Update the analyzer for merge_request title and description

New analyzers allows to better search the code syntax in the
merge request title and description. Also, adding an elastic migration
to use Zero downtime reindexing to reindex all epics.

Changelog: added
MR: https://gitlab.com/gitlab-org/gitlab/-/merge_requests/142550
EE: true

ee/elastic/docs/20240201164432_reindex_merge_request_to_update_analyzer.yml

+---
+name: ReindexMergeRequestToUpdateAnalyzer
+version: '20240201164432'
+description: This migration reindexes the merge_requests index to start using new analyzers for description and title
+group: group::global search
+milestone: '16.9'
+introduced_by_url: https://gitlab.com/gitlab-org/gitlab/-/merge_requests/142550
+obsolete: false
+marked_obsolete_by_url:
+marked_obsolete_in_milestone:

ee/elastic/migrate/20240201164432_reindex_merge_request_to_update_analyzer.rb

+# frozen_string_literal: true
+
+class ReindexMergeRequestToUpdateAnalyzer < Elastic::Migration
+  def migrate
+    Elastic::ReindexingTask.create!(targets: %w[MergeRequest], options: { skip_pending_migrations_check: true })
+  end
+
+  def completed?
+    true
+  end
+end

ee/lib/elastic/latest/merge_request_config.rb

@@ -21,8 +21,8 @@ module MergeRequestConfig
         indexes :id, type: :integer
         indexes :iid, type: :integer
 
-        indexes :title, type: :text, index_options: 'positions'
-        indexes :description, type: :text, index_options: 'positions'
+        indexes :title, type: :text, index_options: 'positions', analyzer: :title_analyzer
+        indexes :description, type: :text, index_options: 'positions', analyzer: :code_analyzer
         indexes :state, type: :keyword
         indexes :project_id, type: :integer
         indexes :author_id, type: :integer

ee/spec/elastic/migrate/20240201164432_reindex_merge_request_to_update_analyzer_spec.rb

+# frozen_string_literal: true
+
+require 'spec_helper'
+require_relative 'migration_shared_examples'
+require File.expand_path('ee/elastic/migrate/20240201164432_reindex_merge_request_to_update_analyzer.rb')
+
+RSpec.describe ReindexMergeRequestToUpdateAnalyzer, feature_category: :global_search do
+  let(:version) { 20240201164432 }
+  let(:migration) { described_class.new(version) }
+
+  it 'does not have migration options set', :aggregate_failures do
+    expect(migration).not_to be_batched
+    expect(migration).not_to be_retry_on_failure
+  end
+
+  describe '#migrate' do
+    it 'creates reindexing task with correct target and options' do
+      expect { migration.migrate }.to change { Elastic::ReindexingTask.count }.by(1)
+      task = Elastic::ReindexingTask.last
+      expect(task.targets).to eq(%w[MergeRequest])
+      expect(task.options).to eq({ 'skip_pending_migrations_check' => true })
+    end
+  end
+
+  describe '#completed?' do
+    it 'always returns true' do
+      expect(migration.completed?).to eq(true)
+    end
+  end
+end

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

@@ -776,6 +776,30 @@
       expect(results.merge_requests_count).to eq 2
     end
 
+    it_behaves_like 'can search by title for miscellaneous cases', 'merge_requests'
+
+    context 'when description has code snippets' do
+      include_context 'with code examples' do
+        before do
+          code_examples.values.uniq.each.with_index do |code, idx|
+            sha = Digest::SHA256.hexdigest(code)
+            create :merge_request, target_branch: "feature#{idx}", source_project: project_1, target_project: project_1,
+              title: sha, description: code
+          end
+
+          ensure_elasticsearch_index!
+        end
+
+        it 'finds all examples' do
+          code_examples.each do |query, description|
+            sha = Digest::SHA256.hexdigest(description)
+            merge_requests = described_class.new(user, query, limit_project_ids).objects('merge_requests')
+            expect(merge_requests.map(&:title)).to include(sha)
+          end
+        end
+      end
+    end
+
     it 'returns empty list when merge requests are not found' do
       results = described_class.new(user, 'security', limit_project_ids)
 

ee/spec/support/shared_examples/services/search_service_shared_examples.rb

@@ -126,10 +126,14 @@
 
   def create_records!(type)
     case type
-    when 'issues'
-      create_list(:issue, records_count, project: searched_project)
     when 'epics'
       create_list(:epic, records_count, group: searched_group)
+    when 'issues'
+      create_list(:issue, records_count, project: searched_project)
+    when 'merge_requests'
+      records = []
+      records_count.times { |i| records << create(:merge_request, source_branch: i, source_project: searched_project) }
+      records
     end
   end