From dce9599ef53dcdeb035b339510b759d71ad10f1e Mon Sep 17 00:00:00 2001
From: Grzegorz Bizon <grzesiek.bizon@gmail.com>
Date: Mon, 16 Jul 2018 13:40:55 +0200
Subject: [PATCH] Implement action mailer delivery metrics observer

---
 config/initializers/action_mailer_hooks.rb    |  9 +++--
 .../email/hook/delivery_metrics_observer.rb   | 31 ++++++++++++++++
 .../hook/delivery_metrics_observer_spec.rb    | 35 +++++++++++++++++++
 3 files changed, 73 insertions(+), 2 deletions(-)
 create mode 100644 lib/gitlab/email/hook/delivery_metrics_observer.rb
 create mode 100644 spec/lib/gitlab/email/hook/delivery_metrics_observer_spec.rb

diff --git a/config/initializers/action_mailer_hooks.rb b/config/initializers/action_mailer_hooks.rb
index f792e2dc68e..f1b3c1f8ae8 100644
--- a/config/initializers/action_mailer_hooks.rb
+++ b/config/initializers/action_mailer_hooks.rb
@@ -3,5 +3,10 @@ unless Gitlab.config.gitlab.email_enabled
   ActionMailer::Base.logger = nil
 end
 
-ActionMailer::Base.register_interceptor(::Gitlab::Email::Hook::AdditionalHeadersInterceptor)
-ActionMailer::Base.register_interceptor(::Gitlab::Email::Hook::EmailTemplateInterceptor)
+ActionMailer::Base.register_interceptors(
+  ::Gitlab::Email::Hook::AdditionalHeadersInterceptor,
+  ::Gitlab::Email::Hook::EmailTemplateInterceptor,
+  ::Gitlab::Email::Hook::DeliveryMetricsObserver
+)
+
+ActionMailer::Base.register_observer(::Gitlab::Email::Hook::DeliveryMetricsObserver)
diff --git a/lib/gitlab/email/hook/delivery_metrics_observer.rb b/lib/gitlab/email/hook/delivery_metrics_observer.rb
new file mode 100644
index 00000000000..1c2985f6045
--- /dev/null
+++ b/lib/gitlab/email/hook/delivery_metrics_observer.rb
@@ -0,0 +1,31 @@
+module Gitlab
+  module Email
+    module Hook
+      class DeliveryMetricsObserver
+        extend Gitlab::Utils::StrongMemoize
+
+        def self.delivering_email(_message)
+          delivery_attempts_counter.increment
+        end
+
+        def self.delivered_email(_message)
+          delivered_emails_counter.increment
+        end
+
+        def self.delivery_attempts_counter
+          strong_memoize(:delivery_attempts_counter) do
+            Gitlab::Metrics.counter(:gitlab_emails_delivery_attempts_total,
+                                    'Counter of total emails delivery attempts')
+          end
+        end
+
+        def self.delivered_emails_counter
+          strong_memoize(:delivered_emails_counter) do
+            Gitlab::Metrics.counter(:gitlab_emails_delivered_total,
+                                    'Counter of total emails delievered')
+          end
+        end
+      end
+    end
+  end
+end
diff --git a/spec/lib/gitlab/email/hook/delivery_metrics_observer_spec.rb b/spec/lib/gitlab/email/hook/delivery_metrics_observer_spec.rb
new file mode 100644
index 00000000000..4497d4002da
--- /dev/null
+++ b/spec/lib/gitlab/email/hook/delivery_metrics_observer_spec.rb
@@ -0,0 +1,35 @@
+require 'spec_helper'
+
+describe Gitlab::Email::Hook::DeliveryMetricsObserver do
+  let(:email) do
+    ActionMailer::Base.mail(to: 'test@example.com',
+                            from: 'info@example.com',
+                            body: 'hello')
+  end
+
+  context 'when email has been delivered' do
+    it 'increments both email delivery metrics' do
+      expect(described_class.delivery_attempts_counter).to receive(:increment)
+      expect(described_class.delivered_emails_counter).to receive(:increment)
+
+      email.deliver_now
+    end
+  end
+
+  context 'when email has not been delivered due to an error' do
+    before do
+      allow(email.delivery_method).to receive(:deliver!)
+        .and_raise(StandardError, 'Some SMTP error')
+    end
+
+    it 'increments only delivery attempt metric' do
+      expect(described_class.delivery_attempts_counter)
+        .to receive(:increment)
+      expect(described_class.delivered_emails_counter)
+        .not_to receive(:increment)
+
+      expect { email.deliver_now }
+        .to raise_error(StandardError, 'Some SMTP error')
+    end
+  end
+end