Skip auto-inserting Rack::Lint on Rack 3+

lib/pitchfork.rb

@@ -97,7 +97,18 @@ def builder(ru, op)
         Rack::Builder.new do
           use(Rack::ContentLength)
           use(Pitchfork::Chunked)
-          use(Rack::Lint) if ENV["RACK_ENV"] == "development"
+          # Rack 3 dropped `rewind` from the strict `rack.input` surface, so
+          # `Rack::Lint::Wrapper::InputWrapper` no longer exposes it. Rails
+          # middleware (cookies, body parsers) and third-party Rack middleware
+          # still call `request.body.rewind` (or read the body twice), so
+          # inserting `Rack::Lint` here in development silently breaks every
+          # body-bearing request the moment a config switches a dev `web:`
+          # process to Pitchfork. The dev-affordance value of Lint is no longer
+          # worth the trap on Rack 3+, so only auto-insert under Rack 2.
+          # See https://github.com/Shopify/pitchfork/issues/177.
+          if ENV["RACK_ENV"] == "development" && (!defined?(Rack::RELEASE) || Rack::RELEASE < "3")
+            use(Rack::Lint)
+          end
           use(Rack::TempfileReaper)
           run inner_app
         end.to_app

test/integration/test_rack_lint.rb

@@ -0,0 +1,31 @@
+# frozen_string_literal: true
+require 'integration_test_helper'
+
+class RackLintTest < Pitchfork::IntegrationTest
+  # `fails-rack-lint.ru` returns a header named "status", which Rack::Lint
+  # rejects with a 500. On Rack 3+, Pitchfork no longer auto-inserts Rack::Lint
+  # (it drops `rewind` from `rack.input` in a way that breaks Rails middleware,
+  # see https://github.com/Shopify/pitchfork/issues/177), so the response should
+  # come back from the app unchanged.
+  def test_no_auto_rack_lint_on_rack_3
+    skip("Only relevant on Rack 3+") unless defined?(Rack::RELEASE) && Rack::RELEASE >= "3"
+
+    addr, port = unused_port
+
+    # lint: true keeps the default RACK_ENV (which the Pitchfork CLI sets to
+    # "development"). On Rack 2 this would inject Rack::Lint and the request
+    # below would 500; on Rack 3+ the gate skips the insert and the app's body
+    # comes through.
+    pid = spawn_server(app: File.join(ROOT, "test/integration/fails-rack-lint.ru"), lint: true, config: <<~CONFIG)
+      listen "#{addr}:#{port}"
+    CONFIG
+
+    assert_healthy("http://#{addr}:#{port}")
+
+    response = Net::HTTP.get_response(URI("http://#{addr}:#{port}"))
+    assert_equal 200, response.code.to_i
+    assert_equal "Rack::Lint wasn't there if you see this", response.body.strip
+
+    assert_clean_shutdown(pid)
+  end
+end