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 openSUSE#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) }
     rescue Project::UnknownObjectError, Package::UnknownObjectError
       # object got removed already
       return
@@ -52,4 +52,16 @@ 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 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(event)
+    event.expanded_payload.merge('faillog' => reencode_faillog(event.expanded_payload.delete('faillog')))
+  end
 end

src/api/spec/mailers/another_event_mailer_spec.rb

@@ -0,0 +1,37 @@
+require 'rails_helper'
+
+RSpec.describe EventMailer, type: :mailer do
+  describe '#event' 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
+      double(:event,
+             expanded_payload: expanded_payload,
+             custom_headers: {},
+             template_name: 'build_fail',
+             originator: from,
+             subject: 'Test email',
+             created_at: Time.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