Add VAPID public key to instance serializer (#28006)

Co-authored-by: Renaud Chaput <renchap@gmail.com>

app/serializers/rest/application_serializer.rb

@@ -2,7 +2,10 @@
 
 class REST::ApplicationSerializer < ActiveModel::Serializer
   attributes :id, :name, :website, :scopes, :redirect_uri,
-             :client_id, :client_secret, :vapid_key
+             :client_id, :client_secret
+
+  # NOTE: Deprecated in 4.3.0, needs to be removed in 5.0.0
+  attribute :vapid_key
 
   def id
     object.id.to_s

app/serializers/rest/instance_serializer.rb

@@ -48,6 +48,10 @@ class REST::InstanceSerializer < ActiveModel::Serializer
         status: object.status_page_url,
       },
 
+      vapid: {
+        public_key: Rails.configuration.x.vapid_public_key,
+      },
+
       accounts: {
         max_featured_tags: FeaturedTag::LIMIT,
       },

spec/serializers/rest/instance_serializer_spec.rb

@@ -10,5 +10,11 @@ describe REST::InstanceSerializer do
     it 'returns recent usage data' do
       expect(serialization['usage']).to eq({ 'users' => { 'active_month' => 0 } })
     end
+
+    it 'returns the VAPID public key' do
+      expect(serialization['configuration']['vapid']).to eq({
+        'public_key' => Rails.configuration.x.vapid_public_key,
+      })
+    end
   end
 end