Prevent 2nd Elasticsearch query for project searches

We memoize the result object so that when it is later evaluated the same
Elasticsearch result can be used for the `total_count`.

We also extract a shared test examples for the 3 different kinds of
Elastic search results to reduce the chance of regression here.

This memoize approach to solving this problem was originally introduced
in https://gitlab.com/gitlab-org/gitlab/-/merge_requests/13120

Note that some of the methods being memoized have arguments and as such
the memoization is not logically consistent from the code perspective.
We must memoize the results without the page argument because
the current page we're on - if we were to memoize with dynamic
parameters we would end up hitting ES twice for any page that's not page
1, and that's something we want to avoid.  It is safe to memoize the
page we get here because the method is always called before
the equivalent `#_count` method.

Read more at https://gitlab.com/gitlab-org/gitlab/-/issues/219209

ee/changelogs/unreleased/219209-remove-extra-elasticsearch-query-in-project-searches.yml

+---
+title: Prevent 2nd Elasticsearch query for project searches
+merge_request: 36672
+author:
+type: performance

ee/lib/gitlab/elastic/project_search_results.rb

@@ -30,28 +30,33 @@ module Gitlab
         return Kaminari.paginate_array([]) if project.empty_repo? || query.blank?
         return Kaminari.paginate_array([]) unless root_ref?
 
+        strong_memoize(:blobs) do
           project.repository.__elasticsearch__.elastic_search_as_found_blob(
             query,
             page: (page || 1).to_i,
             per: per_page
           )
         end
+      end
 
       def wiki_blobs(page: 1, per_page: DEFAULT_PER_PAGE)
         return Kaminari.paginate_array([]) unless Ability.allowed?(@current_user, :read_wiki, project)
 
         if project.wiki_enabled? && !project.wiki.empty? && query.present?
+          strong_memoize(:wiki_blobs) do
             project.wiki.__elasticsearch__.elastic_search_as_wiki_page(
               query,
               page: (page || 1).to_i,
               per: per_page
             )
+          end
         else
           Kaminari.paginate_array([])
         end
       end
 
       def notes
+        strong_memoize(:notes) do
           opt = {
             project_ids: limit_project_ids,
             current_user: @current_user,
@@ -60,6 +65,7 @@ module Gitlab
 
           Note.elastic_search(query, options: opt)
         end
+      end
 
       def commits(page: 1, per_page: DEFAULT_PER_PAGE, preload_method: nil)
         return Kaminari.paginate_array([]) unless Ability.allowed?(@current_user, :download_code, project)
@@ -69,12 +75,14 @@ module Gitlab
         else
           # We use elastic for default branch only
           if root_ref?
+            strong_memoize(:commits) do
               project.repository.find_commits_by_message_with_elastic(
                 query,
                 page: (page || 1).to_i,
                 per_page: per_page,
                 preload_method: preload_method
               )
+            end
           else
             offset = per_page * ((page || 1) - 1)
 

ee/spec/lib/gitlab/elastic/group_search_results_spec.rb

@@ -38,4 +38,10 @@ RSpec.describe Gitlab::Elastic::GroupSearchResults do
       end
     end
   end
+
+  context 'query performance' do
+    let(:query) { '*' }
+
+    include_examples 'does not hit Elasticsearch twice for objects and counts', %w|projects notes blobs wiki_blobs commits issues merge_requests milestones|
+  end
 end

ee/spec/lib/gitlab/elastic/project_search_results_spec.rb

@@ -240,4 +240,18 @@ RSpec.describe Gitlab::Elastic::ProjectSearchResults, :elastic do
       end
     end
   end
+
+  context 'query performance' do
+    let(:project) { create(:project, :public, :repository, :wiki_repo) }
+
+    before do
+      # wiki_blobs method checks to see if there is a wiki page before doing
+      # the search
+      create(:wiki_page, wiki: project.wiki)
+    end
+
+    let(:results) { described_class.new(user, '*', project) }
+
+    include_examples 'does not hit Elasticsearch twice for objects and counts', %w|notes blobs wiki_blobs commits issues merge_requests milestones|
+  end
 end

ee/spec/lib/gitlab/elastic/search_results_spec.rb

@@ -12,16 +12,6 @@ RSpec.describe Gitlab::Elastic::SearchResults, :elastic, :sidekiq_might_not_need
   let(:project_2) { create(:project, :public, :repository, :wiki_repo) }
   let(:limit_project_ids) { [project_1.id] }
 
-  describe 'counts' do
-    it 'does not hit Elasticsearch twice for result and counts' do
-      expect(Repository).to receive(:find_commits_by_message_with_elastic).with('hello world', anything).once.and_call_original
-
-      results = described_class.new(user, 'hello world', limit_project_ids)
-      expect(results.objects('commits', page: 2)).to be_empty
-      expect(results.commits_count).to eq 0
-    end
-  end
-
   describe '#formatted_count' do
     using RSpec::Parameterized::TableSyntax
 
@@ -1134,4 +1124,10 @@ RSpec.describe Gitlab::Elastic::SearchResults, :elastic, :sidekiq_might_not_need
       end
     end
   end
+
+  context 'query performance' do
+    let(:results) { described_class.new(user, 'hello world', limit_project_ids) }
+
+    include_examples 'does not hit Elasticsearch twice for objects and counts', %w|projects notes blobs wiki_blobs commits issues merge_requests milestones|
+  end
 end

ee/spec/support/shared_examples/lib/gitlab/elastic/search_results_shared_examples.rb

+# frozen_string_literal: true
+
+RSpec.shared_examples 'does not hit Elasticsearch twice for objects and counts' do |scopes|
+  scopes.each do |scope|
+    context "for scope #{scope}", :elastic, :request_store do
+      it 'makes 1 Elasticsearch query' do
+        ::Gitlab::SafeRequestStore.clear!
+
+        results.objects(scope)
+        results.public_send("#{scope}_count")
+
+        expect(::Gitlab::Instrumentation::ElasticsearchTransport.get_request_count).to eq(1)
+      end
+    end
+  end
+end