Check if the database exists for feature gates

Before calling out to the database to check if a feature is enabled or
not, first check if the database exists.

This allows us to use feature checks during the setup of the
application.

app/models/concerns/sha256_attribute.rb

@@ -39,11 +39,7 @@ module Sha256Attribute
     end
 
     def database_exists?
-      ApplicationRecord.connection
-
-      true
-    rescue
-      false
+      Gitlab::Database.exists?
     end
   end
 end

app/models/concerns/sha_attribute.rb

@@ -39,11 +39,7 @@ module ShaAttribute
     end
 
     def database_exists?
-      ApplicationRecord.connection
-
-      true
-    rescue
-      false
+      Gitlab::Database.exists?
     end
   end
 end

lib/feature.rb

@@ -52,6 +52,10 @@ class Feature
     # use `default_enabled: true` to default the flag to being `enabled`
     # unless set explicitly.  The default is `disabled`
     def enabled?(key, thing = nil, default_enabled: false)
+      # During setup the database does not exist yet. So we haven't stored a value
+      # for the feature yet and return the default.
+      return default_enabled unless Gitlab::Database.exists?
+
       feature = Feature.get(key)
 
       # If we're not default enabling the flag or the feature has been set, always evaluate.

lib/gitlab/database.rb

@@ -241,6 +241,14 @@ module Gitlab
       row['version']
     end
 
+    def self.exists?
+      connection
+
+      true
+    rescue
+      false
+    end
+
     private_class_method :database_version
 
     def self.add_post_migrate_path_to_rails(force: false)

spec/lib/feature_spec.rb

@@ -171,6 +171,13 @@ describe Feature do
       end
     end
 
+    it 'returns the default value when the database does not exist' do
+      fake_default = double('fake default')
+      expect(ActiveRecord::Base).to receive(:connection) { raise ActiveRecord::NoDatabaseError, "No database" }
+
+      expect(described_class.enabled?(:a_feature, default_enabled: fake_default)).to eq(fake_default)
+    end
+
     context 'cached feature flag', :request_store do
       let(:flag) { :some_feature_flag }
 

spec/lib/gitlab/database_spec.rb

@@ -396,6 +396,20 @@ describe Gitlab::Database do
     end
   end
 
+  describe '.exists?' do
+    it 'returns true if `ActiveRecord::Base.connection` succeeds' do
+      expect(ActiveRecord::Base).to receive(:connection)
+
+      expect(described_class.exists?).to be(true)
+    end
+
+    it 'returns false if `ActiveRecord::Base.connection` fails' do
+      expect(ActiveRecord::Base).to receive(:connection) { raise ActiveRecord::NoDatabaseError, 'broken' }
+
+      expect(described_class.exists?).to be(false)
+    end
+  end
+
   describe '#true_value' do
     it 'returns correct value' do
       expect(described_class.true_value).to eq "'t'"