fix(rails): move CaptureContext in the middleware stack

This fixes the issue while ensuring we're not adding
Sentry overhead to static file-serving routes, which
is why the middleware was moved historically.

Author: solnic

sentry-rails/lib/sentry/rails/railtie.rb

@@ -9,8 +9,10 @@ module Sentry
   class Railtie < ::Rails::Railtie
     # middlewares can't be injected after initialize
     initializer "sentry.use_rack_middleware" do |app|
-      # placed first so anything logged before CaptureExceptions shares the same trace context
-      app.config.middleware.unshift Sentry::Rails::CaptureContext
+      # placed right after the app-request boundary: early enough that anything logged before
+      # CaptureExceptions shares the same trace context, late enough that file-serving requests,
+      # which never reach CaptureExceptions, do not pay for a hub clone they never use
+      app.config.middleware.insert_after ActionDispatch::Executor, Sentry::Rails::CaptureContext
       # placed after all the file-sending middlewares so we can avoid unnecessary transactions
       app.config.middleware.insert_after ActionDispatch::ShowExceptions, Sentry::Rails::CaptureExceptions
       # need to place as close to DebugExceptions as possible to intercept most of the exceptions, including those raised by middlewares

sentry-rails/spec/sentry/rails_spec.rb

@@ -22,13 +22,26 @@
 
     it "inserts middleware to a correct position" do
       app = Rails.application
-      expect(app.middleware.first).to eq(Sentry::Rails::CaptureContext)
       index_of_executor = app.middleware.find_index { |m| m == ActionDispatch::ShowExceptions }
       expect(app.middleware.find_index(Sentry::Rails::CaptureExceptions)).to eq(index_of_executor + 1)
       index_of_debug_exceptions = app.middleware.find_index { |m| m == ActionDispatch::DebugExceptions }
       expect(app.middleware.find_index(Sentry::Rails::RescuedExceptionInterceptor)).to eq(index_of_debug_exceptions + 1)
     end
 
+    it "establishes the trace context before the request is logged" do
+      middleware = Rails.application.middleware
+
+      expect(middleware.find_index(Sentry::Rails::CaptureContext))
+        .to be < middleware.find_index(Rails::Rack::Logger)
+    end
+
+    it "leaves requests served above the app boundary untouched" do
+      middleware = Rails.application.middleware
+
+      expect(middleware.find_index(Sentry::Rails::CaptureContext))
+        .to be > middleware.find_index(ActionDispatch::Executor)
+    end
+
     it "propagates timezone to cron config" do
       # cron.default_timezone is set to nil by default
       expect(Sentry.configuration.cron.default_timezone).to eq("Etc/UTC")