Fix caching of future broadcast messages

This changes the caching mechanism so we cache both current _and_ future
broadcast messages, then manually filter out those we don't want to
display. This ensures we don't need any additional queries while still
being able to display the right messages at the right time.

Fixes https://gitlab.com/gitlab-org/gitlab-ce/issues/36661

app/models/broadcast_message.rb

@@ -19,11 +19,21 @@ class BroadcastMessage < ActiveRecord::Base
   after_commit :flush_redis_cache
 
   def self.current
-    Rails.cache.fetch(CACHE_KEY) do
-      where('ends_at > :now AND starts_at <= :now', now: Time.zone.now)
-        .reorder(id: :asc)
-        .to_a
+    messages = Rails.cache.fetch(CACHE_KEY) { current_and_future_messages.to_a }
+
+    return messages if messages.empty?
+
+    now_or_future = messages.select(&:now_or_future?)
+
+    # If there are cached entries but none are to be displayed we'll purge the
+    # cache so we don't keep running this code all the time.
+    Rails.cache.delete(CACHE_KEY) if now_or_future.empty?
+
+    now_or_future.select(&:now?)
   end
+
+  def self.current_and_future_messages
+    where('ends_at > :now', now: Time.zone.now).reorder(id: :asc)
   end
 
   def active?
@@ -38,6 +48,18 @@ class BroadcastMessage < ActiveRecord::Base
     ends_at < Time.zone.now
   end
 
+  def now?
+    (starts_at..ends_at).cover?(Time.zone.now)
+  end
+
+  def future?
+    starts_at > Time.zone.now
+  end
+
+  def now_or_future?
+    now? || future?
+  end
+
   def flush_redis_cache
     Rails.cache.delete(CACHE_KEY)
   end

changelogs/unreleased/fix-broadcast-message-caching.yml

+---
+title: Fix caching of future broadcast messages
+merge_request:
+author:
+type: fixed

spec/models/broadcast_message_spec.rb

@@ -53,6 +53,29 @@ describe BroadcastMessage do
 
       2.times { described_class.current }
     end
+
+    it 'includes messages that need to be displayed in the future' do
+      create(:broadcast_message)
+
+      future = create(
+        :broadcast_message,
+        starts_at: Time.now + 10.minutes,
+        ends_at: Time.now + 20.minutes
+      )
+
+      expect(described_class.current.length).to eq(1)
+
+      Timecop.travel(future.starts_at) do
+        expect(described_class.current.length).to eq(2)
+      end
+    end
+
+    it 'does not clear the cache if only a future message should be displayed' do
+      create(:broadcast_message, :future)
+
+      expect(Rails.cache).not_to receive(:delete)
+      expect(described_class.current.length).to eq(0)
+    end
   end
 
   describe '#active?' do