Add rebalance_issues feature flag

This adds a new feature flag: `rebalance_issues`

This flag is scoped by project, and is read in the issue create and
update services.

app/services/issues/base_service.rb

@@ -22,6 +22,7 @@ module Issues
     NO_REBALANCING_NEEDED = ((RelativePositioning::MIN_POSITION * 0.9999)..(RelativePositioning::MAX_POSITION * 0.9999)).freeze
 
     def rebalance_if_needed(issue)
+      return unless Feature.enabled?(:rebalance_issues, issue.project)
       return unless issue
       return if issue.relative_position.nil?
       return if NO_REBALANCING_NEEDED.cover?(issue.relative_position)

config/feature_flags/development/rebalance_issues.yml

+---
+name: rebalance_issues
+introduced_by_url: https://gitlab.com/gitlab-org/gitlab/-/merge_requests/40124 
+rollout_issue_url: https://gitlab.com/gitlab-org/gitlab/-/issues/239344
+group: ''
+type: development
+default_enabled: false

spec/services/issues/create_service_spec.rb

@@ -82,6 +82,24 @@ RSpec.describe Issues::CreateService do
         expect(issue.relative_position).to eq(project.issues.maximum(:relative_position))
       end
 
+      it 'does not rebalance if the flag is disabled' do
+        stub_feature_flags(rebalance_issues: false)
+
+        create(:issue, project: project, relative_position: RelativePositioning::MAX_POSITION)
+        expect(IssueRebalancingWorker).not_to receive(:perform_async).with(Integer)
+
+        expect(issue.relative_position).to eq(project.issues.maximum(:relative_position))
+      end
+
+      it 'does rebalance if the flag is enabled for the project' do
+        stub_feature_flags(rebalance_issues: project)
+
+        create(:issue, project: project, relative_position: RelativePositioning::MAX_POSITION)
+        expect(IssueRebalancingWorker).to receive(:perform_async).with(Integer)
+
+        expect(issue.relative_position).to eq(project.issues.maximum(:relative_position))
+      end
+
       it 'does not rebalance unless needed' do
         expect(IssueRebalancingWorker).not_to receive(:perform_async).with(Integer)
 

spec/services/issues/update_service_spec.rb

@@ -116,6 +116,38 @@ RSpec.describe Issues::UpdateService, :mailer do
         expect(issue.relative_position).to be_between(issue1.relative_position, issue2.relative_position)
       end
 
+      it 'does not rebalance even if needed if the flag is disabled' do
+        stub_feature_flags(rebalance_issues: false)
+
+        range = described_class::NO_REBALANCING_NEEDED
+        issue1 = create(:issue, project: project, relative_position: range.first - 100)
+        issue2 = create(:issue, project: project, relative_position: range.first)
+        issue.update!(relative_position: RelativePositioning::START_POSITION)
+
+        opts[:move_between_ids] = [issue1.id, issue2.id]
+
+        expect(IssueRebalancingWorker).not_to receive(:perform_async).with(issue.id)
+
+        update_issue(opts)
+        expect(issue.relative_position).to be_between(issue1.relative_position, issue2.relative_position)
+      end
+
+      it 'rebalances if needed if the flag is enabled for the project' do
+        stub_feature_flags(rebalance_issues: project)
+
+        range = described_class::NO_REBALANCING_NEEDED
+        issue1 = create(:issue, project: project, relative_position: range.first - 100)
+        issue2 = create(:issue, project: project, relative_position: range.first)
+        issue.update!(relative_position: RelativePositioning::START_POSITION)
+
+        opts[:move_between_ids] = [issue1.id, issue2.id]
+
+        expect(IssueRebalancingWorker).to receive(:perform_async).with(issue.id)
+
+        update_issue(opts)
+        expect(issue.relative_position).to be_between(issue1.relative_position, issue2.relative_position)
+      end
+
       it 'rebalances if needed on the left' do
         range = described_class::NO_REBALANCING_NEEDED
         issue1 = create(:issue, project: project, relative_position: range.first - 100)