Rebalance issues if needed on create and update

This adds a new method in the issues base_service that rebalances the
issue set when the issue is placed in the extreme 0.0001 fringes of the
relative positioning range.

app/services/issues/base_service.rb

@@ -19,6 +19,16 @@ module Issues
 
     private
 
+    NO_REBALANCING_NEEDED = ((RelativePositioning::MIN_POSITION * 0.9999)..(RelativePositioning::MAX_POSITION * 0.9999)).freeze
+
+    def rebalance_if_needed(issue)
+      return unless issue
+      return if issue.relative_position.nil?
+      return if NO_REBALANCING_NEEDED.cover?(issue.relative_position)
+
+      IssueRebalancingWorker.perform_async(issue.id)
+    end
+
     def create_assignee_note(issue, old_assignees)
       SystemNoteService.change_issuable_assignees(
         issue, issue.project, current_user, old_assignees)

app/services/issues/create_service.rb

@@ -30,6 +30,7 @@ module Issues
       user_agent_detail_service.create
       resolve_discussions_with_issue(issuable)
       delete_milestone_total_issue_counter_cache(issuable.milestone)
+      rebalance_if_needed(issuable)
 
       super
     end

app/services/issues/update_service.rb

@@ -83,6 +83,7 @@ module Issues
       raise ActiveRecord::RecordNotFound unless issue_before || issue_after
 
       issue.move_between(issue_before, issue_after)
+      rebalance_if_needed(issue)
     end
 
     # rubocop: disable CodeReuse/ActiveRecord

spec/services/issues/create_service_spec.rb

@@ -75,6 +75,19 @@ RSpec.describe Issues::CreateService do
         expect(Todo.where(attributes).count).to eq 1
       end
 
+      it 'rebalances if needed' do
+        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)
+
+        expect(issue.relative_position).to eq(project.issues.maximum(:relative_position))
+      end
+
       context 'when label belongs to project group' do
         let(:group) { create(:group) }
         let(:group_labels) { create_pair(:group_label, group: group) }

spec/services/issues/update_service_spec.rb

@@ -106,7 +106,7 @@ RSpec.describe Issues::UpdateService, :mailer do
 
         [issue, issue1, issue2].each do |issue|
           issue.move_to_end
-          issue.save
+          issue.save!
         end
 
         opts[:move_between_ids] = [issue1.id, issue2.id]
@@ -116,6 +116,34 @@ RSpec.describe Issues::UpdateService, :mailer do
         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)
+        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 right' do
+        range = described_class::NO_REBALANCING_NEEDED
+        issue1 = create(:issue, project: project, relative_position: range.last)
+        issue2 = create(:issue, project: project, relative_position: range.last + 100)
+        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
+
       context 'when moving issue between issues from different projects' do
         let(:group) { create(:group) }
         let(:subgroup) { create(:group, parent: group) }