DirectFileStore fails on Ruby 4.x: undefined method 'parse' for CGI #329

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1. Sinjo commented on Feb 5, 2026

   

   Sinjo

   on Feb 5, 2026

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   Hello! Thanks for the report. I've got a couple of PRs open that will fix this:

   • Rip out support for end-of-life Ruby versions #328
   • Run tests against Ruby 4.0 and JRuby 10.0 #327

   Once they're merged I'll cut a new release and ping you here!

   👍React with 👍1Reacted by Eduardo Bohrer
2. Sinjo

   self-assigned this

   on Feb 5, 2026
3. ciscou commented on Jul 4, 2026

   

   ciscou

   on Jul 4, 2026

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   same here. I've fixed it on my rails app with this monkey patch (in config/initializers/monkey_patch_prometheus_client.rb)

   Prometheus::Client::DataStores::DirectFileStore.const_get("MetricStore").class_eval do
     def all_values
       stores_data = Hash.new { |hash, key| hash[key] = [] }
   
       # There's no need to call `synchronize` here. We're opening a second handle to
       # the file, and `flock`ing it, which prevents inconsistent reads
       stores_for_metric.each do |file_path|
         store = Prometheus::Client::DataStores::DirectFileStore::FileMappedDict.new(file_path, true)
         store.all_values.each do |(labelset_qs, v, ts)|
           # Labels come as a query string, that we parse with URI.decode_www_form as this:
           # "foo=bar&x=y" => { "foo" => "bar", "x" => "y" }
           # Turn the keys back into symbols
           label_set = URI.decode_www_form(labelset_qs).map do |k, v|
             [k.to_sym, v]
           end.to_h
   
           stores_data[label_set] << [v, ts]
         end
       ensure
         store.close if store
       end
   
       # Aggregate all the different values for each label_set
       aggregate_hash = Hash.new { |hash, key| hash[key] = 0.0 }
       stores_data.each_with_object(aggregate_hash) do |(label_set, values), acc|
         acc[label_set] = aggregate_values(values)
       end
     end
   end

   Might attempt to open a PR later today
4. Sinjo commented on Jul 4, 2026

   

   Sinjo

   on Jul 4, 2026

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   Hey, sorry for the lack of progress on this. I have the code teed up already, but haven't had time to write and run a test I wanted to before shipping it. I don't get employer-sponsored time to work on this gem any more and life is busy.

   ❤️React with ❤️1Reacted by Francis Pérez Padilla
5. ciscou commented on Jul 4, 2026

   

   ciscou

   on Jul 4, 2026

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   @Sinjo I totally understand! No pressure at all, we're very grateful for all your work. For now I'm happy with my monkey patch, I'll remove it when this is fixed upstream 😄
6. nbluis commented on Jul 5, 2026

   

   nbluis

   on Jul 5, 2026

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   @Sinjo dont worry and thanks for your incredible work.

   I was able to work around this by explicitly adding the cgi gem to my Gemfile:

   gem 'cgi', '~> 0.5.1'
   

   Ruby 4 no longer ships CGI.parse in the default bundled cgi — it now lives in the standalone gem. Adding it back restores the method and DirectFileStore works as expected again.
   This should be a good option for anyone hitting this issue until a definitive fix.
7. Sinjo commented on Sep 1, 2026

   

   Sinjo

   on Sep 1, 2026

   Last edited by Sinjo

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   This is fixed in 5.0.0, which I pushed to RubyGems a few days ago.

   If you're curious about the testing I wanted to do, it's here: #330 (comment)
8. Sinjo

   closed this as completedon Sep 1, 2026