Fix emoji paths in relative root configurations

If a site specifies a relative URL root, emoji files would omit the path from
the URL, leading to lots of 404s.

Closes #15642

lib/gitlab/award_emoji.rb

@@ -66,8 +66,17 @@ module Gitlab
     def self.urls
       @urls ||= begin
                   path = File.join(Rails.root, 'fixtures', 'emojis', 'digests.json')
+                  # Construct the full asset path ourselves because
+                  # ActionView::Helpers::AssetUrlHelper.asset_url is slow for hundreds
+                  # of entries since it has to do a lot of extra work (e.g. regexps).
                   prefix = Gitlab::Application.config.assets.prefix
                   digest = Gitlab::Application.config.assets.digest
+                  base =
+                    if defined?(Gitlab::Application.config.relative_url_root) && Gitlab::Application.config.relative_url_root
+                      Gitlab::Application.config.relative_url_root
+                    else
+                      ''
+                    end
 
                   JSON.parse(File.read(path)).map do |hash|
                     if digest
@@ -76,7 +85,7 @@ module Gitlab
                       fname = hash['unicode']
                     end
 
-                    { name: hash['name'], path: "#{prefix}/#{fname}.png" }
+                    { name: hash['name'], path: File.join(base, prefix, "#{fname}.png") }
                   end
                 end
     end

spec/lib/gitlab/award_emoji_spec.rb

@@ -2,6 +2,10 @@ require 'spec_helper'
 
 describe Gitlab::AwardEmoji do
   describe '.urls' do
+    after do
+      Gitlab::AwardEmoji.instance_variable_set(:@urls, nil)
+    end
+
     subject { Gitlab::AwardEmoji.urls }
 
     it { is_expected.to be_an_instance_of(Array) }
@@ -15,6 +19,17 @@ describe Gitlab::AwardEmoji do
         end
       end
     end
+
+    context 'handles relative root' do
+      it 'includes the full path' do
+        allow(Gitlab::Application.config).to receive(:relative_url_root).and_return('/gitlab')
+
+        subject.each do |hash|
+          expect(hash[:name]).to be_an_instance_of(String)
+          expect(hash[:path]).to start_with('/gitlab')
+        end
+      end
+    end
   end
 
   describe '.emoji_by_category' do