Closes puma#2341 - Make sure fallback Rack response are sent to the c…

…lient

Currently, when Client I/O operations raise any exceptions, we build a fallback rack response (either by lowlevel_error_handler or by default puma's hard-coded response), but doesn't do anything with it.
This commit ensures that this response is sent back to client.
Use `bundle exec puma -C test/config/lowlevel_error_handler.rb test/rackup/hello.ru` to see the behavior.

lib/puma/server.rb

@@ -464,7 +464,7 @@ def process_client(client)
         end
         true
       rescue StandardError => e
-        client_error(e, client)
+        client_error(e, client, requests)
         # The ensure tries to close +client+ down
         requests > 0
       ensure
@@ -492,21 +492,29 @@ def with_force_shutdown(client, &block)
     # :nocov:
 
     # Handle various error types thrown by Client I/O operations.
-    def client_error(e, client)
+    def client_error(e, client, requests = 1)
       # Swallow, do not log
       return if [ConnectionError, EOFError].include?(e.class)
 
-      lowlevel_error(e, client.env)
       case e
       when MiniSSL::SSLError
         @log_writer.ssl_error e, client.io
       when HttpParserError
+        status, headers, res_body = lowlevel_error(e, client.env, 400)
+        prepare_response(status, headers, res_body, requests, client)
+
         client.write_error(400)
         @log_writer.parse_error e, client
       when HttpParserError501
+        status, headers, res_body = lowlevel_error(e, client.env, 501)
+        prepare_response(status, headers, res_body, requests, client)
+
         client.write_error(501)
         @log_writer.parse_error e, client
       else
+        status, headers, res_body = lowlevel_error(e, client.env, 500)
+        prepare_response(status, headers, res_body, requests, client)
+
         client.write_error(500)
         @log_writer.unknown_error e, nil, "Read"
       end

test/config/lowlevel_error_handler.rb

@@ -0,0 +1,7 @@
+app do |env|
+  [200, nil, []]
+end
+
+lowlevel_error_handler do |err|
+  [500, {}, ["something wrong happened"]]
+end

test/test_config.rb

@@ -586,7 +586,7 @@ def assert_run_hooks(hook_name, options = {})
 
   def assert_warning_for_hooks_defined_in_single_mode(hook_name)
     out, _ = capture_io do
-      conf = Puma::Configuration.new do |c|
+      Puma::Configuration.new do |c|
         c.send(hook_name)
       end
     end

test/test_puma_server.rb

@@ -1545,4 +1545,19 @@ def test_streaming_enum_body_2
     assert_equal str * loops, resp_body
     assert_operator times.last - times.first, :>, 1.0
   end
+
+  def test_lowlevel_error_handler_response
+    options = {
+      lowlevel_error_handler: ->(_error) do
+        [500, {}, ["something wrong happened"]]
+      end
+    }
+    broken_app = ->(_env) { [200, nil, []] }
+
+    server_run(**options, &broken_app)
+
+    data = send_http_and_read "GET / HTTP/1.1\r\n\r\n"
+
+    assert_match(/something wrong happened/, data)
+  end
 end