Add env var that bypasses Puma cluster validation

We often need to run Puma in single-mode for performance
testing, but the application isn't fully set up yet
to support that. Bypassing this check as a non-published
developer switch makes our life easier.

config/initializers/validate_puma.rb

 # frozen_string_literal: true
 
 if Gitlab::Runtime.puma? && ::Puma.cli_config.options[:workers].to_i == 0
+  return if allow_single_mode?
+
   raise 'Puma is only supported in Cluster-mode: workers > 0'
 end
+
+def allow_single_mode?
+  return false if Gitlab.com?
+
+  Gitlab::Utils.to_boolean(ENV['PUMA_SKIP_CLUSTER_VALIDATION'])
+end

spec/initializers/validate_puma_spec.rb

+# frozen_string_literal: true
+
+require 'spec_helper'
+
+RSpec.describe 'validate puma' do
+  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)
+  end
+
+  context 'for .com' do
+    before do
+      allow(Gitlab).to receive(:com?).and_return(true)
+    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 }.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
+    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
+    end
+  end
+
+  context 'for other environments' do
+    before do
+      allow(Gitlab).to receive(:com?).and_return(false)
+    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
+
+      context 'and PUMA_SKIP_CLUSTER_VALIDATION is false' do
+        let(:skip_validation) { false }
+
+        specify { expect { subject }.to raise_error(String) }
+      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 }
+      end
+    end
+  end
+end