.rb:5:in `<main>'
    from /Users/st/.rbenv/versions/3.1.1/lib/ruby/gems/3.1.0/gems/bootsnap-1.10.3/lib/bootsnap/load_path_cache/core_ext/kernel_require.rb:30:in `require'
    from /Users/st/.rbenv/versions/3.1.1/lib/ruby/gems/3.1.0/gems/bootsnap-1.10.3/lib/bootsnap/load_path_cache/core_ext/kernel_require.rb:30:in `require'
    from /Users/st/.rbenv/versions/3.1.1/lib/ruby/gems/3.1.0/gems/zeitwerk-2.5.4/lib/zeitwerk/kernel.rb:35:in `require'
    from /Users/st/.rbenv/versions/3.1.1/lib/ruby/gems/3.1.0/gems/bootsnap-1.10.3/lib/bootsnap/load_path_cache/core_ext/kernel_require.rb:42:in `require_relative'
    from config.ru:3:in `block in <main>'
    from /Users/st/.rbenv/versions/3.1.1/lib/ruby/gems/3.1.0/gems/rack-2.2.3/lib/rack/builder.rb:116:in `eval'
    from /Users/st/.rbenv/versions/3.1.1/lib/ruby/gems/3.1.0/gems/rack-2.2.3/lib/rack/builder.rb:116:in `new_from_string'
    from /Users/st/.rbenv/versions/3.1.1/lib/ruby/gems/3.1.0/gems/rack-2.2.3/lib/rack/builder.rb:105:in `load_file'
    from /Users/st/.rbenv/versions/3.1.1/lib/ruby/gems/3.1.0/gems/rack-2.2.3/lib/rack/builder.rb:66:in `parse_file'
    from /Users/st/.rbenv/versions/3.1.1/lib/ruby/gems/3.1.0/gems/rack-2.2.3/lib/rack/server.rb:349:in `build_app_and_options_from_config'
    from /Users/st/.rbenv/versions/3.1.1/lib/ruby/gems/3.1.0/gems/rack-2.2.3/lib/rack/server.rb:249:in `app'
    from /Users/st/.rbenv/versions/3.1.1/lib/ruby/gems/3.1.0/gems/rack-2.2.3/lib/rack/server.rb:422:in `wrapped_app'
    from /Users/st/.rbenv/versions/3.1.1/lib/ruby/gems/3.1.0/gems/railties-7.0.2.2/lib/rails/commands/server/server_command.rb:76:in `log_to_stdout'
    from /Users/st/.rbenv/versions/3.1.1/lib/ruby/gems/3.1.0/gems/railties-7.0.2.2/lib/rails/commands/server/server_command.rb:36:in `start'
    from /Users/st/.rbenv/versions/3.1.1/lib/ruby/gems/3.1.0/gems/railties-7.0.2.2/lib/rails/commands/server/server_command.rb:143:in `block in perform'
    from <internal:kernel>:90:in `tap'
    from /Users/st/.rbenv/versions/3.1.1/lib/ruby/gems/3.1.0/gems/railties-7.0.2.2/lib/rails/commands/server/server_command.rb:134:in `perform'
    from /Users/st/.rbenv/versions/3.1.1/lib/ruby/gems/3.1.0/gems/thor-1.2.1/lib/thor/command.rb:27:in `run'
    from /Users/st/.rbenv/versions/3.1.1/lib/ruby/gems/3.1.0/gems/thor-1.2.1/lib/thor/invocation.rb:127:in `invoke_command'
    from /Users/st/.rbenv/versions/3.1.1/lib/ruby/gems/3.1.0/gems/thor-1.2.1/lib/thor.rb:392:in `dispatch'
    from /Users/st/.rbenv/versions/3.1.1/lib/ruby/gems/3.1.0/gems/railties-7.0.2.2/lib/rails/command/base.rb:87:in `perform'
    from /Users/st/.rbenv/versions/3.1.1/lib/ruby/gems/3.1.0/gems/railties-7.0.2.2/lib/rails/command.rb:48:in `invoke'
    from /Users/st/.rbenv/versions/3.1.1/lib/ruby/gems/3.1.0/gems/railties-7.0.2.2/lib/rails/commands.rb:18:in `<main>'
    from /Users/st/.rbenv/versions/3.1.1/lib/ruby/gems/3.1.0/gems/bootsnap-1.10.3/lib/bootsnap/load_path_cache/core_ext/kernel_require.rb:30:in `require'
    from /Users/st/.rbenv/versions/3.1.1/lib/ruby/gems/3.1.0/gems/bootsnap-1.10.3/lib/bootsnap/load_path_cache/core_ext/kernel_require.rb:30:in `require'
    from bin/rails:4:in `<main>'

What I've tried

Googling the 'psych' error message reveals this which may be related. But when I search the entire app for YAML.safe_load (or even just safe_load), there are 0 occurrences of it. (perhaps I should be searching actual gems in my app?).

It was a long shot but based on this comment, I ran gem rdoc --all to update all rdoc documentations. But that didn't help.

CodePudding user response：

The problem is related to the Ruby 3.1 / Psych 4.x incompatibility described in this issue: https://bugs.ruby-lang.org/issues/17866

Ruby 3.0 comes with Psych 3, while Ruby 3.1 comes with Psych 4, which has a major breaking change.

• The new YAML loading methods (Psych 4) do not load aliases unless they get the aliases: true argument.
• The old YAML loading methods (Psych 3) do not support the aliases keyword.

At this point, it seems like anyone, anywhere that wants to load YAML in the same way it worked prior to Ruby 3.1, need to do something like this:

begin
  YAML.load(source, aliases: true, **options)
rescue ArgumentError
  YAML.load(source, **options)
end

as patched by the Rails team.

In your case, I suspect you will need to see which Rails version that matches your major version preference includes this patch before you can upgrade to Ruby 3.1.

Personally, wherever I have control of the code that loads YAML, I am adding an extension like this:

module YAML
  def self.properly_load_file(path)
    YAML.load_file path, aliases: true
  rescue ArgumentError
    YAML.load_file path
  end
end

which lets me do YAML.properly_load_file path to get a semi-transparently working solution for all version.

I hope this behavior is reverted in Ruby 3.1.x / Psych.