Fix Padrino instrumentation causing boot loop

Apply the same boot loop fix as for Sinatra for Padrino from PR #1105.

For Sinatra apps noticed in our test setup that when I added a Sinatra
app to the Rails app, and loaded the Sinatra instrumentation as
specified in our docs it would get stuck in a boot loop. This was caused
by the two different configs overwriting each other every time.

Apply the same fix for Padrino by checking first if AppSignal has
already started and skipping the auto start.

.changesets/padrino-boot-loop.md

@@ -0,0 +1,7 @@
+---
+bump: patch
+type: fix
+---
+
+Fix issue with AppSignal getting stuck in a boot loop when loading the Padrino integration with: `require "appsignal/integrations/padrino"`
+This could happen in nested applications, like a Padrino app in a Rails app. AppSignal will now use the first config AppSignal starts with.

lib/appsignal/integrations/padrino.rb

@@ -13,9 +13,11 @@ def self.init
         Padrino.before_load do
           Appsignal.internal_logger.debug("Loading Padrino (#{Padrino::VERSION}) integration")
 
-          root = Padrino.mounted_root
-          Appsignal.config = Appsignal::Config.new(root, Padrino.env)
-          Appsignal.start
+          unless Appsignal.active?
+            root = Padrino.mounted_root
+            Appsignal.config = Appsignal::Config.new(root, Padrino.env)
+            Appsignal.start
+          end
 
           next unless Appsignal.active?
 

spec/lib/appsignal/integrations/padrino_spec.rb

@@ -24,6 +24,37 @@ def uninstall_padrino_integration
         end
       end
 
+      context "when already active" do
+        before { allow(Appsignal).to receive(:active?).and_return(true) }
+
+        it "does not start AppSignal again" do
+          expect(Appsignal::Config).to_not receive(:new)
+          expect(Appsignal).to_not receive(:start)
+
+          Appsignal::Integrations::PadrinoPlugin.init
+          callbacks[:before_load].call
+        end
+
+        it "adds the instrumentation middleware to Sinatra::Base" do
+          Appsignal::Integrations::PadrinoPlugin.init
+          callbacks[:before_load].call
+
+          middlewares = Padrino.middleware
+          expect(middlewares).to include(
+            [Rack::Events, [[instance_of(Appsignal::Rack::EventHandler)]], nil]
+          )
+          expect(middlewares).to include(
+            [
+              Appsignal::Rack::SinatraBaseInstrumentation,
+              [
+                :instrument_span_name => "process_action.padrino"
+              ],
+              nil
+            ]
+          )
+        end
+      end
+
       context "with active config" do
         before do
           ENV["APPSIGNAL_APP_NAME"] = "My Padrino app name"