Ensure Rack config pragmas are kept during application building

Resolves #173

Prior to these changes, Ruby pragmas (including `frozen-string-literal`)
were inserted in the wrong place.

This might be a breaking change.

Author: viralpraxis

lib/pitchfork.rb

@@ -87,7 +87,12 @@ def builder(ru, op)
         when /\.ru$/
           raw = File.read(ru)
           raw.sub!(/^__END__\n.*/, '')
-          eval("Rack::Builder.new {(\n#{raw}\n)}.to_app", TOPLEVEL_BINDING, ru)
+          lines = raw.lines
+          trailing_comments_index = lines.index { |line| !line.start_with?('#') }
+          prelude = lines[0...trailing_comments_index].join("\n")
+          raw = lines[trailing_comments_index...-1].join("\n")
+
+          eval("#{prelude}\nRack::Builder.new {(\n#{raw}\n)}.to_app", TOPLEVEL_BINDING, ru)
         else
           require ru
           Object.const_get(File.basename(ru, '.rb').capitalize)

test/integration/frozen_string.ru

@@ -0,0 +1,5 @@
+# frozen_string_literal: true
+
+'42'.succ!
+
+run lambda { |_env| [200, {}, []] }

test/integration/test_rack_pragmas.rb

@@ -0,0 +1,16 @@
+# frozen_string_literal: true
+require 'integration_test_helper'
+
+class BrokenAppTest < Pitchfork::IntegrationTest
+  def test_keeps_rack_config_pragmas
+    addr, port = unused_port
+
+    pid = spawn_server("-E", "none", app: File.join(ROOT, "test/integration/frozen_string.ru"), lint: false, config: <<~CONFIG,)
+      listen "#{addr}:#{port}"
+    CONFIG
+
+    assert_stderr "can't modify frozen String: \"42\" (FrozenError)"
+
+    assert_exited pid, 1
+  end
+end