Add issue rebalancing worker

app/services/issue_rebalancing_service.rb

+# frozen_string_literal: true
+
+module Issue
+  class RebalancingService < Issues::BaseService
+    MAX_ISSUE_COUNT = 100_000
+    TooManyIssues = Class.new(StandardError)
+
+    attr_reader :issue
+
+    def initialize(issue)
+      @issue = issue
+    end
+
+    # rubocop: disable CodeReuse/ActiveRecord
+    def execute
+      base = Issue.relative_positioning_query_base(issue)
+
+      n = base.count
+
+      if n > MAX_ISSUE_COUNT
+        raise TooManyIssues, "#{n} issues"
+      end
+
+      gaps = n - 1
+      gap_size = 0
+      ratio = 0.5
+
+      while gap_size < RelativePositioning::MIN_GAP && ratio < 1
+        gap_size = (ratio * Gitlab::Database::MAX_INT_VALUE) / (gaps / 2)
+        ratio += 0.1
+      end
+
+      # If there are 4 billion issues, then we cannot rebalance them
+      if gap_size < RelativePositioning::MIN_GAP
+        raise RelativePositioning::NoSpaceLeft
+      end
+
+      start = 0 - (gaps / 2) * gap_size
+
+      indexed = base.reorder(:relative_position, :id).pluck(:id).each_with_index
+
+      indexed.each_slice(500) do |pairs|
+        values = pairs.map do |id, index|
+          "(#{id}, #{start + (index * gap_size)})"
+        end.join(', ')
+
+        Issue.connection.exec_query(<<~SQL, "rebalance issue positions")
+          WITH cte(cte_id, new_pos) AS (
+           SELECT *
+           FROM (VALUES #{values}) as t (id, pos)
+          )
+          UPDATE #{Issue.table_name}
+          SET relative_position = cte.new_pos
+          FROM cte
+          WHERE cte_id = id
+        SQL
+      end
+    end
+    # rubocop: enable CodeReuse/ActiveRecord
+  end
+end

app/workers/issue_rebalancing_worker.rb

+# frozen_string_literal: true
+
+class IssueRebalancingWorker # rubocop:disable Scalability/IdempotentWorker
+  include ApplicationWorker
+
+  def perform(issue_id)
+    issue = Issue.find(issue_id)
+
+    rebalance(issue)
+  rescue ActiveRecord::RecordNotFound => e
+    Sidekiq.logger.warn(e)
+  end
+
+  private
+
+  def rebalance(issue)
+    IssueRebalancingService.new(issue).execute
+  end
+end