Fix invalid byte sequence in UTF-8 in event mailer

Sometimes the build log contains invalid UTF-8 characters that raise an
exception during email composing.

Fixes #6872

src/api/app/mailers/event_mailer.rb

@@ -26,7 +26,7 @@ def event(subscribers, e)
 
     set_headers
     begin
-      locals = { event: e.expanded_payload }
+      locals = { event: sanitize_expanded_payload(e.expanded_payload) }
     rescue Project::UnknownObjectError, Package::UnknownObjectError
       # object got removed already
       return
@@ -52,4 +52,20 @@ def event(subscribers, e)
       format.text { render template_name, locals: locals } if template_exists?("event_mailer/#{template_name}", formats: [:text])
     end
   end
+
+  private
+
+  # Reencode the fail log replacing invalid UTF-8 characters with the default unicode replacement character: '\ufffd'
+  # source: https://stackoverflow.com/a/24493972
+  def reencode_faillog(faillog)
+    faillog.encode!('UTF-8', 'UTF-8', invalid: :replace)
+  end
+
+  def sanitize_expanded_payload(payload)
+    if payload['faillog']
+      payload.merge('faillog' => reencode_faillog(payload.delete('faillog')))
+    else
+      payload
+    end
+  end
 end

src/api/spec/mailers/event_mailer_spec.rb

@@ -127,5 +127,39 @@
         expect(ActionMailer::Base.deliveries).to be_empty
       end
     end
+
+    context 'when trying to compose an email containing invalid byte sequences' do
+      let(:expanded_payload) do
+        {
+          'project' => 'project_2',
+          'package' => 'package_2',
+          'repository' => 'repository_2',
+          'arch' => 'i585',
+          'faillog' => "invalid byte sequence ->\xD3'"
+        }
+      end
+      let(:from) { create(:confirmed_user) }
+      let(:recipient) { create(:confirmed_user) }
+      let(:event_stub) do
+        instance_double('Event::Base',
+                        expanded_payload: expanded_payload,
+                        custom_headers: {},
+                        template_name: 'build_fail',
+                        originator: from,
+                        subject: 'Test email',
+                        created_at: Time.zone.now)
+      end
+      let(:mail) { EventMailer.event([recipient], event_stub) }
+
+      it 'renders the headers' do
+        expect(mail.subject).to eq('Test email')
+        expect(mail.to).to eq([recipient.email])
+        expect(mail.from).to eq([from.email])
+      end
+
+      it 'renders the body' do
+        expect(mail.body.encoded).to match('Package project_2/package_2 failed to build in repository_2/i585')
+      end
+    end
   end
 end