Introduce snowplow event test helper

By calling the original method, we can make sure that the types
are correct. Snowplow is using a type checking gem that would
otherwise throw an exception in runtime. This is something we want
to catch in tests.

Replace the AsyncEmitter with Emitter. We want to avoid
AsyncEmitter because it does some work in a separate thread, and
we want any failures to occur in the same thread as the test.

doc/development/testing_guide/best_practices.md

@@ -524,6 +524,48 @@ for modifications. If you have no other choice, an `around` block similar to the
 example for global variables, above, can be used, but this should be avoided if
 at all possible.
 
+#### Test Snowplow events
+
+CAUTION: **Warning:**
+Snowplow performs **runtime type checks** by using the [contracts gem](https://rubygems.org/gems/contracts).
+Since Snowplow is **by default disabled in tests and development**, it can be hard to
+**catch exceptions** when mocking `Gitlab::Tracking`.
+
+To catch runtime errors due to type checks, you can enable Snowplow in tests by marking the spec with
+`:snowplow` and use the `expect_snowplow_event` helper which will check for
+calls to `Gitlab::Tracking#event`.
+
+```ruby
+describe '#show', :snowplow do
+  it 'tracks snowplow events' do
+    get :show
+
+    expect_snowplow_event(
+      category: 'Experiment',
+      action: 'start',
+    )
+    expect_snowplow_event(
+      category: 'Experiment',
+      action: 'sent',
+      property: 'property',
+      label: 'label'
+    )
+  end
+end
+```
+
+When you want to ensure that no event got called, you can use `expect_no_snowplow_event`.
+
+```ruby
+  describe '#show', :snowplow do
+    it 'does not track any snowplow events' do
+      get :show
+
+      expect_no_snowplow_event
+    end
+  end
+```
+
 ### Table-based / Parameterized tests
 
 This style of testing is used to exercise one piece of code with a comprehensive

lib/gitlab/tracking.rb

@@ -56,12 +56,19 @@ module Gitlab
 
       def snowplow
         @snowplow ||= SnowplowTracker::Tracker.new(
-          SnowplowTracker::AsyncEmitter.new(Gitlab::CurrentSettings.snowplow_collector_hostname, protocol: 'https'),
+          emitter,
           SnowplowTracker::Subject.new,
           SNOWPLOW_NAMESPACE,
           Gitlab::CurrentSettings.snowplow_app_id
         )
       end
+
+      def emitter
+        SnowplowTracker::AsyncEmitter.new(
+          Gitlab::CurrentSettings.snowplow_collector_hostname,
+          protocol: 'https'
+        )
+      end
     end
   end
 end

spec/spec_helper.rb

@@ -115,6 +115,7 @@ RSpec.configure do |config|
   config.include StubExperiments
   config.include StubGitlabCalls
   config.include StubGitlabData
+  config.include SnowplowHelpers
   config.include NextFoundInstanceOf
   config.include NextInstanceOf
   config.include TestEnv

spec/support/helpers/snowplow_helpers.rb

+# frozen_string_literal: true
+
+module SnowplowHelpers
+  # Asserts call for one snowplow event from `Gitlab::Tracking#event`.
+  #
+  # @param [Hash]
+  #
+  # Examples:
+  #
+  #   describe '#show', :snowplow do
+  #     it 'tracks snowplow events' do
+  #       get :show
+  #
+  #       expect_snowplow_event(category: 'Experiment', action: 'start')
+  #     end
+  #   end
+  #
+  #   describe '#create', :snowplow do
+  #     it 'tracks snowplow events' do
+  #       post :create
+  #
+  #       expect_snowplow_event(
+  #         category: 'Experiment',
+  #         action: 'created',
+  #       )
+  #       expect_snowplow_event(
+  #         category: 'Experiment',
+  #         action: 'accepted',
+  #         property: 'property',
+  #         label: 'label'
+  #       )
+  #     end
+  #   end
+  def expect_snowplow_event(category:, action:, **kwargs)
+    expect(Gitlab::Tracking).to have_received(:event)
+      .with(category, action, **kwargs)
+  end
+
+  # Asserts that no call to `Gitlab::Tracking#event` was made.
+  #
+  # Example:
+  #
+  #   describe '#show', :snowplow do
+  #     it 'does not track any snowplow events' do
+  #       get :show
+  #
+  #       expect_no_snowplow_event
+  #     end
+  #   end
+  def expect_no_snowplow_event
+    expect(Gitlab::Tracking).not_to have_received(:event)
+  end
+end

spec/support/snowplow.rb

+# frozen_string_literal: true
+
+RSpec.configure do |config|
+  config.before(:each, :snowplow) do
+    # Using a high buffer size to not cause early flushes
+    buffer_size = 100
+    # WebMock is set up to allow requests to `localhost`
+    host = 'localhost'
+
+    allow(Gitlab::Tracking)
+      .to receive(:emitter)
+      .and_return(SnowplowTracker::Emitter.new(host, buffer_size: buffer_size))
+
+    stub_application_setting(snowplow_enabled: true)
+
+    allow(Gitlab::Tracking).to receive(:event).and_call_original
+  end
+
+  config.after(:each, :snowplow) do
+    Gitlab::Tracking.send(:snowplow).flush
+  end
+end