Fix Error 500 when visiting build page of project with nil runners_token

Properly ensure that the token exists and add defensively check for a
non-nil value.

Closes #4294

CHANGELOG

@@ -4,6 +4,7 @@ v 8.4.0 (unreleased)
   - The default GitLab logo now acts as a loading indicator
   - Accept 2xx status codes for successful Web hook triggers (Stan Hu)
   - Fix missing date of month in network graph when commits span a month (Stan Hu)
+  - Fix Error 500 when visiting build page of project with nil runners_token (Stan Hu)
   - Expire view caches when application settings change (e.g. Gravatar disabled) (Stan Hu)
   - Don't notify users twice if they are both project watchers and subscribers (Stan Hu)
   - Implement new UI for group page

app/models/ci/build.rb

@@ -206,7 +206,7 @@ module Ci
 
     def trace
       trace = raw_trace
-      if project && trace.present?
+      if project && trace.present? && project.runners_token.present?
         trace.gsub(project.runners_token, 'xxxxxx')
       else
         trace

app/models/project.rb

@@ -50,6 +50,7 @@ class Project < ActiveRecord::Base
   include Sortable
   include AfterCommitQueue
   include CaseSensitivity
+  include TokenAuthenticatable
 
   extend Gitlab::ConfigHelper
 
@@ -193,10 +194,7 @@ class Project < ActiveRecord::Base
     if: ->(project) { project.avatar.present? && project.avatar_changed? }
   validates :avatar, file_size: { maximum: 200.kilobytes.to_i }
 
-  before_validation :set_runners_token_token
-  def set_runners_token_token
-    self.runners_token = SecureRandom.hex(15) if self.runners_token.blank?
-  end
+  add_authentication_token_field :runners_token
 
   mount_uploader :avatar, AvatarUploader
 
@@ -900,4 +898,8 @@ class Project < ActiveRecord::Base
     return true unless forked?
     Gitlab::VisibilityLevel.allowed_fork_levels(forked_from_project.visibility_level).include?(level.to_i)
   end
+
+  def runners_token
+    ensure_runners_token!
+  end
 end

spec/models/ci/build_spec.rb

+require 'spec_helper'
+
+describe Ci::Build, models: true do
+  let(:build) { create(:ci_build) }
+  let(:test_trace) { 'This is a test' }
+
+  describe '#trace' do
+    it 'obfuscates project runners token' do
+      allow(build).to receive(:raw_trace).and_return("Test: #{build.project.runners_token}")
+
+      expect(build.trace).to eq("Test: xxxxxx")
+    end
+
+    it 'empty project runners token' do
+      allow(build).to receive(:raw_trace).and_return(test_trace)
+      # runners_token can't normally be set to nil
+      allow(build.project).to receive(:runners_token).and_return(nil)
+
+      expect(build.trace).to eq(test_trace)
+    end
+  end
+end