Fix pagination on sorts with lots of ties

Postgres and MySQL don't guarantee that pagination with `LIMIT` and
`OFFSET` will work if the ordering isn't unique. From the Postgres docs:

> When using `LIMIT`, it is important to use an `ORDER BY` clause that
> constrains the result rows into a unique order. Otherwise you will get
> an unpredictable subset of the query's rows

Before:

    [1] pry(main)> issues = 1.upto(Issue.count).map { |i| Issue.sort('priority').page(i).per(1).map(&:id) }.flatten
    [2] pry(main)> issues.count
    => 81
    [3] pry(main)> issues.uniq.count
    => 42

After:

    [1] pry(main)> issues = 1.upto(Issue.count).map { |i| Issue.sort('priority').page(i).per(1).map(&:id) }.flatten
    [2] pry(main)> issues.count
    => 81
    [3] pry(main)> issues.uniq.count
    => 81

CHANGELOG

@@ -4,6 +4,7 @@ v 8.10.0 (unreleased)
   - Wrap code blocks on Activies and Todos page. !4783 (winniehell)
   - Add Sidekiq queue duration to transaction metrics.
   - Fix MR-auto-close text added to description. !4836
+  - Fix pagination when sorting by columns with lots of ties (like priority)
   - Implement Subresource Integrity for CSS and JavaScript assets. This prevents malicious assets from loading in the case of a CDN compromise.
 
 v 8.9.0

app/models/concerns/issuable.rb

@@ -112,15 +112,18 @@ module Issuable
     end
 
     def sort(method, excluded_labels: [])
-      case method.to_s
-      when 'milestone_due_asc' then order_milestone_due_asc
-      when 'milestone_due_desc' then order_milestone_due_desc
-      when 'downvotes_desc' then order_downvotes_desc
-      when 'upvotes_desc' then order_upvotes_desc
-      when 'priority' then order_labels_priority(excluded_labels: excluded_labels)
-      else
-        order_by(method)
-      end
+      sorted = case method.to_s
+               when 'milestone_due_asc' then order_milestone_due_asc
+               when 'milestone_due_desc' then order_milestone_due_desc
+               when 'downvotes_desc' then order_downvotes_desc
+               when 'upvotes_desc' then order_upvotes_desc
+               when 'priority' then order_labels_priority(excluded_labels: excluded_labels)
+               else
+                 order_by(method)
+               end
+
+      # Break ties with the ID column for pagination
+      sorted.order(id: :desc)
     end
 
     def order_labels_priority(excluded_labels: [])

spec/models/concerns/issuable_spec.rb

@@ -154,6 +154,20 @@ describe Issue, "Issuable" do
         expect(issues).to match_array([issue1, issue2, issue, issue3])
       end
     end
+
+    context 'when all of the results are level on the sort key' do
+      let!(:issues) do
+        10.times { create(:issue, project: project) }
+      end
+
+      it 'has no duplicates across pages' do
+        sorted_issue_ids = 1.upto(10).map do |i|
+          project.issues.sort('milestone_due_desc').page(i).per(1).first.id
+        end
+
+        expect(sorted_issue_ids).to eq(sorted_issue_ids.uniq)
+      end
+    end
   end