Use events table to feed Burndown data on closed issues

https://gitlab.com/gitlab-org/gitlab-ee/merge_requests/4855 introduces a
change which nullifies issues.closed_at whenever one reopens a issue.
Which is not currently expected by the burndown charts. Hence we're now
using the events.created_at information.

ee/app/models/burndown.rb

@@ -82,10 +82,33 @@ class Burndown
 
   def milestone_issues
     @milestone_issues ||=
-      @milestone.issues
-        .where("state = 'closed' OR (state = 'opened' AND closed_at IS NOT NULL)")
-        .reorder("closed_at ASC")
-        .pluck("closed_at, weight, state")
-        .map {|attrs| Issue.new(*attrs) }
+      begin
+        # We make use of `events` table to get the closed_at timestamp.
+        # `issues.closed_at` can't be used once it's nullified if the issue is
+        # reopened.
+        internal_clause =
+          ::Issue
+            .joins("LEFT OUTER JOIN events e ON issues.id = e.target_id AND e.target_type = 'Issue'")
+            .where(milestone: @milestone)
+            .where("state = 'closed' OR (state = 'opened' AND e.action = #{Event::CLOSED})") # rubocop:disable GitlabSecurity/SqlInjection
+
+        rel =
+          if Gitlab::Database.postgresql?
+            ::Issue
+              .select("*")
+              .from(internal_clause.select('DISTINCT ON (issues.id) issues.id, issues.state, issues.weight, e.created_at AS closed_at'))
+          else
+            ::Issue
+              .select("*")
+              .from(internal_clause.select('issues.id, issues.state, issues.weight, e.created_at AS closed_at'))
+              .group('id')
+              .having('closed_at = MIN(closed_at) OR closed_at IS NULL')
+          end
+
+        rel
+          .order('closed_at ASC')
+          .pluck('closed_at, weight, state')
+          .map { |attrs| Issue.new(*attrs) }
+      end
   end
 end

ee/spec/features/projects/milestones/milestone_spec.rb

@@ -13,6 +13,10 @@ describe 'Milestones on EE' do
     visit project_milestone_path(project, milestone)
   end
 
+  def close_issue(issue)
+    Issues::CloseService.new(issue.project, user, {}).execute(issue)
+  end
+
   context 'burndown charts' do
     let(:milestone) do
       create(:milestone,
@@ -36,11 +40,13 @@ describe 'Milestones on EE' do
         end
       end
 
-      context 'when any closed issues do not have closed_at value' do
+      context 'when a closed issue do not have closed events' do
         it 'shows warning' do
-          create(:issue, issue_params)
+          close_issue(create(:issue, issue_params))
+          close_issue(create(:issue, issue_params))
+
+          # Legacy issue: No closed events being created
           create(:closed_issue, issue_params)
-          create(:closed_issue, issue_params.merge(closed_at: nil))
 
           visit_milestone
 
@@ -50,10 +56,10 @@ describe 'Milestones on EE' do
         end
       end
 
-      context 'when all closed issues do not have closed_at value' do
+      context 'when all closed issues do not have closed events' do
         it 'shows warning and hides burndown' do
-          create(:closed_issue, issue_params.merge(closed_at: nil))
-          create(:closed_issue, issue_params.merge(closed_at: nil))
+          create(:closed_issue, issue_params)
+          create(:closed_issue, issue_params)
 
           visit_milestone
 
@@ -66,7 +72,7 @@ describe 'Milestones on EE' do
       context 'data is accurate' do
         it 'does not show warning' do
           create(:issue, issue_params)
-          create(:closed_issue, issue_params)
+          close_issue(create(:issue, issue_params))
 
           visit_milestone
 

ee/spec/models/burndown_spec.rb

@@ -65,9 +65,9 @@ describe Burndown do
     expect(burndown).to be_accurate
   end
 
-  context "when all closed issues does not have closed_at" do
+  context "when all closed issues does not have closed events" do
     before do
-      milestone.issues.update_all(closed_at: nil)
+      Event.where(target: milestone.issues, action: Event::CLOSED).destroy_all
     end
 
     it "considers closed_at as milestone start date" do
@@ -87,9 +87,9 @@ describe Burndown do
     end
   end
 
-  context "when one or more closed issues does not have closed_at" do
+  context "when one or more closed issues does not have a closed event" do
     before do
-      milestone.issues.closed.first.update(closed_at: nil)
+      Event.where(target: milestone.issues.closed.first, action: Event::CLOSED).destroy_all
     end
 
     it "sets attribute accurate to false" do
@@ -112,11 +112,25 @@ describe Burndown do
 
         # Close issues
         closed = issues.slice(0..count / 2)
-        closed.each(&:close)
+        closed.each { |issue| close_issue(issue) }
 
         # Reopen issues
-        closed.slice(0..count / 4).each(&:reopen)
+        reopened_issues = closed.slice(0..count / 4)
+        reopened_issues.each { |issue| reopen_issue(issue) }
+
+        # This generates an issue with multiple closing events
+        issue_closed_twice = reopened_issues.last
+        close_issue(issue_closed_twice)
+        reopen_issue(issue_closed_twice)
       end
     end
   end
+
+  def close_issue(issue)
+    Issues::CloseService.new(issue.project, user, {}).execute(issue)
+  end
+
+  def reopen_issue(issue)
+    Issues::ReopenService.new(issue.project, user, {}).execute(issue)
+  end
 end