Drop Puma only-clustered validation

We support Puma Single mode now, allowing to set `workers: 0`.
More at: https://gitlab.com/groups/gitlab-org/-/epics/5303.

changelogs/unreleased/300910-drop-puma-clustered-validation.yml

+---
+title: Allow running Puma in Single mode
+merge_request: 53830
+author:
+type: other

config/initializers/validate_puma.rb

 # frozen_string_literal: true
 
-def allow_single_mode?
-  return false if Gitlab.com?
-
-  Gitlab::Utils.to_boolean(ENV['PUMA_SKIP_CLUSTER_VALIDATION'])
+def max_puma_workers
+  Puma.cli_config.options[:workers].to_i
 end
 
-if Gitlab::Runtime.puma? && ::Puma.cli_config.options[:workers].to_i == 0
-  return if allow_single_mode?
+if Gitlab::Runtime.puma? && max_puma_workers == 0
+  raise 'Puma is only supported in Clustered mode (workers > 0)' if Gitlab.com?
 
-  raise 'Puma is only supported in Cluster-mode: workers > 0'
+  warn 'WARNING: Puma is running in Single mode (workers = 0). Some features may not work. Please refer to https://gitlab.com/groups/gitlab-org/-/epics/5303 for info.'
 end

spec/initializers/validate_puma_spec.rb

@@ -3,12 +3,13 @@
 require 'spec_helper'
 
 RSpec.describe 'validate puma' do
+  include RakeHelpers
+
   subject do
     load Rails.root.join('config/initializers/validate_puma.rb')
   end
 
   before do
-    stub_env('PUMA_SKIP_CLUSTER_VALIDATION', skip_validation)
     stub_const('Puma', double)
     allow(Gitlab::Runtime).to receive(:puma?).and_return(true)
     allow(Puma).to receive_message_chain(:cli_config, :options).and_return(workers: workers)
@@ -22,70 +23,44 @@ RSpec.describe 'validate puma' do
     context 'when worker count is 0' do
       let(:workers) { 0 }
 
-      context 'and PUMA_SKIP_CLUSTER_VALIDATION is true' do
-        let(:skip_validation) { true }
-
-        specify { expect { subject }.to raise_error(String) }
-      end
-
-      context 'and PUMA_SKIP_CLUSTER_VALIDATION is false' do
-        let(:skip_validation) { false }
-
-        specify { expect { subject }.to raise_error(String) }
-      end
+      specify { expect { subject }.to raise_error(String) }
     end
 
     context 'when worker count is > 0' do
       let(:workers) { 2 }
 
-      context 'and PUMA_SKIP_CLUSTER_VALIDATION is true' do
-        let(:skip_validation) { true }
-
-        specify { expect { subject }.not_to raise_error }
-      end
-
-      context 'and PUMA_SKIP_CLUSTER_VALIDATION is false' do
-        let(:skip_validation) { false }
-
-        specify { expect { subject }.not_to raise_error }
-      end
+      specify { expect { subject }.not_to raise_error }
     end
   end
 
   context 'for other environments' do
     before do
       allow(Gitlab).to receive(:com?).and_return(false)
+
+      allow(main_object).to receive(:warn)
     end
 
     context 'when worker count is 0' do
       let(:workers) { 0 }
 
-      context 'and PUMA_SKIP_CLUSTER_VALIDATION is true' do
-        let(:skip_validation) { true }
-
-        specify { expect { subject }.not_to raise_error }
-      end
+      specify { expect { subject }.not_to raise_error }
 
-      context 'and PUMA_SKIP_CLUSTER_VALIDATION is false' do
-        let(:skip_validation) { false }
+      it 'warns about running Puma in a Single mode' do
+        expect(main_object).to receive(:warn) do |warning|
+          expect(warning).to include('https://gitlab.com/groups/gitlab-org/-/epics/5303')
+        end
 
-        specify { expect { subject }.to raise_error(String) }
+        subject
       end
     end
 
     context 'when worker count is > 0' do
       let(:workers) { 2 }
 
-      context 'and PUMA_SKIP_CLUSTER_VALIDATION is true' do
-        let(:skip_validation) { true }
-
-        specify { expect { subject }.not_to raise_error }
-      end
-
-      context 'and PUMA_SKIP_CLUSTER_VALIDATION is false' do
-        let(:skip_validation) { false }
+      specify { expect { subject }.not_to raise_error }
 
-        specify { expect { subject }.not_to raise_error }
+      it 'does not issue a warning' do
+        expect(main_object).not_to receive(:warn)
       end
     end
   end