Add a limit to the number of events

We don't know what number to cap this exactly but I think this is a
reasonable first iteration where most regular milestones shouldn't hit

ee/app/services/milestones/burnup_chart_service.rb

@@ -8,6 +8,12 @@
 # so that we can keep track of the issue's state during that point in time and handle the events based on that.
 
 class Milestones::BurnupChartService
+  include Gitlab::Utils::StrongMemoize
+
+  EVENT_COUNT_LIMIT = 50_000
+
+  TooManyEventsError = Class.new(StandardError)
+
   def initialize(milestone)
     raise ArgumentError, 'Milestone must have a start and due date' if milestone.start_date.blank? || milestone.due_date.blank?
 
@@ -18,6 +24,8 @@ class Milestones::BurnupChartService
     @issue_states = {}
     @chart_data = []
 
+    raise TooManyEventsError if resource_events.num_tuples > EVENT_COUNT_LIMIT
+
     resource_events.each do |event|
       case event['event_type']
       when 'milestone'
@@ -152,9 +160,11 @@ class Milestones::BurnupChartService
 
   # rubocop: disable CodeReuse/ActiveRecord
   def resource_events
+    strong_memoize(:resource_events) do
       union = Gitlab::SQL::Union.new([milestone_events, state_events, weight_events]) # rubocop: disable Gitlab/Union
 
-    ActiveRecord::Base.connection.execute("(#{union.to_sql}) ORDER BY created_at")
+      ActiveRecord::Base.connection.execute("(#{union.to_sql}) ORDER BY created_at LIMIT #{EVENT_COUNT_LIMIT + 1}")
+    end
   end
   # rubocop: enable CodeReuse/ActiveRecord
 

ee/spec/services/milestones/burnup_chart_service_spec.rb

@@ -16,6 +16,15 @@ RSpec.describe Milestones::BurnupChartService do
     expect { described_class.new(milestone) }.to raise_error('Milestone must have a start and due date')
   end
 
+  it 'raises an error when the number of events exceeds the limit' do
+    stub_const('Milestones::BurnupChartService::EVENT_COUNT_LIMIT', 1)
+
+    create(:resource_milestone_event, issue: issues[0], milestone: milestone, action: :add, created_at: '2019-12-15')
+    create(:resource_milestone_event, issue: issues[1], milestone: milestone, action: :add, created_at: '2019-12-16')
+
+    expect { chart_data }.to raise_error(described_class::TooManyEventsError)
+  end
+
   it 'aggregates events before the start date to the start date' do
     create(:resource_milestone_event, issue: issues[0], milestone: milestone, action: :add, created_at: '2019-12-15')
     create(:resource_weight_event, issue: issues[0], weight: 2, created_at: '2019-12-18')