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

[nbluis]

Hi 👋
I’m seeing an incompatibility when using prometheus-client with Ruby 4.x, specifically when using Prometheus::Client::DataStores::DirectFileStore.

The same code works correctly on Ruby 3.4.2, but fails on Ruby 4.0.1 with a NoMethodError related to CGI.parse.

Other data stores were not tested yet, but the error clearly originates from DirectFileStore.

Environment

Ruby:
✅ 3.4.2 (works)
❌ 4.0.1 (fails)

Reproduction steps

require 'prometheus/client'
require 'prometheus/client/data_stores/direct_file_store'

Prometheus::Client.config.data_store =
  Prometheus::Client::DataStores::DirectFileStore.new(
    dir: '/tmp/prometheus_direct_file_store'
  )

prometheus = Prometheus::Client.registry

http_requests = Prometheus::Client::Counter.new(
  :http_requests,
  docstring: 'A counter of HTTP requests made',
  labels: [:status_code]
)

prometheus.register(http_requests)

http_requests.increment(labels: { status_code: 200 })

prometheus.metrics.each do |metric|
  puts "Metric: #{metric.name} #{metric.values}"
end

Expected behavior (Ruby 3.4.2)

Metric: http_requests {{} => 2.0, {status_code: "200"} => 15.0}

Actual behavior (Ruby 4.0.1)

/home/ebohrer/.asdf/installs/ruby/4.0.1/lib/ruby/gems/4.0.0/gems/prometheus-client-4.2.5/lib/prometheus/client/data_stores/direct_file_store.rb:139:in 'block (2 levels) in Prometheus::Client::DataStores::DirectFileStore::MetricStore#all_values': undefined method 'parse' for class CGI (NoMethodError)

                  label_set = CGI::parse(labelset_qs).map do |k, vs|
                                 ^^^^^^^
        from /home/ebohrer/.asdf/installs/ruby/4.0.1/lib/ruby/gems/4.0.0/gems/prometheus-client-4.2.5/lib/prometheus/client/data_stores/direct_file_store.rb:135:in 'Array#each'
        from /home/ebohrer/.asdf/installs/ruby/4.0.1/lib/ruby/gems/4.0.0/gems/prometheus-client-4.2.5/lib/prometheus/client/data_stores/direct_file_store.rb:135:in 'block in Prometheus::Client::DataStores::DirectFileStore::MetricStore#all_values'
        from /home/ebohrer/.asdf/installs/ruby/4.0.1/lib/ruby/gems/4.0.0/gems/prometheus-client-4.2.5/lib/prometheus/client/data_stores/direct_file_store.rb:132:in 'Array#each'
        from /home/ebohrer/.asdf/installs/ruby/4.0.1/lib/ruby/gems/4.0.0/gems/prometheus-client-4.2.5/lib/prometheus/client/data_stores/direct_file_store.rb:132:in 'Prometheus::Client::DataStores::DirectFileStore::MetricStore#all_values'
        from /home/ebohrer/.asdf/installs/ruby/4.0.1/lib/ruby/gems/4.0.0/gems/prometheus-client-4.2.5/lib/prometheus/client/metric.rb:80:in 'Prometheus::Client::Metric#values'
        from main.rb:17:in 'block in <main>'
        from main.rb:16:in 'Array#each'
        from main.rb:16:in '<main>'

Fast Analysis

This issue appears to be specific to DirectFileStore, more precisely in
DirectFileStore::MetricStore#all_values.

Ruby 4 removed or changed the availability of CGI.parse.
In Ruby 3.x this method exists, but in Ruby 4.x it raises NoMethodError.

[Sinjo]

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!