Add specs for UsageMetricDefinitionGenerator

spec/generator_helper.rb

+# frozen_string_literal: true
+
+require 'spec_helper'
+
+RSpec.configure do |config|
+  # Redirect stdout so specs don't have so much noise
+  config.before(:all) do
+    $stdout = StringIO.new
+  end
+
+  # Reset stdout
+  config.after(:all) do
+    $stdout = STDOUT
+  end
+end

spec/lib/generators/gitlab/usage_metric_definition_generator_spec.rb

+# frozen_string_literal: true
+
+require 'generator_helper'
+
+RSpec.describe Gitlab::UsageMetricDefinitionGenerator do
+  describe 'Running generator' do
+    let(:key_path) { 'counter.category.event' }
+    let(:dir) { '7d' }
+    let(:options) { [key_path, '--dir', dir, '--pretend'] }
+
+    subject { described_class.start(options) }
+
+    it 'does not raise an error' do
+      expect { subject }.not_to raise_error
+    end
+
+    context 'with a missing directory' do
+      let(:options) { [key_path, '--pretend'] }
+
+      it 'raises an error' do
+        expect { subject }.to raise_error(RuntimeError)
+      end
+    end
+
+    context 'with an invalid directory' do
+      let(:dir) { '8d' }
+
+      it 'raises an error' do
+        expect { subject }.to raise_error(RuntimeError)
+      end
+    end
+
+    context 'with an already existing metric with the same key_path' do
+      before do
+        allow(Gitlab::Usage::MetricDefinition).to receive(:definitions).and_return(Hash[key_path, 'definition'])
+      end
+
+      it 'raises an error' do
+        expect { subject }.to raise_error(RuntimeError)
+      end
+    end
+  end
+end