Reduce Namespace queries in UserReferenceFilter

This changes UserReferenceFilter so it operates using the following
steps:

1. Grab all username references from the input document.
2. Query the corresponding Namespace objects using a single query.
3. Iterate over all nodes to build links while re-using the objects
   queried in step 2.

The impact of these changes is that a comment mentioning 5 different
usernames no longer runs 5 different queries (1 for every username),
instead it only runs a single query.

CHANGELOG

@@ -26,6 +26,7 @@ v 8.9.0 (unreleased)
   - Measure queue duration between gitlab-workhorse and Rails
   - Make authentication service for Container Registry to be compatible with < Docker 1.11
   - Add Application Setting to configure Container Registry token expire delay (default 5min)
+  - Reduce number of SQL queries when rendering user references
 
 v 8.8.3
   - Fix incorrect links on pipeline page when merge request created from fork

lib/banzai/filter/user_reference_filter.rb

@@ -29,7 +29,7 @@ module Banzai
         ref_pattern = User.reference_pattern
         ref_pattern_start = /\A#{ref_pattern}\z/
 
-        each_node do |node|
+        nodes.each do |node|
           if text_node?(node)
             replace_text_when_pattern_matches(node, ref_pattern) do |content|
               user_link_filter(content)
@@ -59,7 +59,7 @@ module Banzai
         self.class.references_in(text) do |match, username|
           if username == 'all'
             link_to_all(link_text: link_text)
-          elsif namespace = Namespace.find_by(path: username)
+          elsif namespace = namespaces[username]
             link_to_namespace(namespace, link_text: link_text) || match
           else
             match
@@ -67,6 +67,31 @@ module Banzai
         end
       end
 
+      # Returns a Hash containing all Namespace objects for the username
+      # references in the current document.
+      #
+      # The keys of this Hash are the namespace paths, the values the
+      # corresponding Namespace objects.
+      def namespaces
+        @namespaces ||=
+          Namespace.where(path: usernames).each_with_object({}) do |row, hash|
+            hash[row.path] = row
+          end
+      end
+
+      # Returns all usernames referenced in the current document.
+      def usernames
+        refs = Set.new
+
+        nodes.each do |node|
+          node.to_html.scan(User.reference_pattern) do
+            refs << $~[:user]
+          end
+        end
+
+        refs.to_a
+      end
+
       private
 
       def urls

spec/lib/banzai/filter/user_reference_filter_spec.rb

@@ -136,4 +136,23 @@ describe Banzai::Filter::UserReferenceFilter, lib: true do
       expect(link.attr('data-user')).to eq user.namespace.owner_id.to_s
     end
   end
+
+  describe '#namespaces' do
+    it 'returns a Hash containing all Namespaces' do
+      document = Nokogiri::HTML.fragment("<p>#{user.to_reference}</p>")
+      filter = described_class.new(document, project: project)
+      ns = user.namespace
+
+      expect(filter.namespaces).to eq({ ns.path => ns })
+    end
+  end
+
+  describe '#usernames' do
+    it 'returns the usernames mentioned in a document' do
+      document = Nokogiri::HTML.fragment("<p>#{user.to_reference}</p>")
+      filter = described_class.new(document, project: project)
+
+      expect(filter.usernames).to eq([user.username])
+    end
+  end
 end