Fix database migrations when Redis is not running

If Redis were not running or USE_DB were set to false, the
application settings retrieval would fail completely. This
change only attempts to use the cache if the system actually
wants to connect to the DB and rescues any failures in talking to
Redis.

Closes #17557

lib/gitlab/current_settings.rb

@@ -9,10 +9,14 @@ module Gitlab
     end
 
     def ensure_application_settings!
+      if connect_to_db?
+        begin
           settings = ::ApplicationSetting.cached
+        # In case Redis isn't running or the Redis UNIX socket file is not available
+        rescue ::Redis::BaseError, ::Errno::ENOENT
+          settings = ::ApplicationSetting.last
+        end
 
-      if !settings && connect_to_db?
-        settings = ::ApplicationSetting.current
         settings ||= ::ApplicationSetting.create_from_defaults unless ActiveRecord::Migrator.needs_migration?
       end
 

spec/lib/gitlab/current_settings_spec.rb

+require 'spec_helper'
+
+describe Gitlab::CurrentSettings do
+  describe '#current_application_settings' do
+    it 'attempts to use cached values first' do
+      allow_any_instance_of(Gitlab::CurrentSettings).to receive(:connect_to_db?).and_return(true)
+      expect(ApplicationSetting).to receive(:cached).and_call_original
+      expect(ApplicationSetting).not_to receive(:last)
+
+      expect(current_application_settings).to be_a(ApplicationSetting)
+    end
+
+    it 'does not attempt to connect to DB or Redis' do
+      allow_any_instance_of(Gitlab::CurrentSettings).to receive(:connect_to_db?).and_return(false)
+      expect(ApplicationSetting).not_to receive(:current)
+      expect(ApplicationSetting).not_to receive(:last)
+
+      expect(current_application_settings).to eq fake_application_settings
+    end
+
+    it 'falls back to DB if Redis fails' do
+      allow_any_instance_of(Gitlab::CurrentSettings).to receive(:connect_to_db?).and_return(true)
+      expect(ApplicationSetting).to receive(:cached).and_raise(::Redis::BaseError)
+      expect(ApplicationSetting).to receive(:last).and_call_original
+
+      expect(current_application_settings).to be_a(ApplicationSetting)
+    end
+  end
+end